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Influence of Pain Associated with Musculoskeletal Disorders on Grip Force Timing
SCIENTIFIC/CLINICAL ARTICLE JHT READ
FOR
CREDIT ARTICLE #200.
Influence of Pain Associated with Musculoskeletal Disorders on Grip Force Timing Na Jin Seo, PhD Department of Industrial Engineering, College of Engineering & Applied Science, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin
Bhagwant S. Sindhu, PhD, OTR/L Department of Occupational Science & Technology, College of Health Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin
Orit Shechtman, PhD, OTR/L Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida
Musculoskeletal and connective tissue disorders represent 20% of all conditions that limit daily activity.1 The upper extremity is affected in 20% of all nonfatal work-related injuries and illnesses.2 People with upper extremity musculoskeletal disorders are limited in their daily activities by reduced grip strength3,4 and reduced ability to properly control grip force.5 Reduced grip strength among people with upper extremity musculoskeletal disorders has been attributed to both physical and psychosocial factors. Physical factors include reduction in the number of contracting muscle fibers,6 reduction in the firing rate of motor units,6 and changes in muscle fiber types.6e11 Psychosocial factors include pain,12e14 fear of pain, and fear of reinjury.15 People with upper extremity musculoskeletal disorders often experience pain.16 Pain can reduce grip force, which has been attributed to decreases in voluntary muscle activity seen as decreases in force generation, electromyographic activity,4,12e14,17e23 motor Part of the findings was presented at the Annual Meeting of the Society of Neuroscience, San Diego, CA, November 13, 2010. Correspondence and reprint requests to Na Jin Seo, PhD, 3200 N Cramer Street, Milwaukee, WI 53211; e-mail:. 0894-1130/$ - see front matter Ó 2011 Hanley & Belfus, an imprint of Elsevier Inc. All rights reserved. doi:10.1016/j.jht.2011.06.004
ABSTRACT: Study Design: Retrospective repeated-measures design. Introduction: Pain is a common symptom associated with musculoskeletal conditions. Purpose: This study examined if pain resulting from a unilateral upper extremity musculoskeletal injury compromises the person’s ability to rapidly initiate and release handgrip. Methods: Delays in initiating and releasing a handgrip were determined for 28 individuals with ‘‘low pain’’ and 12 individuals with ‘‘high pain’’ in the injured upper extremity. All participants had no pain in the uninjured upper extremity. Results: The high-pain group was 10% slower in initiating and releasing a grip than the low-pain group, in both injured and uninjured upper extremities, for both maximal and submaximal grips. In addition, delay in grip initiation was, on average, 8% longer for the injured than for the uninjured upper extremity. Conclusions: Unilateral musculoskeletal pain appears to delay grip initiation and relaxation bilaterally, perhaps due to a centrally mediated mechanism. Level of Evidence: n/a. J HAND THER. 2011;24:335–44.
unit discharge rates,20,21 g-motor neuron activity,22 and the ability to maintain a grip force.19,23 In addition to reduced ability to generate force, pain subsequent to musculoskeletal injuries affects the ability of a person to properly control force generation. Use of an optimal amount of force required for specific tasks5 and coordination of motor unit recruitment to generate muscle force in an intended direction24 appear to be compromised in people with musculoskeletal pain. Evidence exists that pain may also affect one’s ability to execute motor actions in a timely manner. Slower reaction time has been reported among people who experience chronic pain in the low back25,26 and lower extremities,27 as well as in people with fibromyalgia.28 In addition, acute pain induced by injecting hypertonic saline into biceps muscles has been shown to slow down reaction time for elbow flexion.29 However, it is not clear if pain in the upper extremity is associated with delays specific to handgrip force application, that is, delays in initiating a grip force exertion and in relaxing from it. Rapid gripping and releasing is necessary in many workrelated activities and daily situations. A few examples are rapid grasping of a handrail to retain balance and prevent a fall, quickly turning a steering wheel to avoid an accident, catching a flying ball, holding onto a coffee cup when the arm is pushed by another OctobereDecember 2011 335
person, releasing one stirrer to grasp another while cooking multiple dishes, and releasing a hot object before the skin burns. Consequently, slowed grip initiation and grip release may limit the ability of a person to perform daily living tasks and work-related activities. Furthermore, slowed grip initiation and release in people with pain subsequent to upper extremity musculoskeletal disorders may reduce their ability to prevent additional injury. The present study investigated if upper extremity musculoskeletal disorders/injuries and the pain associated with these injuries result in a compromised ability to perform rapid gripping and rapid releasing of grip. The main objective was to determine if delays in grip initiation and release are affected by the following independent variables: 1) the level of pain (low-pain vs. high-pain groups), 2) injury (injured vs. uninjured upper extremity), and 3) the level of grip effort, that is, the level of force exertion during gripping (maximal vs. submaximal grips). Painrelated grip delays for maximal and submaximal grips were examined because the speed of grip force development may be affected by the level of force exertion during gripping.30 In addition to the main objective, two secondary objectives were pursued. We examined if the three independent variables mentioned above (level of pain, injury, and level of grip effort) affect grip force and the consistency (or variability) in delays of grip initiation and release.
characteristics, including mean age, mean pain rating, and cause, type, side, and site of injury, are summarized in Table 1. The pain rating was significantly different between the low- and high-pain groups (p , 0.01 using the two-sample t-test). The age was not significantly different between the two groups (p ¼ 0.4 using the two-sample t-test). The incidence of dominant upper extremity injury was not significantly different between the two groups (p ¼ 0.3 using the two-sample t-test). All participants had sustained their injury at least a month before the experiment. In both low- and high-pain groups, 75% of the participants experienced traumatic injuries; 25% had experienced cumulative trauma disorders. The major types of injury were fracture and tendon tear (Table 1). The experimental protocol was reviewed and approved by the institutional review board. All study participants gave informed consent before the experiments.
Procedure Participants were instructed to grip an electronic Jamar dynamometer (Thought Technology Ltd, Montreal, QC) at a prescribed effort level on hearing TABLE 1. Participant Characteristics for the Low- and High-Pain Groups
Characteristic
METHODS Study Participants Forty individuals (20 men and 20 women) with unilateral upper extremity musculoskeletal injuries participated in this study. All participants experienced no pain in their uninjured upper extremity and pain in their injured upper extremity. They were divided accordingly into ‘‘low-pain’’ and ‘‘high-pain’’ groups. Before performing grips, participants rated their current pain intensity using a visual analog scale (VAS) ranging from 0 (no pain) to 10 (worst pain possible).31e33 Participants who rated the pain in their injured upper extremity as less than five were classified as the low-pain group; participants who rated their injured upper extremity pain at five or greater were classified as the high-pain group.34 The reason for using five as a dividing point is that people with a pain rating equal to or greater than five were reported to exhibit lower function and were described as having ‘‘significant pain,’’35 whereas people with a pain rating lower than five were reported to be able to complete daily tasks.36 Twenty-eight participants were classified as the low-pain group, and the other 12 participants were classified as the high-pain group. The participant 336
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Gender Men Women
Statistical Low-Pain High-Pain Comparison Group Group (Two-Sample (n ¼ 28) (n ¼ 12) t-test) 15 13
5 7
Age (yr)
37 (13)
40 (9)
p ¼ 0.4
Pain rating
p , 0.01
2.1 (1.1)
6.0 (1.3)
Injury side (n) Dominant side Nondominant side
16 12
10 2
Cause of injury (%) Traumatic Cumulative
75 25
75 25
36 29 7 7 7 4
42 17 8 8 17 8
7 4
0 0
39 39 21
42 25 33
Type of injury (%) Fracture Tendon tear Sprain Cut Nerve compression Repetitive stress injury Ganglion cyst Arthritis Site of pain (%) Hand Wrist Forearm/elbow
p ¼ 0.3
For participant ages and pain ratings, the mean (standard deviation) is shown. The ages, pain ratings, and injury side were statistically compared between the low- and high-pain groups using two-sample t-tests.
a computer-generated sound and relax the gripping muscles when hearing another computer-generated sound (Figure 1). During this grip-and-release procedure, grip force was recorded at a sampling rate of 2,048 Hz. Both injured and uninjured hands were tested. Three grip effort levels were tested including maximal perceived effort, 50% of maximal perceived effort, and submaximal effort at an imagined augmented pain level (referred hereafter as max, submax-50%, and submax-pain, respectively). For max, subjects were instructed to exert their maximum effort. For submax-50%, subjects were instructed to exert 50% of their maximal effort. For submax-pain, subjects were instructed to pretend that their pain was at a level of two points higher on the VAS than their current pain level. During the experiment, subjects were seated with the elbow flexed to 908 and the forearm and wrist held in a neutral position.37 The grip span of the Jamar dynamometer was 49 mm (the second position) for all subjects. Each subject performed a total of 24 grips such that each hand and grip effort level combination was tested four times and the mean across the four repetitions was calculated. The sequence of testing the hand and grip effort level conditions was randomized. Each grip exertion lasted approximately 6 seconds and a minimum of 2 minutes rest was given between consecutive grip exertions to prevent muscle fatigue.38 To ensure that the gripping tasks did not impose significant stress and pain on the participants during the experiment, their current pain intensity was recorded again after the last maximal grip using the same VAS. No significant change was observed in the pain level before and after the experiment (p ¼ 0.6 using the paired t-test).
Data Analysis The grip force data were analyzed to detect grip initiation and release for each grip trial using a custom-designed computer program (MATLAB v7.0.2; The MathWorks, Inc., Natick, MA). Delay in grip initiation was determined as the time interval between the auditory cue for gripping and when grip force exceeded the threshold (Figure 1). The threshold was defined as the sum of the mean and three standard deviations (SDs) of the grip force data in a 0.5-second window during the initial waiting period (before the auditory cue for gripping).39 Delay in grip release was determined as the time interval between the auditory cue for release and when grip force fell below the threshold (Figure 1). Grip force for each grip trial was determined as the highest mean grip force in a 1-second period during grip exertion. Grip force was quantified because grip strength may be reduced after musculoskeletal injuries,3,4 which may be due to pain12e14 or fear of pain.15 The main statistical analysis involved two mixeddesign analyses of variance (ANOVAs), one for delay in grip initiation and another for delay in grip release. The two ANOVAs were used to determine if delay in grip initiation and delay in grip release were significantly affected by pain level (high- vs. low-pain group), injury (injured vs. uninjured hand), grip effort level (max, submax-50%, submax-pain), and their second-order interactions. As secondary analysis, another mixed-design ANOVA was performed to determine if grip force was significantly affected by pain level, injury, and grip effort level. In addition, the consistency (or variability) in delays of grip initiation and release was studied by performing a multivariate ANOVA (MANOVA) on the SDs of these delays across four grip repetitions. The MANOVA was used to determine if the SDs of the delays in grip initiation and release significantly varied with the three independent variables, namely pain level, injury, grip effort level, as well as their second-order interactions. All statistical analyses were conducted using statistical software Minitab 16 (Minitab Inc., State College, PA). The p-value of 0.05 was considered significant.
RESULTS Delay in Grip Initiation FIGURE 1. Grip force during a sample grip-and-release trial at maximum effort for one subject’s injured hand. Delay in grip initiation was determined as the time interval between the ‘‘go’’ cue and when the grip force was above the threshold. Delay in grip release was determined as the time interval between the ‘‘stop’’ cue and when the grip force was below the threshold. The threshold was determined as the mean plus three times the standard deviation of the grip force data during the initial resting period.
Delay in grip initiation significantly varied with pain level, injury, and grip effort level (p , 0.05). None of the second-order interactions were significant (p . 0.05). Specifically, mean delay in grip initiation was 10% longer for the high-pain group than for the low-pain group (for both injured and uninjured hands and all three grip effort levels pooled; Figure 2A; p , 0.01). The mean delay in grip OctobereDecember 2011 337
FIGURE 2. Mean 6 standard error delay in grip initiation (A), delay in grip release (B), and grip force (C) for the injured and contralateral uninjured upper extremities for the low- (n ¼ 28) and high-pain (n ¼ 12) groups. Significant effects of injury (injured vs. uninjured hand) and pain level (low- vs. high-pain group) were found for delays in grip initiation (A) and for grip force (C). A significant effect of pain level was found for delays in grip release (B). initiation was 8% longer for the injured hand than for the uninjured hand (for both low- and high-pain groups and all three grip effort þ levels pooled; Figure 2A; p ¼ 0.03). When examining each pain group individually, the low-pain group had 9% longer delay for the injured hand, whereas the highpain group had 6% longer delay for the injured than the uninjured hand (Figure 2A). Delay in grip initiation significantly varied by grip effort level (Figure 3A; p ¼ 0.04). The mean delay in grip initiation was 9% longer for the submax-50% than for the maximum effort, and 11% longer for the submaxpain than for the maximum effort. The nonsignificant second-order interactions indicated that greater delays in grip initiation were consistently found for the high- vs. low-pain group, the injured vs. uninjured hand, and the submaximal vs. maximal grip efforts.
Delay in Grip Release Delay in grip release significantly varied with pain level (p , 0.01) but not for injury, grip effort level, and any of the second-order interactions between pain level, injury, and grip effort level (p . 0.05). 338
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The mean delay in grip release was 10% longer for the high-pain group than for the low-pain group (for both injured and uninjured hands and all three levels of grip effort pooled; Figure 2B; p , 0.01). Nonsignificant second-order interactions indicated that the slowness in grip release found for the highpain group compared with that of the low-pain group was consistent for both the injured and uninjured hands and for all three levels of grip effort.
Grip Force Grip force significantly varied for pain level, injury, and grip effort level. Grip force was 22% lower for the high-pain group than for the low-pain group (Figure 2C; p , 0.01). Grip force was 22% lower for the injured hand than for the uninjured hand (Figure 2C; p , 0.01). The mean grip force for the two submaximal grips (submax-50% and submaxpain) was approximately half of that for the maximum grip (Figure 3B; p , 0.01).
Consistency in Grip Delays The SDs of the delays in grip initiation and release for the four grip repetitions did not vary significantly
FIGURE 3. Mean 6 standard error delay in grip initiation (A), delay in grip release (B), and grip force (C) for the injured and contralateral uninjured upper extremities for the three grip effort levels. The three grip effort levels were maximum as denoted by ‘‘max,’’ 50% of maximum as denoted by ‘‘submax-50%,’’ and submaximum with imagined augmented pain as denoted by ‘‘submax-pain’’ (n ¼ 40; low- and high-pain groups pooled).
for pain level, injury, grip effort level, and their second-order interactions (p . 0.05). The MANOVA results indicated that variability in grip delays is consistent between the two pain groups, injured and uninjured hands, and the three levels of grip effort. The mean SDs of the delay in grip initiation across the four grip repetitions were as follows: for the lowpain group, 111 milliseconds for the injured hand and 114 milliseconds for the uninjured hand; for the high-pain group, 113 milliseconds for the injured hand and 108 milliseconds for the uninjured hand. The mean SDs of the delay in grip release across the four grip repetitions were as follows: for the lowpain group, 145 milliseconds for the injured hand and 103 milliseconds for the uninjured hand; for the high-pain group, 128 milliseconds for the injured hand and 103 milliseconds for the uninjured hand.
DISCUSSION Pain is the most common symptom associated with musculoskeletal conditions.16 The findings of the present study revealed that the injured upper extremity of people in the high-pain group exhibited longer delays in grip initiation and release when compared with those of people in the low-pain group. An
additional and somewhat surprising finding was that the bilateral effect of injury-related pain. In other words, people in the high-pain group exhibited a slowdown of both grip initiation and release in the contralateral uninjured upper extremity.
Delay in Grip Initiation Low- vs. High-Pain Groups Our findings indicate prolonged bilateral delays for people with musculoskeletal pain. Uninjured individuals with a similar age range to our participants are reported to have mean movement initiation times of 228e230 milliseconds when measured under the simple reaction time paradigm.40,41 Compared with healthy people, the low-pain group had, on average, 52 milliseconds longer delay in grip initiation, and the high-pain group had, on average, 81 milliseconds longer delay (with the injured and uninjured hands pooled). In addition, we found that pain level influences the delay in grip initiation. Mean delay in grip initiation for the injured upper extremity was 25 milliseconds longer for the high-pain group (317 milliseconds) than the low-pain group (293 milliseconds). Similarly, mean delay in grip initiation for OctobereDecember 2011 339
the uninjured upper extremity was 32 milliseconds longer for the high-pain group (301 milliseconds) than for the low-pain group (269 milliseconds). Slower grip initiation in both the injured and uninjured hands was observed for people in the high-pain group compared with the low-pain group (Figure 2A). The longer delay in grip initiation for the injured upper extremity for the high-pain group (Figure 2A) may be explained by one of two mechanisms. The first is avoidance of sudden muscle contraction as an instinctive protective mechanism against pain. This may be due to pain or fear of pain18,24,42 and could serve to promote healing. The second possible mechanism is pain-related reduction in the quality of sensory feedback.5 Reduced sensory feedback due to pain can alter the timing of motor activity5,43 and may prolong the delay in grip initiation. The longer delay in grip initiation for people in the high-pain group compared with those in the lowpain group was seen not only for the injured upper extremity but also for the uninjured upper extremity (Figure 2A). This bilateral effect of unilateral pain is somewhat surprising and may suggest a centrally mediated mechanism.44 Slater et al.4 have reported a centrally mediated sensory effect of upper extremity musculoskeletal pain. They found that people with lateral epicondylitis displayed an enlarged referred pain area and increased deep tissue sensitivity bilaterally, that is, in the upper extremity with pain and in the contralateral pain-free upper extremity.4 To the best of our knowledge, the present study is the first to show a possible centrally mediated motor effect of pain. Injured vs. Uninjured Hands Delay in grip initiation was, on average, 8% longer for the injured compared with the uninjured hands for both the low- and high-pain groups (Figure 2A). The finding of longer delay in grip initiation for the injured hand compared with the pain-free uninjured hand was expected. Slowed grip initiation in the presence of pain is consistent with previous studies reporting slowed reaction time for people with pain in the lower limb, low back, and elbow.25e29 Maximal vs. Submaximal Grip Effort Delay in grip initiation was longer for the submaximal than for the maximal grip efforts (Figure 3A). Similar delays in grip initiation were observed for submax-50% and submax-pain (Figure 3A). The increased delay for submaximal grips is in agreement with previous studies reporting slower speed of force development for submaximal than for maximal grips.15,30,45e49 Slower speed of force development for submaximal grip effort levels has been attributed to the greater amount of sensory feedback required 340
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by the central nervous system for control of submaximal compared with maximal grips.30,45,50e53 During repeated submaximal contractions, the brain uses massive afferent feedback to monitor and precisely control the frequency of motor unit firing and recruitment and the number and types of motor units. On the other hand, during repeated maximal contractions, all motor units are activated at the highest rates. Thus, the longer delay in grip initiation for submaximal compared with maximal contractions observed in the present study may be attributed to the longer time it takes for the central nervous system to process the greater amounts of feedback for force control. In summary, delay in grip initiation appears to be force dependent, that is, the greater the exerted force, the shorter the delay. This was consistent with injury because the stronger uninjured hand produced greater force with a shorter delay in grip initiation compared with the weaker injured hand (Figure 2A).
Delay in Grip Release Low- vs. High-Pain Groups The present study indicates prolonged delays in grip release for people with musculoskeletal pain for both injured and uninjured hands. Healthy individuals of similar age are reported to have mean delay in grip release of 441 milliseconds.39 Compared with healthy people, the low-pain group had, on average, 261 milliseconds longer delay in grip release and the high-pain group had, on average, 328 milliseconds longer delay (with the injured and uninjured hands pooled). In addition, we found that pain level influences the delay in grip release. The mean delay in grip release for the injured upper extremity was 66 milliseconds longer for the high-pain group (775 milliseconds) than the low-pain group (709 milliseconds). Similarly, the mean delay in grip delay for the uninjured upper extremity was 68 milliseconds longer for the high-pain group (762 milliseconds) than for the low-pain group (694 milliseconds). The 10% longer delay in grip release for people in the high-pain group than for those in the low-pain group indicated pain-related lag not only in muscle contraction but also in muscle relaxation (Figure 2B). The pain-related protective mechanism that plays a role in delaying grip initiation may also delay grip release. Specifically, the protective mechanism that aims to minimize sudden muscle movement and associated pain18,24,42 may contribute to slowing down motor unit de-recruitment and, thus, muscle relaxation. Injured vs. Uninjured Hands Both high- and low-pain groups had mean delays in grip release (768 and 701 milliseconds, respectively),
which were longer than that previously reported for healthy people with a similar age range (441 milliseconds).39 Delay in grip release was not different between the injured and uninjured upper extremities (Figure 2B). The lack of difference in grip release time between the two sides may be due to the bilateral effect of pain, that is, a centrally mediated mechanism slowing down grip release similar to that discussed for grip initiation. The lack of difference may also stem from the fact that grip release does not seem to be force dependent (i.e., is not dependent on the amount of force exerted during grip). This is also demonstrated by the lack of difference between maximal and submaximal grips (Figure 3B). To summarize, delay in grip initiation appears to be both pain dependent and force dependent, whereas delay in grip release appears to be only pain dependent. Maximal vs. Submaximal Grip Effort Delay in grip release did not significantly vary for different grip effort levels (Figure 3B). This suggests that feedback-based force control is used during grip initiation but not during grip release. The data also suggest that de-recruitment of many motor units takes approximately as long as de-recruitment of a partial pool of motor units.
Grip Force Consistent with previous studies,3,4,12e15 reduced grip strength was observed for people in the high-pain group than those in the low-pain group, bilaterally, and for the injured than uninjured upper extremities (Figure 2C). Reduced grip strength due to musculoskeletal injury and pain may be attributed to decreased muscle contractions,4,12e14,17e23 fear of pain and reinjury,15 as well as disuse-related muscle atrophy.6 Grip force for the uninjured hand was 13% lower for people in the high-pain group than those in the low-pain group (Figure 2C). The same pain-related protective mechanism that slowed down grip initiation and release may also have reduced grip force bilaterally. Imagining augmented pain resulted in similar grip force as in submax-50% (Figure 3C).
Consistency in Grip Delays Variability in grip delays was not significantly different between the two pain groups, between the injured and uninjured upper extremities, and among the three levels of grip effort. This indicated that prolonged grip delays due to increased pain levels were observed consistently rather than occasionally. Likewise, prolonged delay in grip initiation was consistently observed for the injured hand compared with the uninjured hand, and for
submaximal grip efforts compared with maximal grip efforts.
Functional and Clinical Implications Increased delays due to increased pain levels (29 milliseconds for grip initiation and 67 milliseconds for grip release, pooled for both hands and all levels of grip effort) have functional implications and may reduce one’s ability to perform daily tasks and work-related activities that require fast gripping and fast release. For instance, it takes, on average, 60 milliseconds for healthy persons to activate forearm muscles to stabilize their grip in response to a perturbation to their grip.54 The mean time for healthy persons to detect a floor perturbation and activate both upper and lower limb muscles to restore their balance is 88e90 milliseconds.55 It is conceivable that increased grip delays with increased pain may limit a person’s ability to rapidly respond to an impending situation and prevent a future injury. Prolonged delays in grip initiation and release not only have direct functional implications but also provide information on a person’s global motor function. According to Chae et al.,56 ‘‘delays in initiation and termination of muscle contraction simply reflect the global motor dysfunction in the central nervous system.’’ For instance, although mean differences in simple reaction time across different age groups were only 30 milliseconds (220 6 30 milliseconds for people in their 20s, 230 6 40 milliseconds for people in their early 40s, and 250 6 60 milliseconds for people in their early 60s), reaction time closely correlated with age and associated biomarkers such as forced expiratory volume at one second, grip strength, vision, physical and mental health, and lifestyle.41 In addition, muscle initiation/relaxation times and grip force timing have been shown to correlate with functional scores in patient populations.56e58 Motor impairment in people with brain injury was characterized by mean delay in grip force modulation by 16e122 milliseconds compared with age-matched controls.58 Thus, the mean differences in delay between our low- and high-pain groups (29 milliseconds and 67 milliseconds for the grip initiation and release, respectively) appear to be considerable. Therapies and interventions to shorten delays in grip39,59,60 and/or enhance grip strength may help people with musculoskeletal disorders perform daily activities more efficiently.
LIMITATIONS We have not controlled for possible confounding variables for grip timing such as injury type, age, preinjury muscle mass, fitness level, and possible muscle atrophy from disuse postinjury. Matching of OctobereDecember 2011 341
the injury site, diagnosis, gender, and age between the high- and low-pain groups may give a more definitive conclusion on the effect of pain on grip timing. Furthermore, a large population data set including all potential factors, pain level, and hand function scores may provide a more detailed statistical description of the effect of musculoskeletal injuries and associated pain on function.
CONCLUSION Musculoskeletal pain appears not only to reduce grip strength but also to slow down grip initiation and grip release of both injured and uninjured upper extremities. Slowed grip initiation and release can adversely impact one’s functional abilities and reduce one’s ability to rapidly respond to prevent an event leading to a possible injury. Slowed grip initiation and release may be attributable to psychological factors such as fear of pain and reinjury, as well as physiological factors such as reduced grip force and slowed recruitment and de-recruitment of motor units. The present study furthers the understanding of grip initiation and release delays in people with upper extremity musculoskeletal injuries and associated pain. Such knowledge is expected to contribute to enhancing rehabilitation and ergonomics practices for those affected individuals. Acknowledgments This study was partially funded by the University of Wisconsin-Milwaukee College of Engineering & Applied Science and the Evelyn Mackin Grant from the American Society of Hand Therapists. Partial equipment support was provided by Thought Technology Ltd, Montreal, Quebec, Canada. The authors report no conflict of interest.
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30. Kroemer KH, Marras WS. Towards an objective assessment of the ‘‘maximal voluntary contraction’’ component in routine muscle strength measurements. Eur J Appl Physiol Occup Physiol. 1980;45:1–9. 31. Butler PV. Linear analogue self-assessment and procrustean measurement: a critical review of visual analogue scaling in pain assessment. J Clin Psychol Med Settings. 1997;4: 111–29. 32. Jensen MP, Karoly P. Self-report scales and procedures for assessing pain in adults. In: Turk DC, Melzack R (eds). Handbook of Pain Assessment. 2nd ed. New York, NY: Guilford Press, 2001. pp. 15–34. 33. Coll AM, Ameen JR, Mead D. Postoperative pain assessment tools in day surgery: literature review. J Adv Nurs. 2004;46: 124–33. 34. Zelman DC, Hoffman DL, Seifeldin R, Dukes EM. Development of a metric for a day of manageable pain control: derivation of pain severity cut-points for low back pain and osteoarthritis. Pain. 2003;106:35–42. 35. Cleeland CS, Ryan KM. Pain assessment: global use of the Brief Pain Inventory. Ann Acad Med Singapore. 1994;23:129–38. 36. Chae J, Mascarenhas D, Yu DT, et al. Poststroke shoulder pain: its relationship to motor impairment, activity limitation, and quality of life. Arch Phys Med Rehabil. 2007;88:298–301. 37. Fess EE. Grip Strength. In: Casanova JS (ed). Clinical Assessment Recommendations. 2nd ed. Chicago, IL: The American Society of Hand Therapists, 1992. pp. 41–5. 38. Caldwell LS, Chaffin DB, Dukes-Dobos FN, et al. A proposed standard procedure for static muscle strength testing. Am Ind Hyg Assoc J. 1974;35:201–6. 39. Seo NJ, Rymer WZ, Kamper DG. Delays in grip initiation and termination in persons with stroke: effects of arm support and active muscle stretch exercise. J Neurophysiol. 2009;101: 3108–15. 40. Howes D, Boller F. Simple reaction time: evidence for focal impairment from lesions of the right hemisphere. Brain. 1975;98: 317–32. 41. Anstey KJ, Dear K, Christensen H, Jorm AF. Biomarkers, health, lifestyle, and demographic variables as correlates of reaction time performance in early, middle, and late adulthood. Q J Exp Psychol A. 2005;58:5–21. 42. Kroemer KH. Designing for Muscular Strength of Various Populations. AMRL TR, vol 72. Wright-Patterson AFB, OH: Air Force Aerospace Medical Research Lab, 1974. p. 46. 43. Monzee J, Lamarre Y, Smith AM. The effects of digital anesthesia on force control using a precision grip. J Neurophysiol. 2003;89:672–83. 44. Graven-Nielsen T, Lund H, Arendt-Nielsen L, DanneskioldSamsoe B, Bliddal H. Inhibition of maximal voluntary contraction force by experimental muscle pain: a centrally mediated mechanism. Muscle Nerve. 2002;26:708–12.
45. Astrand PO, Rodahl K (eds). Textbook of Work Physiology. 2nd ed. New York, NY: McGraw-Hill, 1977. 46. Gilbert JC, Knowlton RG. Simple method to determine sincerity of effort during a maximal isometric test of grip strength. Am J Phys Med. 1983;62:135–44. 47. Shechtman O. Using the coefficient of variation to detect sincerity of effort of grip strength: a literature review. J Hand Ther. 2000;13:25–32. 48. Shechtman O, Taylor C. The use of the rapid exchange grip test in detecting sincerity of effort, part ii: validity of the test. J Hand Ther. 2000;13:203–10. 49. Shechtman O, Sindhu BS, Davenport PW. Using the force-time curve to detect maximal grip strength effort. J Hand Ther. 2007; 20:37–48. 50. Edgerton VR. Mammalian muscle-fiber types and their adaptability. Am Zool. 1978;18:113–25. 51. Milner-Brown HS, Stein RB, Yemm R. Changes in firing rate of human motor units during linearly changing voluntary contractions. J Physiol. 1973;230:371–90. 52. Milner-Brown HS, Stein RB, Yemm R. The orderly recruitment of human motor units during voluntary isometric contractions. J Physiol. 1973;230:359–70. 53. Houk JC. Regulation of stiffness by skeletomotor reflexes. Annu Rev Physiol. 1979;41:99–114. 54. Motawar BR, Hur P, Seo NJ. Roles of cutaneous sensation and gloves with different coefficients of friction on fall recovery during simulated ladder falls. The 35th Annual Meeting of the American Society of Biomechanics. 2011; Long Beach, CA. 55. McIlroy WE, Maki BE. Early activation of arm muscles follows external perturbation of upright stance. Neurosci Lett. 1995; 184:177–80. 56. Chae J, Yang G, Park BK, Labatia I. Delay in initiation and termination of muscle contraction, motor impairment, and physical disability in upper limb hemiparesis. Muscle Nerve. 2002; 25:568–75. 57. Chae J, Quinn A, El-Hayek K, Santing J, Berezovski R, Harley M. Delay in initiation and termination of tibialis anterior contraction in lower-limb hemiparesis: relationship to lowerlimb motor impairment and mobility. Arch Phys Med Rehabil. 2006;87:1230–4. 58. Nowak DA, Grefkes C, Dafotakis M, Kust J, Karbe H, Fink GR. Dexterity is impaired at both hands following unilateral subcortical middle cerebral artery stroke. Eur J Neurosci. 2007; 25:3173–84. 59. Tsao H, Hodges PW. Immediate changes in feedforward postural adjustments following voluntary motor training. Exp Brain Res. 2007;181:537–46. 60. Seo NJ, Fischer HC, Bogey RA, Rymer WZ, Kamper DG. Effect of a serotonin antagonist on delay in grip muscle relaxation for persons with chronic hemiparetic stroke. Clin Neurophysiol. 2011;122:796–802.
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JHT Read for Credit Quiz: Article #200
Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to JHTReadforCredit.com. There is only one best answer for each question. #1. Reduced grip strength for people with upper extremity musculoskeletal disorders is related to a. increase in motor unit firing rate b. muscle atrophy c. fear of pain or fear of re-injury d. amplified EMG activity #2. The participants of this study were a. 28 people with moderate pain and 12 people with intense pain b. 28 people with pain resulting from musculoskeletal injury and 12 healthy individuals without history of musculoskeletal problems c. 28 older adults who frequently fall and 12 young adults d. 20 women and 20 men with history of musculoskeletal problems but without pain #3. The methodology of data collection in this study was a. written questionnaires b. retrospective chart reviews c. muscle activity measured using EMG
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d. time course of grip force measured using a force transducer #4. This study found that a. pain speeds up gripping b. pain speeds up grip release c. pain in one upper extremity affects the contralateral upper extremity as well d. grip timing is affected by pain, but not by the intensity of pain #5. The authors think that a. people with musculoskeletal injury and associated pain tend to overexert themselves more than people without musculoskeletal injury and associated pain b. unilateral pain has a bilateral effect on motor function through a centrally-mediated mechanism c. fear of pain may help speed up a person’s reaction to impending situations and thus protects future injuries d. delay in grip initiation is independent of grip force levels
When submitting to the HTCC for re-certification, please batch your JHT RFC certificates in groups of 3 or more to get full credit.
FOR
CREDIT ARTICLE #200.
Influence of Pain Associated with Musculoskeletal Disorders on Grip Force Timing Na Jin Seo, PhD Department of Industrial Engineering, College of Engineering & Applied Science, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin
Bhagwant S. Sindhu, PhD, OTR/L Department of Occupational Science & Technology, College of Health Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin
Orit Shechtman, PhD, OTR/L Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida
Musculoskeletal and connective tissue disorders represent 20% of all conditions that limit daily activity.1 The upper extremity is affected in 20% of all nonfatal work-related injuries and illnesses.2 People with upper extremity musculoskeletal disorders are limited in their daily activities by reduced grip strength3,4 and reduced ability to properly control grip force.5 Reduced grip strength among people with upper extremity musculoskeletal disorders has been attributed to both physical and psychosocial factors. Physical factors include reduction in the number of contracting muscle fibers,6 reduction in the firing rate of motor units,6 and changes in muscle fiber types.6e11 Psychosocial factors include pain,12e14 fear of pain, and fear of reinjury.15 People with upper extremity musculoskeletal disorders often experience pain.16 Pain can reduce grip force, which has been attributed to decreases in voluntary muscle activity seen as decreases in force generation, electromyographic activity,4,12e14,17e23 motor Part of the findings was presented at the Annual Meeting of the Society of Neuroscience, San Diego, CA, November 13, 2010. Correspondence and reprint requests to Na Jin Seo, PhD, 3200 N Cramer Street, Milwaukee, WI 53211; e-mail:
ABSTRACT: Study Design: Retrospective repeated-measures design. Introduction: Pain is a common symptom associated with musculoskeletal conditions. Purpose: This study examined if pain resulting from a unilateral upper extremity musculoskeletal injury compromises the person’s ability to rapidly initiate and release handgrip. Methods: Delays in initiating and releasing a handgrip were determined for 28 individuals with ‘‘low pain’’ and 12 individuals with ‘‘high pain’’ in the injured upper extremity. All participants had no pain in the uninjured upper extremity. Results: The high-pain group was 10% slower in initiating and releasing a grip than the low-pain group, in both injured and uninjured upper extremities, for both maximal and submaximal grips. In addition, delay in grip initiation was, on average, 8% longer for the injured than for the uninjured upper extremity. Conclusions: Unilateral musculoskeletal pain appears to delay grip initiation and relaxation bilaterally, perhaps due to a centrally mediated mechanism. Level of Evidence: n/a. J HAND THER. 2011;24:335–44.
unit discharge rates,20,21 g-motor neuron activity,22 and the ability to maintain a grip force.19,23 In addition to reduced ability to generate force, pain subsequent to musculoskeletal injuries affects the ability of a person to properly control force generation. Use of an optimal amount of force required for specific tasks5 and coordination of motor unit recruitment to generate muscle force in an intended direction24 appear to be compromised in people with musculoskeletal pain. Evidence exists that pain may also affect one’s ability to execute motor actions in a timely manner. Slower reaction time has been reported among people who experience chronic pain in the low back25,26 and lower extremities,27 as well as in people with fibromyalgia.28 In addition, acute pain induced by injecting hypertonic saline into biceps muscles has been shown to slow down reaction time for elbow flexion.29 However, it is not clear if pain in the upper extremity is associated with delays specific to handgrip force application, that is, delays in initiating a grip force exertion and in relaxing from it. Rapid gripping and releasing is necessary in many workrelated activities and daily situations. A few examples are rapid grasping of a handrail to retain balance and prevent a fall, quickly turning a steering wheel to avoid an accident, catching a flying ball, holding onto a coffee cup when the arm is pushed by another OctobereDecember 2011 335
person, releasing one stirrer to grasp another while cooking multiple dishes, and releasing a hot object before the skin burns. Consequently, slowed grip initiation and grip release may limit the ability of a person to perform daily living tasks and work-related activities. Furthermore, slowed grip initiation and release in people with pain subsequent to upper extremity musculoskeletal disorders may reduce their ability to prevent additional injury. The present study investigated if upper extremity musculoskeletal disorders/injuries and the pain associated with these injuries result in a compromised ability to perform rapid gripping and rapid releasing of grip. The main objective was to determine if delays in grip initiation and release are affected by the following independent variables: 1) the level of pain (low-pain vs. high-pain groups), 2) injury (injured vs. uninjured upper extremity), and 3) the level of grip effort, that is, the level of force exertion during gripping (maximal vs. submaximal grips). Painrelated grip delays for maximal and submaximal grips were examined because the speed of grip force development may be affected by the level of force exertion during gripping.30 In addition to the main objective, two secondary objectives were pursued. We examined if the three independent variables mentioned above (level of pain, injury, and level of grip effort) affect grip force and the consistency (or variability) in delays of grip initiation and release.
characteristics, including mean age, mean pain rating, and cause, type, side, and site of injury, are summarized in Table 1. The pain rating was significantly different between the low- and high-pain groups (p , 0.01 using the two-sample t-test). The age was not significantly different between the two groups (p ¼ 0.4 using the two-sample t-test). The incidence of dominant upper extremity injury was not significantly different between the two groups (p ¼ 0.3 using the two-sample t-test). All participants had sustained their injury at least a month before the experiment. In both low- and high-pain groups, 75% of the participants experienced traumatic injuries; 25% had experienced cumulative trauma disorders. The major types of injury were fracture and tendon tear (Table 1). The experimental protocol was reviewed and approved by the institutional review board. All study participants gave informed consent before the experiments.
Procedure Participants were instructed to grip an electronic Jamar dynamometer (Thought Technology Ltd, Montreal, QC) at a prescribed effort level on hearing TABLE 1. Participant Characteristics for the Low- and High-Pain Groups
Characteristic
METHODS Study Participants Forty individuals (20 men and 20 women) with unilateral upper extremity musculoskeletal injuries participated in this study. All participants experienced no pain in their uninjured upper extremity and pain in their injured upper extremity. They were divided accordingly into ‘‘low-pain’’ and ‘‘high-pain’’ groups. Before performing grips, participants rated their current pain intensity using a visual analog scale (VAS) ranging from 0 (no pain) to 10 (worst pain possible).31e33 Participants who rated the pain in their injured upper extremity as less than five were classified as the low-pain group; participants who rated their injured upper extremity pain at five or greater were classified as the high-pain group.34 The reason for using five as a dividing point is that people with a pain rating equal to or greater than five were reported to exhibit lower function and were described as having ‘‘significant pain,’’35 whereas people with a pain rating lower than five were reported to be able to complete daily tasks.36 Twenty-eight participants were classified as the low-pain group, and the other 12 participants were classified as the high-pain group. The participant 336
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Gender Men Women
Statistical Low-Pain High-Pain Comparison Group Group (Two-Sample (n ¼ 28) (n ¼ 12) t-test) 15 13
5 7
Age (yr)
37 (13)
40 (9)
p ¼ 0.4
Pain rating
p , 0.01
2.1 (1.1)
6.0 (1.3)
Injury side (n) Dominant side Nondominant side
16 12
10 2
Cause of injury (%) Traumatic Cumulative
75 25
75 25
36 29 7 7 7 4
42 17 8 8 17 8
7 4
0 0
39 39 21
42 25 33
Type of injury (%) Fracture Tendon tear Sprain Cut Nerve compression Repetitive stress injury Ganglion cyst Arthritis Site of pain (%) Hand Wrist Forearm/elbow
p ¼ 0.3
For participant ages and pain ratings, the mean (standard deviation) is shown. The ages, pain ratings, and injury side were statistically compared between the low- and high-pain groups using two-sample t-tests.
a computer-generated sound and relax the gripping muscles when hearing another computer-generated sound (Figure 1). During this grip-and-release procedure, grip force was recorded at a sampling rate of 2,048 Hz. Both injured and uninjured hands were tested. Three grip effort levels were tested including maximal perceived effort, 50% of maximal perceived effort, and submaximal effort at an imagined augmented pain level (referred hereafter as max, submax-50%, and submax-pain, respectively). For max, subjects were instructed to exert their maximum effort. For submax-50%, subjects were instructed to exert 50% of their maximal effort. For submax-pain, subjects were instructed to pretend that their pain was at a level of two points higher on the VAS than their current pain level. During the experiment, subjects were seated with the elbow flexed to 908 and the forearm and wrist held in a neutral position.37 The grip span of the Jamar dynamometer was 49 mm (the second position) for all subjects. Each subject performed a total of 24 grips such that each hand and grip effort level combination was tested four times and the mean across the four repetitions was calculated. The sequence of testing the hand and grip effort level conditions was randomized. Each grip exertion lasted approximately 6 seconds and a minimum of 2 minutes rest was given between consecutive grip exertions to prevent muscle fatigue.38 To ensure that the gripping tasks did not impose significant stress and pain on the participants during the experiment, their current pain intensity was recorded again after the last maximal grip using the same VAS. No significant change was observed in the pain level before and after the experiment (p ¼ 0.6 using the paired t-test).
Data Analysis The grip force data were analyzed to detect grip initiation and release for each grip trial using a custom-designed computer program (MATLAB v7.0.2; The MathWorks, Inc., Natick, MA). Delay in grip initiation was determined as the time interval between the auditory cue for gripping and when grip force exceeded the threshold (Figure 1). The threshold was defined as the sum of the mean and three standard deviations (SDs) of the grip force data in a 0.5-second window during the initial waiting period (before the auditory cue for gripping).39 Delay in grip release was determined as the time interval between the auditory cue for release and when grip force fell below the threshold (Figure 1). Grip force for each grip trial was determined as the highest mean grip force in a 1-second period during grip exertion. Grip force was quantified because grip strength may be reduced after musculoskeletal injuries,3,4 which may be due to pain12e14 or fear of pain.15 The main statistical analysis involved two mixeddesign analyses of variance (ANOVAs), one for delay in grip initiation and another for delay in grip release. The two ANOVAs were used to determine if delay in grip initiation and delay in grip release were significantly affected by pain level (high- vs. low-pain group), injury (injured vs. uninjured hand), grip effort level (max, submax-50%, submax-pain), and their second-order interactions. As secondary analysis, another mixed-design ANOVA was performed to determine if grip force was significantly affected by pain level, injury, and grip effort level. In addition, the consistency (or variability) in delays of grip initiation and release was studied by performing a multivariate ANOVA (MANOVA) on the SDs of these delays across four grip repetitions. The MANOVA was used to determine if the SDs of the delays in grip initiation and release significantly varied with the three independent variables, namely pain level, injury, grip effort level, as well as their second-order interactions. All statistical analyses were conducted using statistical software Minitab 16 (Minitab Inc., State College, PA). The p-value of 0.05 was considered significant.
RESULTS Delay in Grip Initiation FIGURE 1. Grip force during a sample grip-and-release trial at maximum effort for one subject’s injured hand. Delay in grip initiation was determined as the time interval between the ‘‘go’’ cue and when the grip force was above the threshold. Delay in grip release was determined as the time interval between the ‘‘stop’’ cue and when the grip force was below the threshold. The threshold was determined as the mean plus three times the standard deviation of the grip force data during the initial resting period.
Delay in grip initiation significantly varied with pain level, injury, and grip effort level (p , 0.05). None of the second-order interactions were significant (p . 0.05). Specifically, mean delay in grip initiation was 10% longer for the high-pain group than for the low-pain group (for both injured and uninjured hands and all three grip effort levels pooled; Figure 2A; p , 0.01). The mean delay in grip OctobereDecember 2011 337
FIGURE 2. Mean 6 standard error delay in grip initiation (A), delay in grip release (B), and grip force (C) for the injured and contralateral uninjured upper extremities for the low- (n ¼ 28) and high-pain (n ¼ 12) groups. Significant effects of injury (injured vs. uninjured hand) and pain level (low- vs. high-pain group) were found for delays in grip initiation (A) and for grip force (C). A significant effect of pain level was found for delays in grip release (B). initiation was 8% longer for the injured hand than for the uninjured hand (for both low- and high-pain groups and all three grip effort þ levels pooled; Figure 2A; p ¼ 0.03). When examining each pain group individually, the low-pain group had 9% longer delay for the injured hand, whereas the highpain group had 6% longer delay for the injured than the uninjured hand (Figure 2A). Delay in grip initiation significantly varied by grip effort level (Figure 3A; p ¼ 0.04). The mean delay in grip initiation was 9% longer for the submax-50% than for the maximum effort, and 11% longer for the submaxpain than for the maximum effort. The nonsignificant second-order interactions indicated that greater delays in grip initiation were consistently found for the high- vs. low-pain group, the injured vs. uninjured hand, and the submaximal vs. maximal grip efforts.
Delay in Grip Release Delay in grip release significantly varied with pain level (p , 0.01) but not for injury, grip effort level, and any of the second-order interactions between pain level, injury, and grip effort level (p . 0.05). 338
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The mean delay in grip release was 10% longer for the high-pain group than for the low-pain group (for both injured and uninjured hands and all three levels of grip effort pooled; Figure 2B; p , 0.01). Nonsignificant second-order interactions indicated that the slowness in grip release found for the highpain group compared with that of the low-pain group was consistent for both the injured and uninjured hands and for all three levels of grip effort.
Grip Force Grip force significantly varied for pain level, injury, and grip effort level. Grip force was 22% lower for the high-pain group than for the low-pain group (Figure 2C; p , 0.01). Grip force was 22% lower for the injured hand than for the uninjured hand (Figure 2C; p , 0.01). The mean grip force for the two submaximal grips (submax-50% and submaxpain) was approximately half of that for the maximum grip (Figure 3B; p , 0.01).
Consistency in Grip Delays The SDs of the delays in grip initiation and release for the four grip repetitions did not vary significantly
FIGURE 3. Mean 6 standard error delay in grip initiation (A), delay in grip release (B), and grip force (C) for the injured and contralateral uninjured upper extremities for the three grip effort levels. The three grip effort levels were maximum as denoted by ‘‘max,’’ 50% of maximum as denoted by ‘‘submax-50%,’’ and submaximum with imagined augmented pain as denoted by ‘‘submax-pain’’ (n ¼ 40; low- and high-pain groups pooled).
for pain level, injury, grip effort level, and their second-order interactions (p . 0.05). The MANOVA results indicated that variability in grip delays is consistent between the two pain groups, injured and uninjured hands, and the three levels of grip effort. The mean SDs of the delay in grip initiation across the four grip repetitions were as follows: for the lowpain group, 111 milliseconds for the injured hand and 114 milliseconds for the uninjured hand; for the high-pain group, 113 milliseconds for the injured hand and 108 milliseconds for the uninjured hand. The mean SDs of the delay in grip release across the four grip repetitions were as follows: for the lowpain group, 145 milliseconds for the injured hand and 103 milliseconds for the uninjured hand; for the high-pain group, 128 milliseconds for the injured hand and 103 milliseconds for the uninjured hand.
DISCUSSION Pain is the most common symptom associated with musculoskeletal conditions.16 The findings of the present study revealed that the injured upper extremity of people in the high-pain group exhibited longer delays in grip initiation and release when compared with those of people in the low-pain group. An
additional and somewhat surprising finding was that the bilateral effect of injury-related pain. In other words, people in the high-pain group exhibited a slowdown of both grip initiation and release in the contralateral uninjured upper extremity.
Delay in Grip Initiation Low- vs. High-Pain Groups Our findings indicate prolonged bilateral delays for people with musculoskeletal pain. Uninjured individuals with a similar age range to our participants are reported to have mean movement initiation times of 228e230 milliseconds when measured under the simple reaction time paradigm.40,41 Compared with healthy people, the low-pain group had, on average, 52 milliseconds longer delay in grip initiation, and the high-pain group had, on average, 81 milliseconds longer delay (with the injured and uninjured hands pooled). In addition, we found that pain level influences the delay in grip initiation. Mean delay in grip initiation for the injured upper extremity was 25 milliseconds longer for the high-pain group (317 milliseconds) than the low-pain group (293 milliseconds). Similarly, mean delay in grip initiation for OctobereDecember 2011 339
the uninjured upper extremity was 32 milliseconds longer for the high-pain group (301 milliseconds) than for the low-pain group (269 milliseconds). Slower grip initiation in both the injured and uninjured hands was observed for people in the high-pain group compared with the low-pain group (Figure 2A). The longer delay in grip initiation for the injured upper extremity for the high-pain group (Figure 2A) may be explained by one of two mechanisms. The first is avoidance of sudden muscle contraction as an instinctive protective mechanism against pain. This may be due to pain or fear of pain18,24,42 and could serve to promote healing. The second possible mechanism is pain-related reduction in the quality of sensory feedback.5 Reduced sensory feedback due to pain can alter the timing of motor activity5,43 and may prolong the delay in grip initiation. The longer delay in grip initiation for people in the high-pain group compared with those in the lowpain group was seen not only for the injured upper extremity but also for the uninjured upper extremity (Figure 2A). This bilateral effect of unilateral pain is somewhat surprising and may suggest a centrally mediated mechanism.44 Slater et al.4 have reported a centrally mediated sensory effect of upper extremity musculoskeletal pain. They found that people with lateral epicondylitis displayed an enlarged referred pain area and increased deep tissue sensitivity bilaterally, that is, in the upper extremity with pain and in the contralateral pain-free upper extremity.4 To the best of our knowledge, the present study is the first to show a possible centrally mediated motor effect of pain. Injured vs. Uninjured Hands Delay in grip initiation was, on average, 8% longer for the injured compared with the uninjured hands for both the low- and high-pain groups (Figure 2A). The finding of longer delay in grip initiation for the injured hand compared with the pain-free uninjured hand was expected. Slowed grip initiation in the presence of pain is consistent with previous studies reporting slowed reaction time for people with pain in the lower limb, low back, and elbow.25e29 Maximal vs. Submaximal Grip Effort Delay in grip initiation was longer for the submaximal than for the maximal grip efforts (Figure 3A). Similar delays in grip initiation were observed for submax-50% and submax-pain (Figure 3A). The increased delay for submaximal grips is in agreement with previous studies reporting slower speed of force development for submaximal than for maximal grips.15,30,45e49 Slower speed of force development for submaximal grip effort levels has been attributed to the greater amount of sensory feedback required 340
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by the central nervous system for control of submaximal compared with maximal grips.30,45,50e53 During repeated submaximal contractions, the brain uses massive afferent feedback to monitor and precisely control the frequency of motor unit firing and recruitment and the number and types of motor units. On the other hand, during repeated maximal contractions, all motor units are activated at the highest rates. Thus, the longer delay in grip initiation for submaximal compared with maximal contractions observed in the present study may be attributed to the longer time it takes for the central nervous system to process the greater amounts of feedback for force control. In summary, delay in grip initiation appears to be force dependent, that is, the greater the exerted force, the shorter the delay. This was consistent with injury because the stronger uninjured hand produced greater force with a shorter delay in grip initiation compared with the weaker injured hand (Figure 2A).
Delay in Grip Release Low- vs. High-Pain Groups The present study indicates prolonged delays in grip release for people with musculoskeletal pain for both injured and uninjured hands. Healthy individuals of similar age are reported to have mean delay in grip release of 441 milliseconds.39 Compared with healthy people, the low-pain group had, on average, 261 milliseconds longer delay in grip release and the high-pain group had, on average, 328 milliseconds longer delay (with the injured and uninjured hands pooled). In addition, we found that pain level influences the delay in grip release. The mean delay in grip release for the injured upper extremity was 66 milliseconds longer for the high-pain group (775 milliseconds) than the low-pain group (709 milliseconds). Similarly, the mean delay in grip delay for the uninjured upper extremity was 68 milliseconds longer for the high-pain group (762 milliseconds) than for the low-pain group (694 milliseconds). The 10% longer delay in grip release for people in the high-pain group than for those in the low-pain group indicated pain-related lag not only in muscle contraction but also in muscle relaxation (Figure 2B). The pain-related protective mechanism that plays a role in delaying grip initiation may also delay grip release. Specifically, the protective mechanism that aims to minimize sudden muscle movement and associated pain18,24,42 may contribute to slowing down motor unit de-recruitment and, thus, muscle relaxation. Injured vs. Uninjured Hands Both high- and low-pain groups had mean delays in grip release (768 and 701 milliseconds, respectively),
which were longer than that previously reported for healthy people with a similar age range (441 milliseconds).39 Delay in grip release was not different between the injured and uninjured upper extremities (Figure 2B). The lack of difference in grip release time between the two sides may be due to the bilateral effect of pain, that is, a centrally mediated mechanism slowing down grip release similar to that discussed for grip initiation. The lack of difference may also stem from the fact that grip release does not seem to be force dependent (i.e., is not dependent on the amount of force exerted during grip). This is also demonstrated by the lack of difference between maximal and submaximal grips (Figure 3B). To summarize, delay in grip initiation appears to be both pain dependent and force dependent, whereas delay in grip release appears to be only pain dependent. Maximal vs. Submaximal Grip Effort Delay in grip release did not significantly vary for different grip effort levels (Figure 3B). This suggests that feedback-based force control is used during grip initiation but not during grip release. The data also suggest that de-recruitment of many motor units takes approximately as long as de-recruitment of a partial pool of motor units.
Grip Force Consistent with previous studies,3,4,12e15 reduced grip strength was observed for people in the high-pain group than those in the low-pain group, bilaterally, and for the injured than uninjured upper extremities (Figure 2C). Reduced grip strength due to musculoskeletal injury and pain may be attributed to decreased muscle contractions,4,12e14,17e23 fear of pain and reinjury,15 as well as disuse-related muscle atrophy.6 Grip force for the uninjured hand was 13% lower for people in the high-pain group than those in the low-pain group (Figure 2C). The same pain-related protective mechanism that slowed down grip initiation and release may also have reduced grip force bilaterally. Imagining augmented pain resulted in similar grip force as in submax-50% (Figure 3C).
Consistency in Grip Delays Variability in grip delays was not significantly different between the two pain groups, between the injured and uninjured upper extremities, and among the three levels of grip effort. This indicated that prolonged grip delays due to increased pain levels were observed consistently rather than occasionally. Likewise, prolonged delay in grip initiation was consistently observed for the injured hand compared with the uninjured hand, and for
submaximal grip efforts compared with maximal grip efforts.
Functional and Clinical Implications Increased delays due to increased pain levels (29 milliseconds for grip initiation and 67 milliseconds for grip release, pooled for both hands and all levels of grip effort) have functional implications and may reduce one’s ability to perform daily tasks and work-related activities that require fast gripping and fast release. For instance, it takes, on average, 60 milliseconds for healthy persons to activate forearm muscles to stabilize their grip in response to a perturbation to their grip.54 The mean time for healthy persons to detect a floor perturbation and activate both upper and lower limb muscles to restore their balance is 88e90 milliseconds.55 It is conceivable that increased grip delays with increased pain may limit a person’s ability to rapidly respond to an impending situation and prevent a future injury. Prolonged delays in grip initiation and release not only have direct functional implications but also provide information on a person’s global motor function. According to Chae et al.,56 ‘‘delays in initiation and termination of muscle contraction simply reflect the global motor dysfunction in the central nervous system.’’ For instance, although mean differences in simple reaction time across different age groups were only 30 milliseconds (220 6 30 milliseconds for people in their 20s, 230 6 40 milliseconds for people in their early 40s, and 250 6 60 milliseconds for people in their early 60s), reaction time closely correlated with age and associated biomarkers such as forced expiratory volume at one second, grip strength, vision, physical and mental health, and lifestyle.41 In addition, muscle initiation/relaxation times and grip force timing have been shown to correlate with functional scores in patient populations.56e58 Motor impairment in people with brain injury was characterized by mean delay in grip force modulation by 16e122 milliseconds compared with age-matched controls.58 Thus, the mean differences in delay between our low- and high-pain groups (29 milliseconds and 67 milliseconds for the grip initiation and release, respectively) appear to be considerable. Therapies and interventions to shorten delays in grip39,59,60 and/or enhance grip strength may help people with musculoskeletal disorders perform daily activities more efficiently.
LIMITATIONS We have not controlled for possible confounding variables for grip timing such as injury type, age, preinjury muscle mass, fitness level, and possible muscle atrophy from disuse postinjury. Matching of OctobereDecember 2011 341
the injury site, diagnosis, gender, and age between the high- and low-pain groups may give a more definitive conclusion on the effect of pain on grip timing. Furthermore, a large population data set including all potential factors, pain level, and hand function scores may provide a more detailed statistical description of the effect of musculoskeletal injuries and associated pain on function.
CONCLUSION Musculoskeletal pain appears not only to reduce grip strength but also to slow down grip initiation and grip release of both injured and uninjured upper extremities. Slowed grip initiation and release can adversely impact one’s functional abilities and reduce one’s ability to rapidly respond to prevent an event leading to a possible injury. Slowed grip initiation and release may be attributable to psychological factors such as fear of pain and reinjury, as well as physiological factors such as reduced grip force and slowed recruitment and de-recruitment of motor units. The present study furthers the understanding of grip initiation and release delays in people with upper extremity musculoskeletal injuries and associated pain. Such knowledge is expected to contribute to enhancing rehabilitation and ergonomics practices for those affected individuals. Acknowledgments This study was partially funded by the University of Wisconsin-Milwaukee College of Engineering & Applied Science and the Evelyn Mackin Grant from the American Society of Hand Therapists. Partial equipment support was provided by Thought Technology Ltd, Montreal, Quebec, Canada. The authors report no conflict of interest.
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JHT Read for Credit Quiz: Article #200
Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to JHTReadforCredit.com. There is only one best answer for each question. #1. Reduced grip strength for people with upper extremity musculoskeletal disorders is related to a. increase in motor unit firing rate b. muscle atrophy c. fear of pain or fear of re-injury d. amplified EMG activity #2. The participants of this study were a. 28 people with moderate pain and 12 people with intense pain b. 28 people with pain resulting from musculoskeletal injury and 12 healthy individuals without history of musculoskeletal problems c. 28 older adults who frequently fall and 12 young adults d. 20 women and 20 men with history of musculoskeletal problems but without pain #3. The methodology of data collection in this study was a. written questionnaires b. retrospective chart reviews c. muscle activity measured using EMG
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d. time course of grip force measured using a force transducer #4. This study found that a. pain speeds up gripping b. pain speeds up grip release c. pain in one upper extremity affects the contralateral upper extremity as well d. grip timing is affected by pain, but not by the intensity of pain #5. The authors think that a. people with musculoskeletal injury and associated pain tend to overexert themselves more than people without musculoskeletal injury and associated pain b. unilateral pain has a bilateral effect on motor function through a centrally-mediated mechanism c. fear of pain may help speed up a person’s reaction to impending situations and thus protects future injuries d. delay in grip initiation is independent of grip force levels
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