The Effect of Stretching on Muscle Responses and Postural Sway Responses During Computerized Dynamic Posturography in Women and Men

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ORIGINAL ARTICLE

The Effect of Stretching on Muscle Responses and Postural Sway Responses During Computerized Dynamic Posturography in Women and Men Nancy L. Lewis, ScD, PT, Jean-Michel Brismée, ScD, PT, C. Roger James, PhD, Phillip S. Sizer, PhD, PT, Steven F. Sawyer, PhD, PT ABSTRACT. Lewis NL, Brismée JM, James CR, Sizer PS, Sawyer SF. The effect of stretching on muscle responses and postural sway responses during computerized dynamic posturography in women and men. Arch Phys Med Rehabil 2009;90: 454-62. Objective: To evaluate the effect of stretching on the parameters of postural sway and on the kinematic variables associated with balance control in women and men. Design: Mixed repeated measures design with 2 levels. Setting: Research laboratory. Participants: Fifteen women and fifteen men (mean age 23.4⫾2.2). Intervention: Two separate sessions of (1) lower extremity stretching and (2) no-stretching, immediately prior to balance testing with simultaneous surface electromyographic (EMG) recordings of muscle responses. Main Outcome Measures: EMG latencies and average amplitudes for 4 lower extremity muscles for the preferred stance limb during computerized dynamic posturography (CDP) tests, specifically the Postural Evoked Response Test, Adaptation Test, Motor Control Test, Sensory Organization Test, and Unilateral Stance Test. Results: Analyses of variance indicated no significant main effect for stretching and 2 significant main effects for gender for the Motor Control Test (P⫽.021) and latency of tibialis anterior (P⫽.009). Analyses of covariance with covariants of height and active knee extension revealed no significant main effect of stretching or of gender on muscles responses or CDP performance. Conclusions: In both women and men, lower extremity stretching did not significantly affect muscle responses or performance during CDP. Key Words: Balance; Electromyography; Rehabilitation; Stretch. © 2009 by the American Congress of Rehabilitation Medicine

OSTURAL CONTROL IS INHERENT in human moveP ment and has a universal role in rehabilitation and training, not only in the person recovering from illness or injury, but additionally in the elite athlete seeking to improve performance. Factors that alter postural control responses may affect the quality and safety of performance during both routine functional movement and athletic performance. Increased postural sway is generally associated with an increased risk of falling. This is commonly associated with aging populations1 and populations with neurological impairment,2 but is applicable to healthy populations as well, especially during unstable, dynamic tasks.3,4 Increased postural sway has been associated with multiple ankle sprains in basketball players,5 and increased postural sway during unilateral stance has been associated with nearly a 7-fold increase in risk for ankle sprains among high school basketball players.6 Postural control and balance responses require muscle activation and force production to increase stiffness of body segments and to return the center of gravity to a centralized position over the base of support.7 The sensorimotor system and its ability to activate musculature can be altered by the mechanical and neural effects of stretching.8-10 These alterations may affect postural control and balance through a change in the peripheral afferent system, the central nervous system, or both.9,10 More specifically, the mechanical and neural effects of stretching may affect the latency or magnitude of muscle forces production by changing reflexive responses mediated through short spinal pathways, automatic postural responses mediated through both spinal and central pathways, or volitional control mediated through the central and cognitive pathways. Stretching has been shown to decrease maximum voluntary contractions11,12 and peak torque at slower velocities.13-17 This has led some authors to recommend that stretching be avoided prior to performances that require high power muscle force production.11,12 However, few studies have investigated postural control and balance after a bout of stretching.9,18,19 Additionally, gender-based differences have been identified in the viscoelastic properties of the tendon structures20 and in List of Abbreviations

From the Center for Rehabilitation Research, Texas Tech University Health Sciences Center, Lubbock, Texas. Supported by the Texas Tech University Health Sciences Center School of Allied Health Sciences students’ funding. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Reprint requests to Jean-Michel Brismée, ScD, PT, Department of Rehabilitation Sciences, Center of Rehabilitation Research, School of Allied Health Sciences, Texas Tech University Health Sciences Center, 3601 4th Street, Mail Stop 6223, Lubbock, Texas 79430, e-mail: [email protected]. 0003-9993/09/9003-00379$36.00/0 doi:10.1016/j.apmr.2008.09.570

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ANCOVA ANOVA CDP COG EMG ICC MCT RMS SOT SPSS ULS

analysis of covariance analysis of variance computerized dynamic posturography center of gravity electromyograph intraclass correlation coefficient Motor Control Test root mean square Sensory Organization Test Statistical Program for the Social Sciences Unilateral Stance Test

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the muscle architecture,21 which indicates that the musculotendinous unit in women may be more compliant. This could represent a different effect of stretching on postural sway between women and men. Different methods have been used to assess the effect of stretching on balance and postural control. Behm et al19 used a wobble board to identify the effects of an acute bout of stretching upon balance performance, whereas others have used force plates to measure postural sway18 and surface electromyography to measure muscle responses.9 CDP provides reliable and valid measurement of posture control and balance22,23 with a more comprehensive measurement of postural control and responses. CDP quantifies and characterizes multiple postural control parameters by measuring postural sway amplitude, path, area, and velocity during a variety of externally driven platform rotations and translations while the subject stands on the platform. Postural control is objectified by measurement of kinematic variables and specific strategies.24 The temporal and amplitude parameters of specific muscle responses recorded through surface EMG during posturography additionally characterize the effect of stretching on postural control and balance performance. The short and medium latency responses (reflexive) occur in the gastrocnemius and the long latency response (automatic postural response) occurs in the anterior tibialis, although activity of the thigh musculature has not been reported in previous studies of muscle responses during perturbation.9,22 This combination of postural sway analysis and EMG analysis allows the measurement of the effect of stretching on postural sway, reflexive and automatic postural responses, and muscle responses of the lower extremities during a single experimental set-up. Although previous studies indicated a significant effect of stretching on muscle responses and on balance performance, these studies focused on a limited aspect of balance performance or measurement.9,18,19 We have found no study that has investigated the effect of stretching on postural control and balance with a comprehensive measurement of postural control parameters. The aim of this study was to evaluate the effect of stretching on the parameters of postural sway and on the kinematic variables associated with balance control in women and men. More specifically, we measured the effect of a bout of lower extremity stretching on the following parameters and variables: (1) the changes in the short, medium, and long muscle latencies and amplitudes in response to rotation perturbations in standing; (2) postural sway parameters measured during CDP; and (3) the differences between women and men for muscles responses and postural sway parameters. METHODS Study Design Our study was a mixed 2 (stretching versus no-stretching) ⫻ 2 (women versus men) design with repeated measures for the stretching versus no-stretching treatment. Each subject attended 2 sessions on 2 separate days. Testing order was systematically counterbalanced for the sequence of the stretching and no-stretching treatment and by gender. For example, the first woman and the first man were tested first with no-stretching, and during the next session both were tested after lower extremity stretching; the second woman and the second man were tested after stretching during the first session and with no-stretching during the next session; and so on.

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Subjects and Procedure After approval of the study by the university’s institutional review board, we recruited 45 subjects, ages 18 to 35 years, at local universities. After explanation of the study to the subjects, those who elected to participate signed the informed consent form. Exclusion criteria consisted of current acute illness or pain that would affect participation, a history of back or lower extremity injury that required surgical intervention or had been persistent (no symptoms within the last 6 months), balance difficulties or vestibular dysfunction, non-correctable visual impairment, use of prescription medication known to affect the central nervous system, body mass that prohibited correct placement of electrodes or safety harness, pregnancy, alcohol intake within 24 hours of testing, illegal substance use, and inability to stand unilaterally on each lower extremity for 10 seconds on a level surface. Additionally, persons who were ranked as sedentary or as actively involved in training or actively participating in competitive sports, as defined by a typical activity classification,25 were excluded. The activity classification for inclusion ranged from the participation in occasional jogging, biking, or swimming with no participation in jumping, turning, or twisting sports to the participation in recreational jumping, turning, and twisting sports. The first session included orientation to the project, review of inclusion and exclusion criteria, expectations of participants, review of risks, and review of consent for participation. Hamstring flexibility was measured, as was height and weight. Stance limb preference was subject-defined by verbally instructing the subject to stand on 1 lower extremity. The first limb that was chosen to stand on was considered to be the preferred stance limb. Measurement of flexibility for hamstrings included goniometric measurement of active knee extension (from 90° knee flexion toward 0° flexion) of the preferred stance limb26 (fig 1). Testing began at the conclusion of this review and preliminary data collection. During both testing sessions, skin electrodes for the vastus lateralis, semitendinosus, medial gastrocnemius, and anterior tibialis were applied to both lower extremities.27 Skin preparation included dry shaving, if needed, followed by cleansing with alcohol. The electrodes were secured and then outlined with felt tip marker for consistent placement during the second testing sessions. All measurements and electrode placements were performed by 1 investigator, who was not blinded to the subject group assignment. The 2 testing sessions were separated by at least 2 days to minimize any fatigue from repeated testing. Each subject was tested at the same general time of day for each testing session (morning, afternoon, evening). Subjects were requested to maintain similar schedules and activity levels for each day of testing and to comply with exclusion criteria regarding substance and medication use. During the stretch session, the quadriceps, hamstrings, and plantar flexors of both lower extremities were passively stretched with the subject supine for the hamstrings and plantar flexors and prone for quadriceps (fig 2). The subject controlled the amount of stretch by verbally indicating the point of painless stretch, which has been reported as the limit of muscle stretch without an increase in EMG activation of the agonist or the antagonist.28,29 Consistent with Behm et al,19 the stretching duration cycle included three 45-second stretches of the quadriceps, hamstrings and plantar flexors, with a 15-second recovery period allowed between stretches. The total time for the stretching routine was approximately 18 minutes. Testing during the no-stretch session began after the subject rested quietly supine for an equivalent period of time. Arch Phys Med Rehabil Vol 90, March 2009

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The Adaptation Test protocol measured postural sway responses and modification of postural responses to both toes up and toes down platform rotations. The MCT protocol used forward and backward platform translations to induce small (2.8°/s), medium (6.0°/s), and large (8.0°/s) sway of the COG, and then measured the onset of the balance correction responses. The SOT protocol measured postural sway based on somatosensory, visual, and vestibular input during 6 different sensory and surface conditions. The 6 conditions included: (1) eyes open with fixed surface and visual surround, (2) eyes closed with a fixed surface, (3) eyes open with fixed surface and sway referenced visual surround, (4) eyes open with sway referenced surface and fixed visual surround, (5) eyes closed with a sway referenced surface, and (6) eyes open with sway referenced surface and visual surround. The ULS protocol measured mean COG sway velocity during unilateral stance (right and left) under 2 conditions, eyes open and eyes closed, and resulted in 1 composite score for all conditions and 1 score for each condition. Because the effect of a bout of stretching on the muscle response amplitude, Hoffman reflex, and muscle lengthening may be transient, possibly limiting the length of time to detect changes,10,26,30 the order of testing was consistent among all subjects. The Postural Evoked Response Test protocol was administered first, followed by the Adaptation Test, MCT, SOT, and ULS protocols.

Fig 1. Measurement of active knee extension. (A) An electronic goniometer was positioned 2.54cm caudal to the fibular head, aligned between the fibular head and lateral malleolus, and secured to the lateral lower leg with self-adhering wrap. The subject maintained the thigh in contact with the crossbar, constructed with polyvinylchloride pipe, with the hip pre-positioned at 90° and the contralateral thigh on the surface. (B) The subject actively extended the knee while the investigator manually insured no movement of the thigh of contralateral lower extremity. The subject performed 6 active knee extensions with a 60-second rest period between repetitions with the terminal range for the sixth trial recorded adapted from Spernoga et al, 2001.26 (Reprinted with permission).

Immediately after either stretching or lying supine, the subject was assisted to the SMART EquiTesta platform and secured with the safety harness. Electrode placement and recordings were verified and testing was begun. Outcome Measurement While the subject stood on the platform, we completed all CDP testing using the standardized NeuroCom protocols for Postural Evoked Response Test, the Adaptation Test, the MCT, the SOT, and the ULS protocols. The Postural Evoked Response Test measured the latencies of muscle responses to a series of platform rotations (toes up, 4° at 50°/second). The test used 8 EMG electrode setups with pre-amplified electrodes. Muscle responses were recorded on both lower extremities during the Postural Evoked Response Test. The raw EMG data were sampled at 1000Hz with the SMART EquiTest system. Arch Phys Med Rehabil Vol 90, March 2009

Data Management We completed data acquisition and reduction for the selected dependent variables for the Adaptation Test, MCT, SOT, and ULS protocols through the SMART EquiTest software, and converted the data to a text file and copied it to a spreadsheet for data management. For the Adaptation Test, the calculated means of the 5 toes up scores (Adaptation Test Mean Toes Up) and of the 5 toes down scores (Adaptation Test Mean Toes Down) were selected for statistical analysis. The MCT comprehensive score for response latencies was used for analysis. The MCT comprehensive score is the average of all the responses recorded during medium and large translations. The SOT resulted in 6 equilibrium scores, including a comprehensive score, a visual score, a vestibular score, a somatosensory score, and a visual preference score, all of which were analyzed statistically. The equilibrium scores are calculated scores, based on a comparison of the maximum anterior posterior angular displacement of the COG obtained during testing and a normal value of 12.5 degrees of total range before falling. The ULS tests resulted in 5 mean COG sway velocity scores for statistical analysis, including a comprehensive score and a score for each of the following conditions: (1) eyes open left unilateral stance, (2) eyes closed left unilateral stance, (3) eyes open right unilateral stance, and (4) eyes closed right unilateral stance. The Postural Evoked Response Test EMG data were converted to a text file and imported into custom data reduction program, MATLAB Version 7.1.0.124.b Electromyographic data were full-wave rectified and passed through a low pass Butterfield filter at 50Hz.31 One investigator visually reviewed each recording and determined a period of quiet baseline prior to the stimulus onset. Using a sliding window of 25ms and with a threshold of 3 SD above the baseline, muscle onsets were quantified by the custom algorithm followed by visual review and correction (if needed) by the investigator32 (fig 3). Recordings were rejected if the onset could not be distinguished from baseline activity, if the onset was within 25ms of the stimulus, or the onset was greater than 700ms from the stimulus.33 The average of all accepted recordings for each subject for each session for the preferred stance limb was used for data analyses for onset latencies measured in ms and RMS

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measured in microvolts, the measure for amplitude of muscle response. Normalization of the EMG RMS to the baseline RMS was completed and the normalized RMS (% baseline EMG RMS) was used for statistical analysis. Intra-rater reliability testing for the EMG data analysis was calculated using intraclass correlation coefficient (ICC model 3, 1). Statistical Analyses We used descriptive statistics to analyze the subjects’ demographic characteristics. Inferential statistics were conducted using SPSS Version 16.0.c Normality assumptions were tested for each outcome variable. The analysis for main effects and interactions on the selected dependent variables for each protocol was determined by 2⫻2 ANOVA for the independent variables (1) of treatments of stretching versus no-stretching and (2) gender. A Bonferroni correction factor was used for each test based on number of variables used for families of statistical analysis. The alpha levels for the Adaptation Test, MCT, SOT, Postural Evoked Response Test muscle latencies and Postural Evoked Response Test RMS values were respectively, 0.025, 0.05, 0.01, 0.01, and 0.012. Pearson product correlation analyses of covariant factors of height33 and active knee extension were completed for the variables, with significant differences identified by the 2⫻2 ANOVA. Regression analyses of the significant correlations were used to verify confounding relationships with the dependent variables. Then 2⫻2 ANCOVA was completed to determine the effect of stretching treatment and gender while controlling for the covariant factors significantly correlated with the dependent variables. Additionally, independent t tests for height and active knee extension were completed to determine if height and active knee extension were significantly different between genders within the study population. RESULTS Forty-five subjects, 24 men and 21 women, participated in the study. Four did not return for the second testing session and were excluded from the study. Four were excluded due to the occurrence of adverse physiological symptoms (bradycardia, diaphoresis, nausea, and syncope) during the testing procedures. All symptoms resolved with termination of testing and a brief period of lying supine. Due to technical and operational problems, the results from the first 7 participants were not included in the data analysis. Data of 15 women and 15 men with a mean age of 23.4⫾2.2 years, were analyzed (table 1). Intra-rater reliability coefficient using ICC (model 3, 1) for the EMG data was 1.00 for all RMS values, and varied from 0.30 to 0.87 for the muscle latencies except for 1 negative value of ⫺0.48 (table 2). Based on the ANOVA results for the main experimental question regarding the effect of stretching on postural control, stretching did not significantly alter postural control on the majority of the outcome measures (tables 3, 4). However, the initial ANOVA analysis did indicate a significant main effect of gender for the MCT comprehensive score (F1,28⫽5.945,

4™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™ Fig. 2. Passive stretching for lower extremities. (A) Plantar flexors stretched using passive dorsiflexion of the ankle of the subject with subject supine and with the knee extended. (B) Hamstrings stretched by passive flexion of the hip with knee extended and contralateral thigh stabilized manually by the investigator. (C) Quadriceps femoris stretched by passive flexion of the knee with the subject prone until the knee was lifted from the surface. The ipsilateral hip was stabilized manually by the investigator.

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COMPUTERIZED DYNAMIC POSTUROGRAPHY IN WOMEN AND MEN, Lewis Table 2: Interclass Correlation for EMG Latency and RMS Values Characteristic

Right

Left

GA Latency 1 GA Latency 2 TA Latency HAM Latency QUAD Latency GA RMS TA RMS HAM RMS QUAD RMS

0.83 0.71 0.63 0.87 1.00 1.00 1.00 1.00 1.00

0.43 ⫺0.48 0.50 0.30 1.00 1.00 1.00 1.00 1.00

Abbreviations: GA, gastrocnemius; HAM, hamstring; QUAD, quadriceps; TA, tibialis anterior.

Fig 3. EMG recording (from medial gastrocnemius muscle). Stimulus onset indicates the initiation of platform rotation (toes up, 4° at 50°/s). Baseline root mean square was determined by an area manually selected by the investigator. Muscle response onset threshold is indicated by the horizontal line, which indicates 3 SDs above baseline root mean square selected as the onset threshold, based on Dedrick et al.32

P⫽.02) and tibialis anterior long latency response (F1,28⫽ 7.806, P⫽.009). No significant interaction effects for stretching and gender were noted. Both height33 and hypermobility34 have been shown to influence postural sway. Using Pearson product correlation analysis, significant correlations were identified between the MCT comprehensive score and height (P⫽.036), between MCT comprehensive score and active knee extension (P⫽.027), and between EMG latency of tibialis anterior and height (P⫽.008). Subsequent ANCOVA results, controlling for the covariant factors of height and active knee extension, revealed no significant main effect of stretching treatment or of gender on the dependent variables (table 5). Significant differences between women and men were identified for height (P⬍.001) and for active knee extension (P⫽.047) by independent t tests. The statistically significant differences identified with ANOVA between women and men on the outcome variables cited above were not present with ANCOVA, when controlling for confounding factors of height and flexibility. DISCUSSION This study represents the first known investigation into the effect of stretching on postural control and balance using a comprehensive measurement. No statistically significant effect

of lower extremity stretching on muscle responses, postural control, and balance was found in our study. Comparison of our results with previous studies of stretching and postural control and balance9,18,19 reveals 2 important factors that warrant consideration: the duration of stretching effect and the nature of the tasks. If the immediate mechanical and neural effects of stretching abate within a short time frame, the practical effect on performance becomes inconsequential. Recovery of maximum voluntary torque and EMG values has been noted within 15 minutes after 1 hour of passive stretches of the plantar flexors with almost complete recovery of Hoffman reflex peak-to-peak amplitude within 4 minutes.10 The lengthening effect of muscle after three 30-second static hamstring stretches was greatest immediately after the termination of the stretch with a decreased effect at 15 minutes.30 The stretching routine included bilateral stretches for 3 muscle groups for a total of approximately 18 minutes. The transition after the stretching period was limited to walking less than 8 feet to the platform and standing for verification of electrode recording and attachment of the safety harness, which typically took 4 to 6 minutes. Each test sequence required verification of foot placement on the platform and verbal instructions to the subject. The entire testing sequence typically required 25 to 30 minutes. In contrast, Nagano et al18 and Chong and Do9 reported significant effects of stretching on postural sway and muscle responses respectively, when testing immediately followed stretching of the gastrocnemius. Differences in elapsed time between the stretching routines and the measurement of postural sway and muscle responses may account for the difference in outcomes among our study and previous studies. The nature of performance or skill determines the mechanical and neural functions that are required in the performance. Behm et al19 identified a significant negative effect of an acute bout of stretching upon balance performance during a 30second wobble board test, in which balance performance was

Table 1: Study Population Characteristics Population Characteristics

Women

Men

Total

Number of subjects (n) Age Height (inches) Weight (pounds) Active knee extension (degrees) Testing order (1st test) Preferred stance limb

15 23.7⫾1.5 63.9⫾4.5 141.3⫾32.9 22.7⫾12.1 S⫽6; NS⫽9 R⫽10; L⫽5

15 23.1⫾2.7 70.4⫾3.9 178.5⫾27.1 31.1⫾10 S⫽9; NS⫽6 R⫽8; L⫽7

30 23.4⫾2.2 67.1⫾5.3 159.9⫾35.2 26.9⫾11.7 S⫽15; NS ⫽15 R⫽18; L⫽12

Abbreviations: L, left; NS, no-stretching treatment; R, right; S, stretch treatment.

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Table 3: Mean Values and ANOVA Results for Dependent Variables Within Repeated Measures of Stretch and No Stretch Treatments

Variable

Sensory Organization Test score SOT Comp SOT SOM SOT VIS SOT VEST SOT PREF Adaptation Test score ADT Mean Toes Up ADT Mean Toes Down Motor Control Test score MCT Comp Unilateral Stance Test Score ULS Mean EOL ULS Mean ECL ULS Mean EOR ULS Mean ECR ULS Mean Comp Postural Evoked Response Test muscle latencies (msec) EMG Lat1 Pref GA EMG Lat2 Pref GA EMG Lat Pref HS EMG Lat Pref TA EMG Lat Pref QUAD Postural Evoked Response Test muscle root mean square (% baseline) EMG RMS Pref GA EMG RMS Pref HS EMG RMS Pref TA EMG RMS Pref QUAD

No Stretch Total (n⫽30) Mean ⫾ SD

Stretch Total (n⫽30) Mean ⫾ SD

F

P

Partial ␩2

Observed Power

77.4⫾7.4 97.1⫾2.7 85.4⫾8.8 68.9⫾11.7 99.3⫾7.3

77.9⫾6.9 96.9⫾3.5 85.3⫾10.2 70.0⫾9.5 98.3⫾7.3

0.248 0.057 0.008 0.884 0.364

0.625 0.813 0.929 0.355 0.551

0.019 0.002 0.000 0.031 0.013

0.08 0.06 0.05 0.15 0.09

48.0⫾7.7 48.1⫾9.3

47.8⫾10.5 46.7⫾10.0

0.010 0.829

0.922 0.370

0.000 0.029

0.05 0.14

147.5⫾13.7

147.9⫾14.7

0.114

0.738

0.004

0.06

1.08⫾2.19 4.50⫾3.82 0.83⫾0.98 5.08⫾3.65 2.89⫾2.06

1.30⫾2.11 5.78⫾3.41 0.62⫾0.16 4.79⫾3.48 3.14⫾1.89

0.734 3.825 1.359 0.122 0.882

0.399 0.061 0.254 0.729 0.356

0.026 0.120 0.046 0.004 0.031

0.13 0.47 0.20 0.06 0.15

50.8⫾11.0 89.1⫾13.9 96.6⫾18.9 120.9⫾23.0 159.6⫾39.5

50.3⫾8.8 90.7⫾12.6 106.1⫾30.8 121.1⫾20.1 155.1⫾49.7

0.060 0.325 4.232 0.011 0.437

0.808 0.573 0.049 0.918 0.514

0.002 0.011 0.131 0.000 0.015

0.06 0.09 0.51 0.05 0.10

219.4⫾73.6 240.2⫾123.9 1890⫾1038 227.6⫾184.3

205.5⫾83.9 234.4⫾128.8 1712⫾1066 203.4⫾73.8

0.630 0.116 1.111 0.725

0.434 0.736 0.301 0.403

0.022 0.004 0.038 0.025

0.12 0.06 0.18 0.13

Abbreviations: ADT Mean Toes Up, Adaptation Test mean for toes up; ADT Mean Toes Down, Adaptation Test mean for toes down; EMG Lat Pref GA, short latency gastrocnemius; EMG Lat2 Pref GA, medium latency gastrocnemius; EMG Lat Pref HS, latency hamstring; EMG Lat Pref TA, latency tibialis anterior; EMG Lat Pref QUAD, latency quadriceps; EMG RMS Pref GA, root mean square gastrocnemius; EMG RMS Pref HS, root mean square hamstring; EMG RMS Pref TA, root mean square tibialis anterior; EMG RMS Pref QUAD, root mean square quadriceps. MCT Comp, motor control test comprehensive score; SOT Comp, sensory organization test comprehensive score; SOT SOM, sensory organization test somatosensory score; SOT VIS, sensory organization test visual score; SOT VEST, sensory organization test vestibular score; SOT PREF, sensory organization test preference score; ULS Mean EO, unilateral stance test left limb eyes open; ULS Mean ECL, unilateral stance test left limb eyes closed; ULS Mean EOR, unilateral stance test right limb eyes open; ULS Mean ECR, unilateral stance test right limb eyes closed; ULS Mean Comp, unilateral stance test comprehensive score.

measured by calculating the ratio of contact with the floor to no-contact time. Inherent to the wobble board task is an expected and predictable activation with components of cognitive control and continuous feedforward-feedback mechanisms that allow attenuation and amplification of responses. The task for the posturography testing in our study was quite different. Standing on unstable surfaces, as in Behm, has been shown to increase postural sway with a decrease in movement sway.4 Latash et al4 defines movement sway as spontaneous changes in postural sway during voluntary shifts of center of pressures. Movement sway during voluntary shifts differs from postural sway in direction, frequency, and amplitude with postural sway as the background for movement sway.4 A wobble board is a sway referenced surface that is also multi-directional, which indicates components of a more complex task. Multi-axial control requires increased medial lateral activation.4,24 The results of our study would suggest that any effect of stretching on balance extending beyond the 4 to 6 minute interval between stretching and balance testing is not due to changes in reflexive and automatic postural responses mediated predominantly through spinal responses as measured by CDP, and is not due to the changes in the responses of the quadriceps and

hamstrings. These findings do not exclude changes (either attenuation or potentiation) in the central nervous system of postural control with a more complex task. Another aspect of our study was the difference in the effect of stretching on postural control and balance between women and men. Architectural and mechanical properties suggest that the musculotendinous unit in men is more resistant to stretching and the musculotendinous unit in women is more compliant.20 Less stiffness in the musculotendinous unit contributes to an increase in electromechanical delay in the series elastic component of force production and could change the sensitivity of the afferent fibers. Gender differences in the MCT and in the long latencies of tibialis anterior were noted in our study using the results of the ANOVA only. This reflects a consistency between the latency of the tibialis anterior and the functional measure, MCT comprehensive score, for the corrective postural response to perturbation, which would support a gender difference. However, height is correlated with postural sway,33 and hypermobility34 has been associated with increased postural sway. When considering these factors in our study group, correlations of height and active knee extension to the MCT were noted, and height was correlated with tibialis anterior Arch Phys Med Rehabil Vol 90, March 2009

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COMPUTERIZED DYNAMIC POSTUROGRAPHY IN WOMEN AND MEN, Lewis Table 4: Mean Values and ANOVA Results for Dependent Variables Between Women and Men Women (n⫽15) Men (n⫽15) Mean Mean Value ⫾ SD Value ⫾ SD

Variable

Sensory Organization Test score SOT Comp SOT SOM SOT VIS SOT VEST SOT PREF Adaptation Test score ADT Mean Toes Up ADT Mean Toes Down Motor Control Test Score MCT Comp Unilateral Stance Test scores ULS Mean EOL ULS Mean ECL ULS Mean EOR ULS Mean ECR ULS Mean Comp Postural Evoked Response Test muscle latencies (msec) EMG Lat1 Pref GA EMG Lat2 Pref GA EMG Lat Pref HS EMG Lat Pref TA EMG Lat Pref QUAD Postural Evoked Response Test muscle root mean square (%baseline) EMG RMS Pref GA EMG RMS Pref HS EMG RMS Pref TA EMG RMS Pref QUAD

76.6⫾6.6 97.0⫾2.7 84.7⫾10.1 67.9⫾10.4 98.5⫾8.0

78.7⫾7.5 97.10⫾3.6 86.0⫾8.8 71.7⫾10.6 99.1⫾6.5

49.7⫾7.8 49.3⫾9.8

46.1⫾10.0 45.4⫾9.2

142.1⫾15.7

153.3⫾9.7

F

0.754 0.027 0.211 1.217 0.086

P

Partial Observed ␩2 Power

0.393 0.870 0.650 0.279 0.772

0.026 0.001 0.007 0.042 0.003

0.13 0.05 0.07 0.19 0.06

1.752 0.196 1.506 0.230

0.059 0.051

0.25 0.22

5.945 0.021* 0.175

0.65

1.72⫾2.87 5.34⫾3.85 0.88⫾0.97 5.26⫾3.85 3.31⫾2.34

0.66⫾0.70 4.94⫾3.50 0.57⫾0.13 4.61⫾3.23 2.72⫾1.48

2.080 0.117 3.076 0.399 0.777

0.160 0.735 0.090 0.533 0.386

0.069 0.004 0.099 0.014 0.027

0.29 0.06 0.405 0.09 0.14

50.6⫾10.4 89.7⫾13.1 100.9⫾30.4 111.4⫾20.4 149.9⫾31.3

50.5⫾9.4 90.1⫾13.5 101.9⫾20.6 130.6⫾18.0 165.6⫾54.0

0.001 0.013 0.017 7.806 1.203

0.973 0.910 0.898 0.009† 0.282

0.000 0.000 0.001 0.218 0.041

0.05 0.05 0.05 0.74 0.19

232.9⫾86.7 236.1⫾134.4 2129⫾1125 240.8⫾10.0

192.0⫾64.6 238.5⫾117.9 1473.7⫾861.0 190.2⫾53.1

0.630 0.003 3.949 1.421

0.434 0.958 0.057 0.243

0.022 0.000 0.124 0.048

0.12 0.05 0.48 0.21

Abbreviations: see table 3. *Significant at ␣⫽.05; † Significant at ␣⫽.01.

latency. Within this study population, women had shorter heights (P⬍.001) and had greater active knee extension (P⫽.047). Subsequent ANCOVA did not establish significant Table 5: ANCOVA for MCT Comprehensive Score and Latency of Tibialis Anterior With Covariates of Height and Active Knee Extension ANCOVA Summary

MCT Comp Treatment Treatment ⫻ active knee extension Active knee extension Gender Treatment ⫻ gender MCT Comp Treatment Treatment ⫻ height Height Gender Treatment ⫻ gender EMG Lat Pref TA Treatment Treatment ⫻ height Height Gender Treatment ⫻ gender

F

P

Partial ␩2

Observed Power

0.815

0.375

0.029

0.14

0.737 4.300 2.701 0.509

0.398 0.048 0.112 0.482

0.027 0.137 0.091 0.017

0.13 0.52 0.35 0.11

0.371 0.398 1.950 1.119 0.211

0.547 0.534 0.174 0.299 0.650

0.014 0.015 0.067 0.040 0.008

0.09 0.09 0.27 0.18 0.07

0.006 0.005 2.358 1.609 0.529

0.941 0.945 0.136 0.215 0.473

0.000 0.000 0.080 0.053 0.019

0.05 0.05 0.32 0.23 0.11

Abbreviations: see table 3.

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effects of gender when controlling for the confounding variables of height and flexibility (active knee extension). The differences between women and men in this study are confounded by the basic anthropometric differences in characteristics and could not be related to the effect of stretching and gender. Our study used a practical combination of lower extremity stretches, which was considered to be a typical stretching routine performed before exercising or participating in an athletic event. This duration and number of repetitions are consistent with Behm et al.19 Although the type of stretching and the inclusion of warm-up vary among studies, 2 factors were considered in the selection of static, passive stretching without warm-up, (1) the dichotomous relationship of active warm-up and passive stretching previously reported,35,36 and (2) the variable impact of muscle history on muscle activation and reflex potentiation.37 Active warm-up is considered to be beneficial to performance, whereas static stretching may have a negative effect on performance. Three limitations of the study should be considered. One is the relatively low statistical power, which increased the possibility of a type 2 error in the statistical analysis. However, the effect sizes were small, which may reflect that the effect is inconsequential (see tables 3, 4). Another limitation is the intrarater reliability of the latency onsets. Baseline activity was variable within and among subjects and hindered determination of an onset burst. The postural displacement during CDP produced relatively low amplitude muscle responses, which hindered the detection of the onset latencies when a variable baseline was present. Specifically, the detection of onset of

COMPUTERIZED DYNAMIC POSTUROGRAPHY IN WOMEN AND MEN, Lewis

tibialis anterior was hindered by cross talk from the gastrocnemius. Our sampling of individual trials for the ICC was small (12 trials) with as many as 7 lacking established onsets for at least 1 muscle group, which further reduced the sampling for the reliability testing portion of the study. Third, the investigator who completed the testing and data analysis was not blinded to the treatment. Although the automated measurement of the CDP system provides objective measures and recordings of performance, the EMG recordings were initially analyzed with use of a custom algorithm software program and, lastly, by visual review and manual correction. Postural control and balance is driven by a complex integrated system of reflexive responses, automatic postural control responses, and volitional control. If stretching reduced the quality or efficacy of postural control, then balance would be affected and the risk of falling and injury would increase, not only for the young elite athlete, but also for the person with injury or pathology. We did not find a significant effect of stretching on postural sway or on muscle responses of the lower extremities, including the reflexive and automatic postural responses, as measured by CDP. However, the complexity of the task and the length of time of the effect of stretching may still pose risk factors. There may be a short window of time after stretching during which postural responses are altered. Immediate performance (within 4 minutes) after stretching and the performance of complex tasks warrant caution. Recommendations are that stretching be followed by other muscle activation activities prior to performance until further research clarifies the temporal aspect of effect of stretching on postural control and balance, not only in young healthy populations but also in populations with pathology and comorbidities. CONCLUSION No significant effect of lower extremity stretching on postural control including muscle responses and balance parameters were detected during CDP in healthy young adults. Acknowledgments: We thank Kyle Lewis, BA, JD, for his assistance during data collection. References 1. Delbaere K, Van den Noortgate N, Bourgois J, Vanderstraeten G, Tine W, Cambier D. The Physical Performance Test as a predictor of frequent fallers: a prospective community-based cohort study. Clin Rehabil 2006;20:83-90. 2. Matinolli M, Korpelainen J, Korpelainen R, Sotaniemi K, Virranniemi M, Myllylä V. Postural sway and falls in Parkinson’s disease: a regression approach. Mov Disord 2007;22:1927-35. 3. Aruin AS, Forrest WR, Latash ML. Anticipatory postural adjustments in conditions of postural instability. Electroencephalogr Clin Neurophysiol 1998;109:350-9. 4. Latash ML, Ferreira SS, Wieczorek SA, Duarte M. Movement sway: changes in postural sway during voluntary shifts of center of pressure. Exp Brain Res 2003;150:314-24. 5. Fu AS, Hui-Chan CW. Ankle joint proprioception and postural control in basketball players with bilateral ankle sprains. Am J Sports Med 2005;33:1174-82. 6. McGuine T, Green J, Best T, Leverson G. Balance as a predictor of ankle injuries in high school basketball players. Clin J Sport Med 2000;10:239-44. 7. Rietdyk S, Patla AE, Winter DA, Ishac MG, Little CE. Balance recovery from medio-lateral perturbations of the upper body during standing. J Biomechanics 1999;32:1149-58. 8. Avela J, Finni T, Liikavainio T, Neimela E, Komi P. Neural and mechanical responses of the triceps surae muscle group after 1 h of repeated fast passive stretches. J Appl Physiol 2004;96:2325-32.

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