- Email: [email protected]
Influence of repeated measurements on isokinetic lifting strength
Clin. Biomech. 1992; 7: 149-152
Cnfluence of repeated measurements Hing strength
A-M Estlander Rehabilitation
PSL, G
Foundation,
Mellin Helsinki,
MD,A Weckström
on isokinetic
PT
Finland
Summary The aim of the study was to evaluate the effect of repeated measurements on isokinetic lifting strength in a sample of patients with low back pain. The measures were performed in series of five consecutive lifts on two occasions 1-13 days apart, and the patients continued their normal living during this period. A group of healthy subjects was used as a control. The hypothesis was that a mere repetition of the lifting procedure would increase the performance, and that the increase would be bigger in the patient group than in the non-patient group. In addition, grip strength was measured on both occasions. It was supposed that grip strength would not change from trial 1 to trial 2. The results showed an increase in Iifting strength from first to second trial in the patient group by an average of 28%, and in the control group by an average of 16%. Grip strength did not increase in either group. The last of five consecutive lifts was the best one in a majority of the cases in both trials. Neither five consecutive lifts nor measurements on two different occasions seem enough to define a reliable baseline for maximal lifting capacity. Relevante Other factors than an increase in muscle strength influence the performance, one being a repetition of the lifting procedure. This must be accounted for when defining a baseline for isokinetic lifting capacity. Key words:
Isokinetic lifting, low back pain, repeated
measures
Introduction Within the field of assessment and treatment of chronic low back pain patients, the interest in non-invasive measuring of back functions, e.g. muscle strength, has increased considerably, and the use of computerized electronic commercial measurement devices has grown accordingly . Measurements of back muscle strength have been used for various purposes. Standards for healthy individuals have been collectedl. Studies comparing low back patients and healthy persons usually show that the muscle strength of patients is inferior to that of healthy persons2-4. Reduction in trunk strength after discectomy has been show&‘, and correlations between Received: 8 December 1990 Accepted: 20 August 1991 Correspondence and reprint requests
Finnish Back Institute, Finland
Salomonkatu
CQ 1991 Butterworth-Heinemann 0268-0033/92/030149-04
Ltd
to: Ann-Mari
Estlander, PsL, 17 B 3 krs, 00100 Helsinki,
muscle density as assessed by CT scan and isokinetic lifting strength have been demonstrated6. This kind of measurement has also been used to discriminate between maxima1 and submaximal effort in healthy subjects7. Measuring trunk muscle strength and lifting capacity with, for example, isokinetic testing devices have been regarded as ‘a quantitative technique for objectively documenting spine function’ and to be a critical component necessary for diagnosis and surgical decisions’. Measurements of spinal functions have also been used to guide treatment and evaluate treatment outcome in rehabilitation programmes for back pain patients’-1°. Increases in lifting capacity during treatment have been interpreted as objective evidente for increased muscle strengths39*‘1. Doubt has, however, been raised about the validity of measures of muscle strength. The influence of repetition on motor skills is a well-known phenomenon12. Trunk muscle performance of patients with low back trouble may be a test of their pain leve1 and tolerance13. Psychological factors, e.g. anticipation and fear of pain
150
Clin. Biomech. 1992; 7: NO 3
due to physical strain, and the patient’s efficacy beliefs related to physical performance and endurance may influence the behaviour14-‘6. Estlander et a1.l’ found that the performance of the patients on an isokinetic lifting test increased as much from the first to the second measurement, during 4 weeks pretreatment, as it did from the second to the last measurement, during 4 weeks of intensive physical training. The authors concluded that, for example, the familiarity of the lifting situation, reduced fear of pain after one successful performance, and improved lifting technique probably influence the results, in addition to improved physical capability. The repeatability of measures of muscle strength has been ignored in most studies. The few reports of test-retest reliability have shown acceptable results17. These reports, however, are based on evaluation of healthy persons. Studies with back pain patients concerning possible changes in performance due to a repetition of the test have not been published. The aim of this study was to evaluate the effect of repeated measurements on isokinetic lifting performance in a sample of patients with low back pain (LBP). A comparison group consisting of healthy subjects was used as control. The hypothesis was that a mere repetition of the lifting procedure would increase the performance, and that the increase would be bigger in the patient group than in the non-patient group. In addition, grip strength was measured at both occasions. It was supposed that the performance on the grip strength test would not change from trial 1 to trial 2. Methods
Ten voluntary subjects (6 men, 4 women) were recruited among patients with LBP undergoing an evaluation of work capacity and need for rehabilitation at the Rehabilitation Foundation in Helsinki. Mean age was 45.8 years (SD 7.38, range 29-55 years). Time since onset of back pain was on average 7.7 years (SD 5.81, range 2-20 years). Two of the patients were working at the time of the assessment, six were sick-listed because of low back trouble, and two were unemployed. The non-patient group consisted of ten age- and sex-matched voluntary employees who did not have low back trouble. Average height and weight was equal in both groups. Isokinetic lifting strength was measured with a
Table 1. Means and
SD
of five consecutive
mechanica1 friction-based centrifugal force-controlled isokinetic device (Digitest OY, SF-40950 Muurame). The speed of isokinetic lift was 0.8 m s-l (& 5% variation, the speed reached in 0.3 s). The lift was performed from floor to above head, and the subject was in a standing position. The lift-force was recorded graphically with a plotter curve. Each trial comprised a series of five consecutive lifts. NO pre-assessment familiarization, time limit, or resting time between lifts were given. The results were expressed in kiloponds by bodyweight. The lifting procedure was repeated l- 13 days after the first trial. During this period the subjects continued their normal way of living. NO encouragement concerning for example physical activities or fitness training was given. The lifting trial was supervised by a physiotherapist with thorough experience of both the task and the device. The instruction was given with exactly the same words at both trials; the subjects were not encouraged to try to improve their performance. They did not get any feedback about their performance until the end of the second trial. The physiotherapist was not blind to whether the subject was a patient or a control subject. Grip strength was measured by a dynamometer device called Vigorimeter (Martin, D-7220 Tuttlingen). This has a ball-shaped air-containing handle from which the attempted grip force is conducted manometrically to the dynamometer (scale 0-200 kPa). The Vigorimeter supplies handles of three sizes. In this study the male subjects used the largest one, and the female subjects the middle-sized one. After one initial warm-up the subjects carried out three consecutive maxima1 squeezing attempts. The mean of the two best trials was calculated. The back pain patients rated their pain at the moment for the trial on a 5-point numerical scale (1 = no pain, 2 = mild, 3 = moderate, 4 = severe, 5 = very severe pain)“. The physiotherapist rated the subjects’ effort on a numerical scale (1 = inadequate, 2 = not very good, 3 = mediocre, 4 = quite good, 5 = very good). The rating was based on the patient’s verba1 and non-verba1 behaviour. Additional descriptive data (e.g. work situation, duration of back pain) were collected by questionnaire. Statistical methods
Means and standard deviations were calculated for the
lifts at trial 1 and trial 2, and increase in performance Health y controls
Low back pain patients
Men (n = 6)
SD
Mean
Women (n =4)
SD
Mean
Trial 7 tkp/bwl
Trial 2 fkp/b wl
56.14 35.61
71.79 28.35
20.09 12.60
25.88 15.80
Trial 1 fkp/bwi
Trial 2 fkp/bwl
28%
106.90 14.37
121.90 15.76
14%
28%
61.95 33.14
73.55 26.74
19%
Increase
Increase
Estlander et al.: Isokinetic Ming
Table 2. Means performance
and SD of maximum
lifts (the best of five consecutive
lifts) at trial 1 and trial 2, and increase
Low back pain patients
Men (n = 6)
SD
Mean
Women (n=4)
SD
Mean
Trial 7 (kp/bw)
Trial 2 (kp/bwl
74.35 22.98
87.29 32.30
27.54 18.18
31.51 20.96
Trial I lkp/b wl
Trial 2 lkp/b wl
17%
119.17 15.15
130.34 15.74
9%
14%
79.04 39.41
84.84 34.19
7%
Increase
Results
The means of the five consecutive lifts on the first and second trial for men and women separately are presented in Table 1. The patients in the LBP group increased their performance from the first to the second trial by an average of 28%. The differente was statistically significant (n = 10, f 3.26, P < 0.01). In the control group, the subjects increased their performance by an average of 14% (men) and 19% (women), and the differente was statistically significant (n = 10, t 2.63, P < 0.05). The correlation coefficient of the
120 r
(39.3) (36.5)
(26+Fz5)
(24.61 ’ ’
20 1 01
I 1
I 2
I 3
I 4
151
in
Health y controls
lifts and grip strength performances. Two-sample and paired t-tests were calculated to analyse differences between men and women, patients and controls, and differences between trials, and Pearson productmoment correlations were calculated.
1
strength
I 5
Consecutivelifts Figure 1. The means of five consecutive lifts at trial 1 and trial 2 in the patient group and the control group (SD in parentheses). 0 patients, trial 1; 0 patients, trial 2; H controls, trial 1; ? ? controls, trial 2.
lncrease
mean performance at trial 1 and trial 2 (test-retest correlation) was in the patient group r = 0.96 and in the control group r = 0.87. The average performance of the healthy subjects was significantly better than that of the patients. The best one of the five consecutive lifts was regarded as the maximum performance. The means of the maximum performance of men and women at both trials are presented in Table 2. In the patient group the increase in maximum performance was 17% for the men and 14% for the women. In the healthy group the increase was 9% for the men and 7% for the women. The differences between trials were not statistically significant. The correlation coefficient of the average maximum lift at trial 1 and trial 2 in both groups was r = 0.93. The means of the five consecutive lifts during trial 1 and trial 2 are presented in Figure 1. During both trials in the patient group, and during trial 1 in the control group, the average performance increased more or less continuously from lift 1 to lift 5, while the average performance of the control subjects during trial 2 increased only slightly. An analysis of the individual performance (means of the five consecutive lifts for each subject) showed that in one case in the patient group, and in two cases in the control group, the second trial was inferior to the first one. The range of increase from trial 1 to trial 2 varied between +2.5% and +83.5%. Regarding the five consecutive lifts in each trial, the fifth or last lift was in most cases the best. Both trials taken together, the fifth lift was the maximum one in 13 of 20 trials in the LBP group and in 12 of 20 trials in the control group. NO differences between trial 1 and trial 2 were found on the effort rating as assessed by the physiotherapist. In only 3 of 20 trials the effort was rated as not very good. According to the pain rating, two patients rated more severe pain at trial 2 than at trial 1; they increased their performance by 60% and 39% respectively. The test of the grip strength showed no significant changes in performance in either group when comparing the first and the second trial (Patient group means; right hand, trial 1 = 85.2, trial 2 = 85.3; left hand, trial 1 = 86.5, trial 2 = 77.6. Control group means; right hand, trial 1 = 101.8, trial 2 = 104.1; left hand, trial 1 = 100.7, trial 2 = 103.6).
152
Clin. Biomech. 1992; 7: NO 3
Discussion The most important results of this study were: (1) an increase in lifting strength from the first to the second trial by an average 28% in the low back pain patients and 16% in the control group, and (2) the last of the five consecutive lifts was the best in a majority of the cases in both trials. The fact that a mere repetition of the lifting procedure leads to an increase in performance suggests that other factors than improved muscle strength influence the results. Measuring lifting strength isokinetically is unfamiliar and artificial from the patients’ point of view, and may be especially prone to factors like improved lifting technique and muscle coordination, reduced fear and anticipation of pain, etc. If so, these results probably do not apply to isometrie testing on one hand or isokinetic testing of more simple movements like those of limbs and trunk on the other. The unchanged performance on the grip strength test suggests that repetition does not improve al1 kinds of physical performance. The increase in performance, calculated as percentage change, was larger in the LBP group than in the control group, and larger when the increase was calculated on the basis of the average performance (the mean of the five consecutive lifts) than when it was calculated on the basis of the maximum lift (the best of the five lifts). The low back pain patients had a lower initial performance. A mathematica1 bias may influence the results: it is easier to get a high change calculated as per cent from the initial leve1 when the initial leve1 is low, and it is also easier to increase a performance which is initially low. This should be considered when reporting treatment effects for example. This study actualizes a statistical problem related to the test-retest correlation coefficient. The test-retest correlations in this study were high, but there was a significant average increase in performance. A systematic change in performance, as was present in this study, is not revealed by the test-retest correlation coefficient , and a comparison of the absolute performances is also required in order not to make erroneous conclusions about the reliability of the test. The phenomenon of submaximal performance at the first trial is wel1 known from for example assessment of lung function by spirometric evaluation. The results of the present study, even if they are based on a smal1 sample, show that a mere repetition of the isokinetic lifting task improves the performance. The results indicate that neither five consecutive lifts within one trial, nor measurements on two different occasions, are enough to define the baseline or the maxima1 lifting
capacity. More research is needed to evaluate methods for establishing baseline levels and maxima1 performance. Caution is indicated with conclusions about, for example, treatment outcome or patient performance versus non-patient performance based on
one single baseline strength.
assessment
of isokinetic
lifting
References 1 Nordin M, Kahanovitz N, Verderame R et al. Normal trunk muscle strength and endurance in women and the effect of exercises and electrical stimulation. Part 1: Normal endurance and trunk muscle strength in 101 women. Spine 1987; 12: 105-11 2 McNeill T, Warwick D, Andersson G, Schultz A. Trunk strengths in attempted flexion, extension, and lateral bending in healthy subjects and patients with low back disorders. Spine 1980; 5: 529-33 3 Suzuki N, Endo S. A quantitative study of trunk muscle strength and fatigability in the low back pain syndrome. Spine 1983; 8: 69-84 4 Kishino ND, Mayer TG, Gatchel RJ et al. Quantification
of lumbar function. Part 4: Isometrie and isokinetic lifting simulation in normal subjects and low-back dysfunction patients. Spine 1985; 10: 921-7 5 Kahanovitz N, Viola K, Gallagher M. Long-term strength assessment of postoperative discectomy patients. Spine 1989; 14: 402-3 6 Mayer TG, Vanharanta
7
8
9
10
11
12
H, Gatchel RJ et al. Comparison of CT scan muscle measurements and isokinetic trunk strength in postoperative patients. Spine 1989; 14: 33-6 Hazard RG, Reid S, Fenwick J, Reeves V. Isokinetic trunk and lifting strength measurements: variability as an indicator of effort. Spine 1988; 13: 54-7 Mayer TG, Gatchel RJ, Kishino N et al. Objective assessment of spine function following industrial injury. A prospective study with comparison group and one-year follow-up. Spine 1985; 10: 482-93 Hazard RG, Fenwick JW, Kalish SM et al. Functional restoration with behavioral support. A one-year prospective study of patients with chronic low back pain. Spine 1989; 14: 157-61 Estlander AM, Mellin G, Vanharanta H, Hupli M. Effects and follow-up of a multimodal treatment program including intensive physical training for low back pain patients. Stand J Rehabil Med 1991; 23: 97-102 Nyman K, Tufveson B. Isometrisk ryggmuskelstyrka hos personer med kroniska lumbala besvär. En utvärdering efter träning. Läkartidningen 1982; 79: 3902-3 Kottke FJ. The training of coordination. Arch Phys Med
RehabiZl980; 61: 551-61
13 Beimborn DS, Morrissey MC. A review of the literature related to trunk muscle performance. Spine 1988; 13: 655-60
14 Lethem J, Slade PD, Troup JDG, Bentley G. Outline of a fear-avoidance model of exaggerated pain perception. 1. Behav Res Ther 1983; 21: 401-8
15 Turk DC, Flor H. Pain > pain behaviors. The utility and limitations of the pain behavior construct. Puin 1987; 31: 277-95
16 Philips HC. Avoidance behaviour and its role in sustaining chronic pain. Behav Res Ther 1987; 25: 273-79 17 Smith SS, Mayer T, Gatchel RJ, Becker T. Quantification of lumbar function. Part 1: Isometrie and multispeed isokinetic trunk strength measures in sagittal and axial planes in normal subjects. Spine 1985; 8: 757-64 18 Estlander AM. Comprehensive treatment of chronic low back pain patients - a preliminary report. In Vinck J et al., eds. Topics in Behavioral Medicine. Annual Series of Research in Behavior Therapy, 1986; 1: 257-67
Cnfluence of repeated measurements Hing strength
A-M Estlander Rehabilitation
PSL, G
Foundation,
Mellin Helsinki,
MD,A Weckström
on isokinetic
PT
Finland
Summary The aim of the study was to evaluate the effect of repeated measurements on isokinetic lifting strength in a sample of patients with low back pain. The measures were performed in series of five consecutive lifts on two occasions 1-13 days apart, and the patients continued their normal living during this period. A group of healthy subjects was used as a control. The hypothesis was that a mere repetition of the lifting procedure would increase the performance, and that the increase would be bigger in the patient group than in the non-patient group. In addition, grip strength was measured on both occasions. It was supposed that grip strength would not change from trial 1 to trial 2. The results showed an increase in Iifting strength from first to second trial in the patient group by an average of 28%, and in the control group by an average of 16%. Grip strength did not increase in either group. The last of five consecutive lifts was the best one in a majority of the cases in both trials. Neither five consecutive lifts nor measurements on two different occasions seem enough to define a reliable baseline for maximal lifting capacity. Relevante Other factors than an increase in muscle strength influence the performance, one being a repetition of the lifting procedure. This must be accounted for when defining a baseline for isokinetic lifting capacity. Key words:
Isokinetic lifting, low back pain, repeated
measures
Introduction Within the field of assessment and treatment of chronic low back pain patients, the interest in non-invasive measuring of back functions, e.g. muscle strength, has increased considerably, and the use of computerized electronic commercial measurement devices has grown accordingly . Measurements of back muscle strength have been used for various purposes. Standards for healthy individuals have been collectedl. Studies comparing low back patients and healthy persons usually show that the muscle strength of patients is inferior to that of healthy persons2-4. Reduction in trunk strength after discectomy has been show&‘, and correlations between Received: 8 December 1990 Accepted: 20 August 1991 Correspondence and reprint requests
Finnish Back Institute, Finland
Salomonkatu
CQ 1991 Butterworth-Heinemann 0268-0033/92/030149-04
Ltd
to: Ann-Mari
Estlander, PsL, 17 B 3 krs, 00100 Helsinki,
muscle density as assessed by CT scan and isokinetic lifting strength have been demonstrated6. This kind of measurement has also been used to discriminate between maxima1 and submaximal effort in healthy subjects7. Measuring trunk muscle strength and lifting capacity with, for example, isokinetic testing devices have been regarded as ‘a quantitative technique for objectively documenting spine function’ and to be a critical component necessary for diagnosis and surgical decisions’. Measurements of spinal functions have also been used to guide treatment and evaluate treatment outcome in rehabilitation programmes for back pain patients’-1°. Increases in lifting capacity during treatment have been interpreted as objective evidente for increased muscle strengths39*‘1. Doubt has, however, been raised about the validity of measures of muscle strength. The influence of repetition on motor skills is a well-known phenomenon12. Trunk muscle performance of patients with low back trouble may be a test of their pain leve1 and tolerance13. Psychological factors, e.g. anticipation and fear of pain
150
Clin. Biomech. 1992; 7: NO 3
due to physical strain, and the patient’s efficacy beliefs related to physical performance and endurance may influence the behaviour14-‘6. Estlander et a1.l’ found that the performance of the patients on an isokinetic lifting test increased as much from the first to the second measurement, during 4 weeks pretreatment, as it did from the second to the last measurement, during 4 weeks of intensive physical training. The authors concluded that, for example, the familiarity of the lifting situation, reduced fear of pain after one successful performance, and improved lifting technique probably influence the results, in addition to improved physical capability. The repeatability of measures of muscle strength has been ignored in most studies. The few reports of test-retest reliability have shown acceptable results17. These reports, however, are based on evaluation of healthy persons. Studies with back pain patients concerning possible changes in performance due to a repetition of the test have not been published. The aim of this study was to evaluate the effect of repeated measurements on isokinetic lifting performance in a sample of patients with low back pain (LBP). A comparison group consisting of healthy subjects was used as control. The hypothesis was that a mere repetition of the lifting procedure would increase the performance, and that the increase would be bigger in the patient group than in the non-patient group. In addition, grip strength was measured at both occasions. It was supposed that the performance on the grip strength test would not change from trial 1 to trial 2. Methods
Ten voluntary subjects (6 men, 4 women) were recruited among patients with LBP undergoing an evaluation of work capacity and need for rehabilitation at the Rehabilitation Foundation in Helsinki. Mean age was 45.8 years (SD 7.38, range 29-55 years). Time since onset of back pain was on average 7.7 years (SD 5.81, range 2-20 years). Two of the patients were working at the time of the assessment, six were sick-listed because of low back trouble, and two were unemployed. The non-patient group consisted of ten age- and sex-matched voluntary employees who did not have low back trouble. Average height and weight was equal in both groups. Isokinetic lifting strength was measured with a
Table 1. Means and
SD
of five consecutive
mechanica1 friction-based centrifugal force-controlled isokinetic device (Digitest OY, SF-40950 Muurame). The speed of isokinetic lift was 0.8 m s-l (& 5% variation, the speed reached in 0.3 s). The lift was performed from floor to above head, and the subject was in a standing position. The lift-force was recorded graphically with a plotter curve. Each trial comprised a series of five consecutive lifts. NO pre-assessment familiarization, time limit, or resting time between lifts were given. The results were expressed in kiloponds by bodyweight. The lifting procedure was repeated l- 13 days after the first trial. During this period the subjects continued their normal way of living. NO encouragement concerning for example physical activities or fitness training was given. The lifting trial was supervised by a physiotherapist with thorough experience of both the task and the device. The instruction was given with exactly the same words at both trials; the subjects were not encouraged to try to improve their performance. They did not get any feedback about their performance until the end of the second trial. The physiotherapist was not blind to whether the subject was a patient or a control subject. Grip strength was measured by a dynamometer device called Vigorimeter (Martin, D-7220 Tuttlingen). This has a ball-shaped air-containing handle from which the attempted grip force is conducted manometrically to the dynamometer (scale 0-200 kPa). The Vigorimeter supplies handles of three sizes. In this study the male subjects used the largest one, and the female subjects the middle-sized one. After one initial warm-up the subjects carried out three consecutive maxima1 squeezing attempts. The mean of the two best trials was calculated. The back pain patients rated their pain at the moment for the trial on a 5-point numerical scale (1 = no pain, 2 = mild, 3 = moderate, 4 = severe, 5 = very severe pain)“. The physiotherapist rated the subjects’ effort on a numerical scale (1 = inadequate, 2 = not very good, 3 = mediocre, 4 = quite good, 5 = very good). The rating was based on the patient’s verba1 and non-verba1 behaviour. Additional descriptive data (e.g. work situation, duration of back pain) were collected by questionnaire. Statistical methods
Means and standard deviations were calculated for the
lifts at trial 1 and trial 2, and increase in performance Health y controls
Low back pain patients
Men (n = 6)
SD
Mean
Women (n =4)
SD
Mean
Trial 7 tkp/bwl
Trial 2 fkp/b wl
56.14 35.61
71.79 28.35
20.09 12.60
25.88 15.80
Trial 1 fkp/bwi
Trial 2 fkp/bwl
28%
106.90 14.37
121.90 15.76
14%
28%
61.95 33.14
73.55 26.74
19%
Increase
Increase
Estlander et al.: Isokinetic Ming
Table 2. Means performance
and SD of maximum
lifts (the best of five consecutive
lifts) at trial 1 and trial 2, and increase
Low back pain patients
Men (n = 6)
SD
Mean
Women (n=4)
SD
Mean
Trial 7 (kp/bw)
Trial 2 (kp/bwl
74.35 22.98
87.29 32.30
27.54 18.18
31.51 20.96
Trial I lkp/b wl
Trial 2 lkp/b wl
17%
119.17 15.15
130.34 15.74
9%
14%
79.04 39.41
84.84 34.19
7%
Increase
Results
The means of the five consecutive lifts on the first and second trial for men and women separately are presented in Table 1. The patients in the LBP group increased their performance from the first to the second trial by an average of 28%. The differente was statistically significant (n = 10, f 3.26, P < 0.01). In the control group, the subjects increased their performance by an average of 14% (men) and 19% (women), and the differente was statistically significant (n = 10, t 2.63, P < 0.05). The correlation coefficient of the
120 r
(39.3) (36.5)
(26+Fz5)
(24.61 ’ ’
20 1 01
I 1
I 2
I 3
I 4
151
in
Health y controls
lifts and grip strength performances. Two-sample and paired t-tests were calculated to analyse differences between men and women, patients and controls, and differences between trials, and Pearson productmoment correlations were calculated.
1
strength
I 5
Consecutivelifts Figure 1. The means of five consecutive lifts at trial 1 and trial 2 in the patient group and the control group (SD in parentheses). 0 patients, trial 1; 0 patients, trial 2; H controls, trial 1; ? ? controls, trial 2.
lncrease
mean performance at trial 1 and trial 2 (test-retest correlation) was in the patient group r = 0.96 and in the control group r = 0.87. The average performance of the healthy subjects was significantly better than that of the patients. The best one of the five consecutive lifts was regarded as the maximum performance. The means of the maximum performance of men and women at both trials are presented in Table 2. In the patient group the increase in maximum performance was 17% for the men and 14% for the women. In the healthy group the increase was 9% for the men and 7% for the women. The differences between trials were not statistically significant. The correlation coefficient of the average maximum lift at trial 1 and trial 2 in both groups was r = 0.93. The means of the five consecutive lifts during trial 1 and trial 2 are presented in Figure 1. During both trials in the patient group, and during trial 1 in the control group, the average performance increased more or less continuously from lift 1 to lift 5, while the average performance of the control subjects during trial 2 increased only slightly. An analysis of the individual performance (means of the five consecutive lifts for each subject) showed that in one case in the patient group, and in two cases in the control group, the second trial was inferior to the first one. The range of increase from trial 1 to trial 2 varied between +2.5% and +83.5%. Regarding the five consecutive lifts in each trial, the fifth or last lift was in most cases the best. Both trials taken together, the fifth lift was the maximum one in 13 of 20 trials in the LBP group and in 12 of 20 trials in the control group. NO differences between trial 1 and trial 2 were found on the effort rating as assessed by the physiotherapist. In only 3 of 20 trials the effort was rated as not very good. According to the pain rating, two patients rated more severe pain at trial 2 than at trial 1; they increased their performance by 60% and 39% respectively. The test of the grip strength showed no significant changes in performance in either group when comparing the first and the second trial (Patient group means; right hand, trial 1 = 85.2, trial 2 = 85.3; left hand, trial 1 = 86.5, trial 2 = 77.6. Control group means; right hand, trial 1 = 101.8, trial 2 = 104.1; left hand, trial 1 = 100.7, trial 2 = 103.6).
152
Clin. Biomech. 1992; 7: NO 3
Discussion The most important results of this study were: (1) an increase in lifting strength from the first to the second trial by an average 28% in the low back pain patients and 16% in the control group, and (2) the last of the five consecutive lifts was the best in a majority of the cases in both trials. The fact that a mere repetition of the lifting procedure leads to an increase in performance suggests that other factors than improved muscle strength influence the results. Measuring lifting strength isokinetically is unfamiliar and artificial from the patients’ point of view, and may be especially prone to factors like improved lifting technique and muscle coordination, reduced fear and anticipation of pain, etc. If so, these results probably do not apply to isometrie testing on one hand or isokinetic testing of more simple movements like those of limbs and trunk on the other. The unchanged performance on the grip strength test suggests that repetition does not improve al1 kinds of physical performance. The increase in performance, calculated as percentage change, was larger in the LBP group than in the control group, and larger when the increase was calculated on the basis of the average performance (the mean of the five consecutive lifts) than when it was calculated on the basis of the maximum lift (the best of the five lifts). The low back pain patients had a lower initial performance. A mathematica1 bias may influence the results: it is easier to get a high change calculated as per cent from the initial leve1 when the initial leve1 is low, and it is also easier to increase a performance which is initially low. This should be considered when reporting treatment effects for example. This study actualizes a statistical problem related to the test-retest correlation coefficient. The test-retest correlations in this study were high, but there was a significant average increase in performance. A systematic change in performance, as was present in this study, is not revealed by the test-retest correlation coefficient , and a comparison of the absolute performances is also required in order not to make erroneous conclusions about the reliability of the test. The phenomenon of submaximal performance at the first trial is wel1 known from for example assessment of lung function by spirometric evaluation. The results of the present study, even if they are based on a smal1 sample, show that a mere repetition of the isokinetic lifting task improves the performance. The results indicate that neither five consecutive lifts within one trial, nor measurements on two different occasions, are enough to define the baseline or the maxima1 lifting
capacity. More research is needed to evaluate methods for establishing baseline levels and maxima1 performance. Caution is indicated with conclusions about, for example, treatment outcome or patient performance versus non-patient performance based on
one single baseline strength.
assessment
of isokinetic
lifting
References 1 Nordin M, Kahanovitz N, Verderame R et al. Normal trunk muscle strength and endurance in women and the effect of exercises and electrical stimulation. Part 1: Normal endurance and trunk muscle strength in 101 women. Spine 1987; 12: 105-11 2 McNeill T, Warwick D, Andersson G, Schultz A. Trunk strengths in attempted flexion, extension, and lateral bending in healthy subjects and patients with low back disorders. Spine 1980; 5: 529-33 3 Suzuki N, Endo S. A quantitative study of trunk muscle strength and fatigability in the low back pain syndrome. Spine 1983; 8: 69-84 4 Kishino ND, Mayer TG, Gatchel RJ et al. Quantification
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