Isometric and isokinetic endurance the forearm complex* NEIL E.
testing
of
MOTZKIN, MD, THOMAS D. CAHALAN, RPT, BERNARD F. MORREY, MD, KAI-NAN AN,† PhD, AND EDMUND Y. S. CHAO, PhD
From the Biomechanics
Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minnesota
a physician could obtain similar information from either testing modality. However, if they are not related, then both isometric and isokinetic endurance testing could provide useful, independent information in terms of functional as-
ABSTRACT The purpose of this study was to investigate the relationship of isometric and isokinetic endurance of the forearm complex under maximum effort in a healthy population. Isometric and isokinetic endurance measurements of elbow flexion and extension of the dominant and nondominant sides were made in 32 healthy male subjects (age range, 21 to 39 years). Both fatigue rates and the amount of time elapsed in reaching fatigue were determined. Analysis of these data reveals that 1) isometric endurance of the forearm complex cannot be used as a predictor of isokinetic endurance, as there is no relationship between the two, 2) dominant and nondominant endurance are related, a result that supports the belief that in the assessment of the endurance of a single diseased upper extremity, the best available comparison is with the contralateral healthy extremity, and 3) healthy males in the age range tested have greater endurance in elbow extension than in flexion.
sessment.
MATERIALS AND METHODS
Subjects Thirty-two adult males, age range, 21 to 39 (mean age, 27.5), with no known pathologic history of the upper extremity musculoskeletal system were studied over a period of 4 months.
Technique Each subject underwent four isometric and four isokinetic endurance tests to measure bilateral elbow flexion and extension (Table 1). The isometric tests were done with a firmly fixed torque cell dynamometer according to a previ-
For functional assessments, knowledge of endurance is as important as knowledge of maximum strength. The most readily available means of assessment is testing of isometric strength. Although isokinetic endurance may be more clinically relevant, the testing apparatus is less available to most clinicians. The primary purpose of this study was to investigate the relationship of isometric and isokinetic endurance of the forearm complex in a healthy population. The issue of their relationship is clinically relevant. If isometric and isokinetic endurance are related, the value of maintaining access to both kinds of testing apparatus is diminished, since
TABLE 1
Time to 50%
peak torque and fatigue rates
*
Presented m part at the 1989 Orthopedic Research Society meeting, Las Vegas, Nevada, and the 1989 Minnesota Orthopedic Society Meeting, Minneapolis, Minnesota t Address correspondence and repnnt requests to Kai-Nan An, PhD, Biomechanics Laboratory, Mayo Clmc/Mayo FoundaUon, Rochester, MN 55905
°
First number mdicates the mean; the numbers in indicate the standard deviation. b Change in percent of peak torque per second.
107
parentheses
108
ously described technique.’ The torque generated and time elapsed were recorded on a Hewlett-Packard 7044A X-Y recorder (Hewlett-Packard, Palo Alto, CA). Subjects were seated on a rigid stool with their upper arms in adduction and slight flexion, shoulders and forearms in neutral rotation, elbows flexed to 90°, using a hand grip (Fig. 1). They were asked to produce a sustained maximal voluntary contraction until the torque generated over time was 50% of the peak torque value. Peak torque was defined as the maximal torque that could be maintained for at least 2 seconds. The isokinetic tests were done with a modified Cybex testing device (Cybex, Ronkonkoma, NY) (Fig. 2). The device was stabilized in the usual fashion, recording the torque generated and time elapsed. Subjects were strapped into the standard Cybex chair with their upper arms stabilized in adduction and slight flexion, shoulders and forearms in neutral rotation, using a hand grip. They were then asked to produce a maximal voluntary contraction through a limited range of motion from 45° to 135° of elbow flexion. Upon
reaching the end of the arc, the subjects returned their forearms to the beginning of the arc and maximally contracted again in the original direction, thereby testing only one function (i.e., flexion or extension) at a time. Subjects continued until the greatest torque generated by individual contractions over time was 50% of the peak torque value. defined as the maximal torque that could three individual contractions. The anof the isokinetic tests was 180 deg/sec. Algular velocity though the frequency of the isokinetic contractions was not rigidly controlled, given the angular velocity (180 deg/sec) and the limited range of motion, from 45° to 135° of elbow flexion, the frequency was approximately one contraction per second. In all of the tests, the elbow was aligned with the torque cell dynamometer or the Cybex device such that the anatomical center of elbow motion was aligned with the axes of rotation of the appropriate machine. The isometric and isokinetic tests were conducted in a random order at least 1 week apart from one another in order to minimize the effects of learning, strengthening, or fatique of one test upon another. The tests were conducted at the same time of day in order to minimize any differences in strength or endurance during different hours of the day.’ All tests and manual digitizing were performed by one of the authors (NEM) in order to minimize any differences in examiner technique and subject motivation. Peak torque
Data
Figure
1. The torque cell
dynamometer used
in the isometric
tests.
was
be maintained
over
analysis
A series of points corresponding to the torque generated and the time elapsed comprised the raw data collected on each subject for each test (Figs. 3A and 4). The data was manually digitized with a Graf/Pen Sonic Digitizer (Science Accessories Corp, Southport, CT) and analyzed with Minitab Data Analysis Software (State College, PA). On the abscissa, the torque as a function of time was recorded with the moment of peak torque count as time zero (Figs. 3B and 4). Tests were continued until time T50, the time to 50% of the peak torque value. On the ordinate, the raw torque values were converted to the percentage of peak torque so that the subjects’ torque values were directly comparable (i.e., every subject fatigued from 100% to 50%). In addition, a linear regression analysis was performed on each test, using the data points from To to TSO. The slope values of the lines so generated were defined as the &dquo;fatigue rate&dquo; (F) so that: F the change of percent peak torque per second (Figs. 3B and 4). The fatigue rates were used to assess endurance. The greater the magnitude of the negative fatigue rate (or the steeper the slope of the line), the less the endurance of a particular muscle. =
Statistical
Figure 2.
The
Cybex device used
in the isokinetic tests.
analysis
Paired t-tests and regression analyses were used to assess significant differences and relationships between fatigue rates, respectively (Table 2). The P 0.05). M, isometric; K, isokinetic; D, dominant, ND, nondominant; F, flexion; E, extension.
32) was sufficient 12 and the paired analyses appropriate for the single variable altered per teSt.8 size (N
=
were
RESULTS Table 1 summarizes the subjects’ endurance under maximum effort. For isometric function, the maximum strength could last an average of 60 to 80 seconds before it dropped to 50% of the peak values. On the other hand, for isokinetic function, the maximum strength before dropping below 50% of the peak value could last for an average of 40 to 60 seconds. Those subjects performing isokinetic, nondominant, and flexion exercises fatigued faster than those performing isometric, dominant, and extension exercises, respectively. Not all of these tendencies were statistically significant, however. In the four tests comparing the forms of contraction (isometric versus isokinetic), there were significant differences in fatigue rates in half of the otherwise identical tests and there were no significant relationships (Table 2A). Significant differences are present when the means of the fatigue rates of the two groups under comparison are different
(i.e.,
a
positive paired t-test). Significant relationships
present when
a nonhorizontal line can reasonably be drawn on a Cartesian coordinate system through a plot of the fatigue rates of the two groups under comparison (i.e., a positive regression analysis). Hence, the hypothesis of this study that there is a direct relationship between isometric and isokinetic endurance testing of the forearm complex are
could not be confirmed. In the four tests
comparing the upper extremity tested nondominant), there were no significant differences in fatigue rates between otherwise identical tests. Also, the relationship between dominant and nondominant fatigue rates was significant in three of the tests (isometric flexion, isometric extension, and isokinetic extension), and approached significance in the fourth test (P < 0.1 for isokinetic flexion) (Table 2B). These results support the assumption that in the assessment of a single diseased upper extremity, the best available comparison is the contralateral healthy upper extremity. To further assess the relationship between dominant and nondominant endurance, the mean ratios of the time elapsed in reaching 50% of peak torque in the tests comparing dominant and nondominant exercises (dominant/nondominant ratios) were determined, and they all approximate 1.0 (range, 1.06 to 1.17); however, the confidence intervals of (dominant
versus
the dominant/nondominant ratios are relatively wide, as much as 30% (Table 3). Hence, although the time to fatigue on the dominant side approximates that time on the nondominant side, a fairly large difference between the two sides can be seen in healthy subjects. In the four tests comparing the muscle group tested (flexion versus extension) there were significant differences in fatigue rates in all of the otherwise identical tests (Table 2C), with testing in flexion eliciting more rapid fatigue than testing in extension (Table 1). Under isometric conditions, the fatigue rate between flexion and extension were signifi-
cantly correlated. DISCUSSION The primary purpose of this study was to compare isometric and isokinetic endurance of the forearm complex. With the data collected, we were also able to compare endurance between the dominant and nondominant upper extremities and between elbow flexion and extension. Endurance has generally been studied to define the level of muscle contraction so that the effort could last for a given period of time. In this study, we defined endurance as the length of time in which a subject, under maximum effort, could maintain a level of torque at or above 50% of his peak torque in either a continual isometric or a repetitive isokinetic maximal effort. Such a definition implies a study of anaerobic, nonoxidative metabolism and the action of Type II (fast twitch) muscle fibers.5This is an accepted definition of endurance.’,&dquo; As a general rule, subjects performing isokinetic, nondominant, and flexion exercises fatigued faster than those performing isometric, dominant, and extension exercises, respectively (Table 1). Not all of these tendencies were statistically significant, however. TABLE 3 Ratios and their confidence intervals of the time elapsed in reaching 50% of peak torque in the four tests comparing dominant and nondominant exercises&dquo; a
a D, dominant; ND, nondominant.
111
Recent studies6 10 have shown correlations between isometric and isokinetic strength tests. However, the correlation between isometric and isokinetic endurance tests has not been previously reported. In the tests comparing the form of contraction (isometric versus isokinetic) (Table 2A), there were no significant relationships in fatigue rates between otherwise identical tests. Hence, our hypothesis of a direct relationship between isometric and isokinetic endurance of the forearm complex could not be confirmed. A primary difference between isometric and isokinetic exercise concerns the production of work. Recall that work F X d). In isometric exercise, d force X distance (W 0; so there is no work production. In isokinetic exercise, d > 0; so work is produced. We postulate that there is a distinct difference in the physiologic mechanisms of endurance between exercises that produce work and those that do not, and that this difference accounts for the difference between isometric and isokinetic endurance. Nonetheless, the precise mechanisms that account for these differences remain ob=
=
=
scure.
A recent study has shown that there are significant differences in isometric strength between dominant and nondominant upper extremities. However, the correlation between dominant and nondominant endurance has not been previously reported. In the tests comparing the upper extremity tested (dominant versus nondominant) (Table 2B), there were no significant differences in fatigue rates between otherwise identical tests, and there were several significant relationships. These results support the assumption that in the assessment of the endurance of a single diseased upper extremity, the best available comparison is the contralateral healthy upper extremity. The mean ratios of the time elapsed in reaching 50% of peak torque in the test comparing dominant and nondominant exercises (dominant/nondominant ratios) all approximate 1.0 (range, 1.06 to 1.17); however, the confidence intervals of the dominant/nondominant ratios are relatively wide, as much as 30% (Table 3). Hence, although the time to fatigue on the dominant side approximates that time on the nondominant side, a fairly large difference between the two sides can be seen in healthy subjects. This is similar to the findings of a recent study which showed small but significant differences between the dominant and nondominant sides of subjects performing isometric flexion and extension strength exercises of the forearm complex.’ In the tests comparing the muscle group tested (flexion versus extension) (Table 2C), there were significant differences in fatigue rates between all of the otherwise identical tests. Given that the magnitude of the fatigue rates of flexion was in each case greater than that of extension (Table 1), this is strong evidence that healthy males in the age range tested have greater endurance in elbow extension than in flexion.
We postulate that this finding depends upon the differin the cumulative muscle fiber composition of the muscles involved in elbow flexion as compared to those involved in extension. The muscle contractions tested in this study (i.e., either a continual isometric or a repetitive isokinetic maximal effort) principally depended upon anaerobic metabolism and Type II, fast twitch muscle fibers. Thus, a muscle group with a higher proportion of Type II muscle fibers might be expected to have greater endurance according to our protocol. We could find no evidence in the literature, however, that shows there is any systematic difference in the muscle fiber composition between these two muscle groups.3, 4,9 In summary, the data presented indicate that isometric endurance of the forearm complex cannot be used as a predictor of isokinetic endurance and there is no significant relationship between the two. The data collected support the assumption that in the assessment of the endurance of a single diseased upper extremity, the best available control is the contralateral side. The data also indicate that healthy subjects have greater endurance in elbow extension than in flexion. ence
ACKNOWLEDGMENTS This study was supported by NIH Grant AR 26287. The authors appreciate the efforts of Linda Romme, Pat Cahill, Dr. Kenton Kaufman, and the staff of the Mayo Orthopedic Biomechanics Laboratory in facilitating the progress and completion of this project.
REFERENCES 1
An KN, Morrey BF, et al Isometric elbow strength in normal individuals Clin Orthop 222 261-266, 1987 2 Burdett RG, Van Swearingen J Reliability of isokinetic muscle endurance tests J Orthop Sports Phys Ther 8 484-488, 1987 3 Clarkson PM, Kroll W, Melchionda AM. Isokinetic strength, endurance, and fiber type composition in elite American paddlers Eur J Appl Physiol 48
Askey LJ,
67-76,1982 4
Elder GCB, Bradbury K, Roberts R Variability of fiber type distributions within human muscles J Appl Physiol 53 1473-1480, 1982 5 Gollnick PD, Armstrong RB, Saubert CW, et al Enzyme activity and fiber composition in skeletal muscle of untrained and trained men. J Appl Physiol 33 312-319,1972 6 Knapik JJ, Ramos MV Isokinetic and isometric torque relationships in the human body Arch Phys Med Rehabil 61 64-67, 1980 7 McGarvey SR, Morrey BF, Askew LJ, et al. Reliability of isometric strength testing Temporal factors and strength variation Clin Orthop 185 301-
305,1984 Morrison DF Multivariate Statistical Methods Second edition New York, McGraw-Hill, 1976 9 Nygaard E, Houston M, Suzuki Y, et al Morphology of the brachial biceps muscle and elbow flexion in man Acta Physiol Scand 117 287-292, 1983 10 Otis JC, Godbold JH Relationship of isokinetic torque to isometric torque. J Orthop Res 1 165-171, 1983 11 Patton RW, Hinson MM, Arnold BR, et al Fatigue curves of isokinetic contractions Arch Phys Med Rehab 59 507-509, 1978 12. Steel RGD,Torrie JH Principles and Procedures of Statistics. A Biometrical Approach Second edition New York, McGraw-Hill, 1980 8
testing
of
MOTZKIN, MD, THOMAS D. CAHALAN, RPT, BERNARD F. MORREY, MD, KAI-NAN AN,† PhD, AND EDMUND Y. S. CHAO, PhD
From the Biomechanics
Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minnesota
a physician could obtain similar information from either testing modality. However, if they are not related, then both isometric and isokinetic endurance testing could provide useful, independent information in terms of functional as-
ABSTRACT The purpose of this study was to investigate the relationship of isometric and isokinetic endurance of the forearm complex under maximum effort in a healthy population. Isometric and isokinetic endurance measurements of elbow flexion and extension of the dominant and nondominant sides were made in 32 healthy male subjects (age range, 21 to 39 years). Both fatigue rates and the amount of time elapsed in reaching fatigue were determined. Analysis of these data reveals that 1) isometric endurance of the forearm complex cannot be used as a predictor of isokinetic endurance, as there is no relationship between the two, 2) dominant and nondominant endurance are related, a result that supports the belief that in the assessment of the endurance of a single diseased upper extremity, the best available comparison is with the contralateral healthy extremity, and 3) healthy males in the age range tested have greater endurance in elbow extension than in flexion.
sessment.
MATERIALS AND METHODS
Subjects Thirty-two adult males, age range, 21 to 39 (mean age, 27.5), with no known pathologic history of the upper extremity musculoskeletal system were studied over a period of 4 months.
Technique Each subject underwent four isometric and four isokinetic endurance tests to measure bilateral elbow flexion and extension (Table 1). The isometric tests were done with a firmly fixed torque cell dynamometer according to a previ-
For functional assessments, knowledge of endurance is as important as knowledge of maximum strength. The most readily available means of assessment is testing of isometric strength. Although isokinetic endurance may be more clinically relevant, the testing apparatus is less available to most clinicians. The primary purpose of this study was to investigate the relationship of isometric and isokinetic endurance of the forearm complex in a healthy population. The issue of their relationship is clinically relevant. If isometric and isokinetic endurance are related, the value of maintaining access to both kinds of testing apparatus is diminished, since
TABLE 1
Time to 50%
peak torque and fatigue rates
*
Presented m part at the 1989 Orthopedic Research Society meeting, Las Vegas, Nevada, and the 1989 Minnesota Orthopedic Society Meeting, Minneapolis, Minnesota t Address correspondence and repnnt requests to Kai-Nan An, PhD, Biomechanics Laboratory, Mayo Clmc/Mayo FoundaUon, Rochester, MN 55905
°
First number mdicates the mean; the numbers in indicate the standard deviation. b Change in percent of peak torque per second.
107
parentheses
108
ously described technique.’ The torque generated and time elapsed were recorded on a Hewlett-Packard 7044A X-Y recorder (Hewlett-Packard, Palo Alto, CA). Subjects were seated on a rigid stool with their upper arms in adduction and slight flexion, shoulders and forearms in neutral rotation, elbows flexed to 90°, using a hand grip (Fig. 1). They were asked to produce a sustained maximal voluntary contraction until the torque generated over time was 50% of the peak torque value. Peak torque was defined as the maximal torque that could be maintained for at least 2 seconds. The isokinetic tests were done with a modified Cybex testing device (Cybex, Ronkonkoma, NY) (Fig. 2). The device was stabilized in the usual fashion, recording the torque generated and time elapsed. Subjects were strapped into the standard Cybex chair with their upper arms stabilized in adduction and slight flexion, shoulders and forearms in neutral rotation, using a hand grip. They were then asked to produce a maximal voluntary contraction through a limited range of motion from 45° to 135° of elbow flexion. Upon
reaching the end of the arc, the subjects returned their forearms to the beginning of the arc and maximally contracted again in the original direction, thereby testing only one function (i.e., flexion or extension) at a time. Subjects continued until the greatest torque generated by individual contractions over time was 50% of the peak torque value. defined as the maximal torque that could three individual contractions. The anof the isokinetic tests was 180 deg/sec. Algular velocity though the frequency of the isokinetic contractions was not rigidly controlled, given the angular velocity (180 deg/sec) and the limited range of motion, from 45° to 135° of elbow flexion, the frequency was approximately one contraction per second. In all of the tests, the elbow was aligned with the torque cell dynamometer or the Cybex device such that the anatomical center of elbow motion was aligned with the axes of rotation of the appropriate machine. The isometric and isokinetic tests were conducted in a random order at least 1 week apart from one another in order to minimize the effects of learning, strengthening, or fatique of one test upon another. The tests were conducted at the same time of day in order to minimize any differences in strength or endurance during different hours of the day.’ All tests and manual digitizing were performed by one of the authors (NEM) in order to minimize any differences in examiner technique and subject motivation. Peak torque
Data
Figure
1. The torque cell
dynamometer used
in the isometric
tests.
was
be maintained
over
analysis
A series of points corresponding to the torque generated and the time elapsed comprised the raw data collected on each subject for each test (Figs. 3A and 4). The data was manually digitized with a Graf/Pen Sonic Digitizer (Science Accessories Corp, Southport, CT) and analyzed with Minitab Data Analysis Software (State College, PA). On the abscissa, the torque as a function of time was recorded with the moment of peak torque count as time zero (Figs. 3B and 4). Tests were continued until time T50, the time to 50% of the peak torque value. On the ordinate, the raw torque values were converted to the percentage of peak torque so that the subjects’ torque values were directly comparable (i.e., every subject fatigued from 100% to 50%). In addition, a linear regression analysis was performed on each test, using the data points from To to TSO. The slope values of the lines so generated were defined as the &dquo;fatigue rate&dquo; (F) so that: F the change of percent peak torque per second (Figs. 3B and 4). The fatigue rates were used to assess endurance. The greater the magnitude of the negative fatigue rate (or the steeper the slope of the line), the less the endurance of a particular muscle. =
Statistical
Figure 2.
The
Cybex device used
in the isokinetic tests.
analysis
Paired t-tests and regression analyses were used to assess significant differences and relationships between fatigue rates, respectively (Table 2). The P 0.05). M, isometric; K, isokinetic; D, dominant, ND, nondominant; F, flexion; E, extension.
32) was sufficient 12 and the paired analyses appropriate for the single variable altered per teSt.8 size (N
=
were
RESULTS Table 1 summarizes the subjects’ endurance under maximum effort. For isometric function, the maximum strength could last an average of 60 to 80 seconds before it dropped to 50% of the peak values. On the other hand, for isokinetic function, the maximum strength before dropping below 50% of the peak value could last for an average of 40 to 60 seconds. Those subjects performing isokinetic, nondominant, and flexion exercises fatigued faster than those performing isometric, dominant, and extension exercises, respectively. Not all of these tendencies were statistically significant, however. In the four tests comparing the forms of contraction (isometric versus isokinetic), there were significant differences in fatigue rates in half of the otherwise identical tests and there were no significant relationships (Table 2A). Significant differences are present when the means of the fatigue rates of the two groups under comparison are different
(i.e.,
a
positive paired t-test). Significant relationships
present when
a nonhorizontal line can reasonably be drawn on a Cartesian coordinate system through a plot of the fatigue rates of the two groups under comparison (i.e., a positive regression analysis). Hence, the hypothesis of this study that there is a direct relationship between isometric and isokinetic endurance testing of the forearm complex are
could not be confirmed. In the four tests
comparing the upper extremity tested nondominant), there were no significant differences in fatigue rates between otherwise identical tests. Also, the relationship between dominant and nondominant fatigue rates was significant in three of the tests (isometric flexion, isometric extension, and isokinetic extension), and approached significance in the fourth test (P < 0.1 for isokinetic flexion) (Table 2B). These results support the assumption that in the assessment of a single diseased upper extremity, the best available comparison is the contralateral healthy upper extremity. To further assess the relationship between dominant and nondominant endurance, the mean ratios of the time elapsed in reaching 50% of peak torque in the tests comparing dominant and nondominant exercises (dominant/nondominant ratios) were determined, and they all approximate 1.0 (range, 1.06 to 1.17); however, the confidence intervals of (dominant
versus
the dominant/nondominant ratios are relatively wide, as much as 30% (Table 3). Hence, although the time to fatigue on the dominant side approximates that time on the nondominant side, a fairly large difference between the two sides can be seen in healthy subjects. In the four tests comparing the muscle group tested (flexion versus extension) there were significant differences in fatigue rates in all of the otherwise identical tests (Table 2C), with testing in flexion eliciting more rapid fatigue than testing in extension (Table 1). Under isometric conditions, the fatigue rate between flexion and extension were signifi-
cantly correlated. DISCUSSION The primary purpose of this study was to compare isometric and isokinetic endurance of the forearm complex. With the data collected, we were also able to compare endurance between the dominant and nondominant upper extremities and between elbow flexion and extension. Endurance has generally been studied to define the level of muscle contraction so that the effort could last for a given period of time. In this study, we defined endurance as the length of time in which a subject, under maximum effort, could maintain a level of torque at or above 50% of his peak torque in either a continual isometric or a repetitive isokinetic maximal effort. Such a definition implies a study of anaerobic, nonoxidative metabolism and the action of Type II (fast twitch) muscle fibers.5This is an accepted definition of endurance.’,&dquo; As a general rule, subjects performing isokinetic, nondominant, and flexion exercises fatigued faster than those performing isometric, dominant, and extension exercises, respectively (Table 1). Not all of these tendencies were statistically significant, however. TABLE 3 Ratios and their confidence intervals of the time elapsed in reaching 50% of peak torque in the four tests comparing dominant and nondominant exercises&dquo; a
a D, dominant; ND, nondominant.
111
Recent studies6 10 have shown correlations between isometric and isokinetic strength tests. However, the correlation between isometric and isokinetic endurance tests has not been previously reported. In the tests comparing the form of contraction (isometric versus isokinetic) (Table 2A), there were no significant relationships in fatigue rates between otherwise identical tests. Hence, our hypothesis of a direct relationship between isometric and isokinetic endurance of the forearm complex could not be confirmed. A primary difference between isometric and isokinetic exercise concerns the production of work. Recall that work F X d). In isometric exercise, d force X distance (W 0; so there is no work production. In isokinetic exercise, d > 0; so work is produced. We postulate that there is a distinct difference in the physiologic mechanisms of endurance between exercises that produce work and those that do not, and that this difference accounts for the difference between isometric and isokinetic endurance. Nonetheless, the precise mechanisms that account for these differences remain ob=
=
=
scure.
A recent study has shown that there are significant differences in isometric strength between dominant and nondominant upper extremities. However, the correlation between dominant and nondominant endurance has not been previously reported. In the tests comparing the upper extremity tested (dominant versus nondominant) (Table 2B), there were no significant differences in fatigue rates between otherwise identical tests, and there were several significant relationships. These results support the assumption that in the assessment of the endurance of a single diseased upper extremity, the best available comparison is the contralateral healthy upper extremity. The mean ratios of the time elapsed in reaching 50% of peak torque in the test comparing dominant and nondominant exercises (dominant/nondominant ratios) all approximate 1.0 (range, 1.06 to 1.17); however, the confidence intervals of the dominant/nondominant ratios are relatively wide, as much as 30% (Table 3). Hence, although the time to fatigue on the dominant side approximates that time on the nondominant side, a fairly large difference between the two sides can be seen in healthy subjects. This is similar to the findings of a recent study which showed small but significant differences between the dominant and nondominant sides of subjects performing isometric flexion and extension strength exercises of the forearm complex.’ In the tests comparing the muscle group tested (flexion versus extension) (Table 2C), there were significant differences in fatigue rates between all of the otherwise identical tests. Given that the magnitude of the fatigue rates of flexion was in each case greater than that of extension (Table 1), this is strong evidence that healthy males in the age range tested have greater endurance in elbow extension than in flexion.
We postulate that this finding depends upon the differin the cumulative muscle fiber composition of the muscles involved in elbow flexion as compared to those involved in extension. The muscle contractions tested in this study (i.e., either a continual isometric or a repetitive isokinetic maximal effort) principally depended upon anaerobic metabolism and Type II, fast twitch muscle fibers. Thus, a muscle group with a higher proportion of Type II muscle fibers might be expected to have greater endurance according to our protocol. We could find no evidence in the literature, however, that shows there is any systematic difference in the muscle fiber composition between these two muscle groups.3, 4,9 In summary, the data presented indicate that isometric endurance of the forearm complex cannot be used as a predictor of isokinetic endurance and there is no significant relationship between the two. The data collected support the assumption that in the assessment of the endurance of a single diseased upper extremity, the best available control is the contralateral side. The data also indicate that healthy subjects have greater endurance in elbow extension than in flexion. ence
ACKNOWLEDGMENTS This study was supported by NIH Grant AR 26287. The authors appreciate the efforts of Linda Romme, Pat Cahill, Dr. Kenton Kaufman, and the staff of the Mayo Orthopedic Biomechanics Laboratory in facilitating the progress and completion of this project.
REFERENCES 1
An KN, Morrey BF, et al Isometric elbow strength in normal individuals Clin Orthop 222 261-266, 1987 2 Burdett RG, Van Swearingen J Reliability of isokinetic muscle endurance tests J Orthop Sports Phys Ther 8 484-488, 1987 3 Clarkson PM, Kroll W, Melchionda AM. Isokinetic strength, endurance, and fiber type composition in elite American paddlers Eur J Appl Physiol 48
Askey LJ,
67-76,1982 4
Elder GCB, Bradbury K, Roberts R Variability of fiber type distributions within human muscles J Appl Physiol 53 1473-1480, 1982 5 Gollnick PD, Armstrong RB, Saubert CW, et al Enzyme activity and fiber composition in skeletal muscle of untrained and trained men. J Appl Physiol 33 312-319,1972 6 Knapik JJ, Ramos MV Isokinetic and isometric torque relationships in the human body Arch Phys Med Rehabil 61 64-67, 1980 7 McGarvey SR, Morrey BF, Askew LJ, et al. Reliability of isometric strength testing Temporal factors and strength variation Clin Orthop 185 301-
305,1984 Morrison DF Multivariate Statistical Methods Second edition New York, McGraw-Hill, 1976 9 Nygaard E, Houston M, Suzuki Y, et al Morphology of the brachial biceps muscle and elbow flexion in man Acta Physiol Scand 117 287-292, 1983 10 Otis JC, Godbold JH Relationship of isokinetic torque to isometric torque. J Orthop Res 1 165-171, 1983 11 Patton RW, Hinson MM, Arnold BR, et al Fatigue curves of isokinetic contractions Arch Phys Med Rehab 59 507-509, 1978 12. Steel RGD,Torrie JH Principles and Procedures of Statistics. A Biometrical Approach Second edition New York, McGraw-Hill, 1980 8
