The normalizing, 80° hip angle-90° starting knee angle combination exhibited significantly less overall between-subject variability in the normalized angle-specific moments than did other hip angle-starting knee angle combinations. Only the average standard deviations for starting knee angles of 30° were not greater (P > 0.05). However, although the normalizing combination exhibited very little variability in the neighborhood of the peak, normalizing, moment, the variability in the latter half of the range of motion approached that of the other combinations.
For these other hip angle-starting knee angle combinations, the between-subject standard deviations in the normalized angle-specific moments were essentially unaffected by hip angle or starting knee angle. Differences between hip angles in the pooled standard deviations at a given knee angle averaged only 1.6 ± 0.9% of the normalizing moment. Similarly, the range across starting knee angles of the pooled standard deviations at a given knee angle averaged 1.4 ± 0.8% of the normalizing moment. Finally, the standard deviations in the normalized angle-specific moments decreased linearly with decreasing knee angle (r2 = 0.84;P < 0.001) when pooled across all but the normalizing hip angle-starting knee angle combination.
Again excluding the normalizing hip angle-starting knee angle combination, the between-subject coefficients of variation of the normalized angle-specific moments were essentially unaffected by hip angle, although effects of the starting knee angle were apparent. Differences between hip angles in the average coefficient of variation at a given knee angle averaged only 1.4 ± 0.8%. However, coefficients of variation at knee angles within the initial 5° of the range of motion of an MVC were, in most cases, notably higher than the average coefficients at these angles for MVC started at larger knee angles. The range, across starting knee angles, of the average coefficients of variation also increased linearly (at 0.2% per degree) over the last 25° of motion. Excepting the initial 5° of motion and the normalizing hip angle-starting knee angle combination, the average coefficients of variation in the angle-specific moments varied quadratically with knee angle (r2 = 0.82;P < 0.001), with the smallest variability observed at knee angles near 53°.
For both measures of variability, the between-subject variability in the normalized angle-specific moments was much larger than the corresponding estimate of within-subject variability. The estimated within-subject standard deviation of the normalized angle-specific moments was 5.5% of the normalizing moment, giving an approximate F-ratio of between- to within-subject variance of 3.7. The average within-subject coefficient of variation in the normalized angle-specific moments was 6.0%.
In most applications of isokinetic strength testing, strength is measured over only a single range of joint motion in a single body position. The results of the present study indicate that the peak and angle-specific moments measured in testing isokinetic knee extension strength can vary significantly with differences in both the range of knee motion and the hip angle employed. In addition, the results indicate that there is considerable variability between individuals in the manner in which these isokinetic knee extension moments vary across different ranges of knee motion and hip angles. The peak knee extension moment measured at 30°·s−1 using a “standard” 80° hip angle-90° starting knee angle protocol only characterized knee extension strength for a different starting knee angle or hip angle to within a standard deviation of about 11% of the measured peak moment, with standard deviations as large as 16% observed.
Before discussing the results, several methodological issues should be addressed. First, in estimating strength variability, it was important that the moment-angle relationships analyzed be directly comparable; each should closely approximate the maximum voluntary capacity for active knee extension moment generation under the prescribed conditions. The best estimate, from the available data, of maximal moment-generating capacity at a given knee angle was the maximum moment measured at that angle. This was the rationale for selecting the maximum moment in combining corresponding trials.
To avoid inflated estimates of variability, it was also necessary to correct the moments for artifactory oscillations introduced by dynamometer accelerations and compliances (14,27). An averaging filter with a window width approximating the average oscillation period was used. As the artifacts represent inertial effects and changes in muscle force that result from oscillations of the leg’s velocity about the isokinetic velocity, they may be assumed to have no mean effect over an oscillation period. Also, based on the isometric moment-angle relationship, it is reasonable that the true isokinetic moments would vary near-linearly over the filter window in the absence of artifacts. The filtered data should, therefore, have reasonably approximated the true isokinetic moments. Although deviations of the filtered moment-angle relationships from the ideal isokinetic relationships undoubtedly existed, these deviations are expected to be of little effect in comparison to the original artifacts.
Electromyography (EMG) was not measured in this study. This is mentioned because differences in measured knee extension strength can reflect differences in actual knee extensor force-generating capacity or differences in the level of neural excitation of these muscles. The magnitude of the opposing moments generated by the knee flexors will also affect the net knee extension moments measured. Without measures of EMG, the extent to which the observed results reflected changes in voluntary neural excitation of the knee extensors and flexors could not be determined. The origin of the results can, therefore, only be hypothesized. However, the lack of EMG is inconsequential to the study’s primary purpose of quantifying variability in maximum voluntary isokinetic knee extension moments.
It was expected that isokinetic knee extension strength would be affected by the angle of the hip, through a change in the strength of the rectus femoris. Because the rectus femoris crosses both the hip and knee, its length at a given knee angle varies with hip angle. As muscle strength is length-dependent, the strength of the rectus femoris at a given knee angle must then also vary with hip angle. Large effects of hip angle on knee extension strength were not expected, however. The rectus femoris contributes only about 17% of the total isokinetic MVC knee extension moment (24). The remaining vast majority of the total extensor moment-generating capacity should have been unaffected by hip angle, as the lengths of the other knee extensors (i.e., the vasti) depend only on the knee angle.
Although effects of hip angle on isokinetic knee extension strength were indeed observed, the significant average decreases of 7–10% in the moments observed at the 0° versus the 80° hip angle were the opposite of what was expected. Theoretical models (15) and in vivo measurements (17) of rectus femoris force-length relationships would predict increases in the moment-generating capacity of the knee extensors at the 0° hip angle within the range of knee angles studied. In addition, studies of isometric and isokinetic strength have found knee extensor moments to either increase or remain unchanged with decreases in hip angle (3,5,8), although most of these studies investigated only a single knee angle or hip angles that differed considerably from those of this study.
The most likely explanation for the unexpected influence of hip angle on isokinetic knee extension strength is that subjects activated their knee extensors to a lesser extent during MVC at the 0° hip angle, causing the measured strength to significantly under-represent the actual strength capacity of the musculature. Effects of hip angle on the maximal voluntary excitation of the knee extensors have, in fact, been observed (13,26). Task familiarity may be a contributing factor, as individuals appear to be able to generate the largest rectus femoris knee moments at the lengths of its most frequent use (16). Observations that the effects of hip angle appeared to vary greatly between subjects (Fig. 2) and that knee extension in the supine position was the less familiar task are consistent with the hypothesis that the effects of hip angle reflected differences in voluntary muscle activation more than differences in muscle moment-generating capacity. Differences in knee flexor activation likely did not contribute to the observed effect, however (7).
Starting knee angle affected isokinetic knee extension strength in a manner that was predictable and consistent with the findings of earlier studies (11,12,28). The observed monotonic decrease in the peak moment as the starting knee angle decreased from 75° reflects a similar change in isometric knee extension moment-generating capacity from its peak at a knee angle near 60° (30). The submaximal moments observed over the initial 5–10° of an MVC reflect the fact that, for isokinetic contractions with a low movement threshold moment, the muscles’ neural excitation, active state (force-generating potential), and force output are all submaximal over the initial portion of the range of motion due to the finite times required for each of these processes (1,2). Finally, the inverse linear relationship between starting knee angle and the moments measured at knee angles less than 25° is consistent with the increased force depression that accompanies increased distances of shortening in skeletal muscle (6).
As the hip angle and range of knee motion employed during isokinetic testing have been shown to affect measured knee extension strength, the question arises as to whether the normal practice of characterizing strength at a given joint velocity by the peak moment measured over a single range of motion in a single body position is appropriate. The answer to this question depends on the accuracy with which the single measured peak moment characterizes the strength of the individual over the full spectrum of ranges of motion and body positions, as well as on the degree of uncertainty in the strength characteristics of the individual that is acceptable.
Variability in strength characteristics between individuals must be expected. Anatomical differences in musculotendon architecture and musculoskeletal geometry will cause variations in moment-angle behaviors between subjects. Sources of these variations include differing joint angle-muscle length relationships, differing muscle length-force capacity relationships, differing muscle force-joint moment relationships, and differences in synergist force capacity. Differing muscle fiber-type compositions will also introduce between-subject variations in the maximum rates of initial moment generation and in the muscles’ capacity for force generation at the shortening velocities associated with the isokinetic motion.
However, the potentially greatest contributors to between-subject differences in measured strength characteristics are differences in the voluntary neural excitation of muscle. Variability will result, not only from differences in excitation in performing a given task, but also from differing changes in excitation between tasks. These differences may be in the initial rate of increase of excitation, in the ability to fully activate the agonist musculature, or in the ability to sustain a voluntary maximum level of excitation. Differences in cocontraction could also be a factor.
In acknowledgment of this between-subject variability, the present study provides quantitative estimates of the errors to expect in employing a single measured peak moment to predict the strength of an individual over different ranges of motion and body positions. The reported measures of variability correspond directly to the error associated with using the group-average moment-angle behavior to predict the subjects’ strength based on their 80° hip angle-90° starting knee angle peak moments. This moment was employed as the reference value because of its common use as a measure of isokinetic knee extension strength.
The between-subject variability in the present study tended to be smaller than the comparable variability reported by Kannus and colleagues (19–21) in the relationships between angle-specific isokinetic knee extension moments and the peak moment measured for the same velocity, range of motion, and hip angle. Their standard errors of estimate averaged approximately 27% (range 12–39%) of the mean angle-specific moment, as compared with the average coefficient of variation of 16% (range 2–25%) for the angle-specific moments in the present study. The larger values in the previous studies are probably attributable to the higher velocities tested (60°·s−1 and 180°·s−1), as well as their restricting their analysis to single angles near the ends of the tested range of motion. It is possible that the differing dynamometers and signal processing methods used could also have contributed to the between-study differences.
In the present study, an isokinetic velocity of 30°·s−1 was studied. This velocity is one that is employed clinically and is among the velocities at which the largest voluntary isokinetic knee extension moments may be generated (18,29), which has the desirable effect of reducing the influence of any small random factors on the results obtained. The slow velocity also provided time for the knee extensors to attain their maximum active state during the MVC, allowing both transient and steady-state behaviors to be observed. Because between-subject variations in the rate of moment generation should significantly affect only the transient behavior, greater coefficients of variation were expected during the initial, transient portion of the MVC than during the latter, steady-state portions. This was indeed the case.
An encouraging finding of the present study was that, qualitatively, the peak knee extension moment measured over a single range of knee motion and at a single hip angle appears to be a reasonable indicator of overall knee extension strength at the tested velocity. Despite significant effects of both hip angle and starting knee angle on knee extension strength, peak extension moments were highly correlated across hip angle-starting knee angle combinations. In addition, a single “standard” peak moment characterized the angle-specific moments over different hip angle-starting knee angle combinations to within an average standard deviation of 11% of the peak moment. Although not negligible, the size of this variability indicates that the effects of hip angle and starting knee angle on strength were qualitatively similar across most subjects. These results would support the continued clinical measurement of knee extension strength over only a single range of motion in a single body position.
On the other hand, where accurate quantitative approximation of an individual’s overall knee extension strength characteristics is needed, the present results are less encouraging. The results indicated that between-subject variability in the normalized peak and angle-specific moments was essentially the same for all hip angle-starting knee angle combinations except the normalizing combination, which exhibited significantly less variability. This would imply that a single, measured, isokinetic peak moment is sufficient to characterize an individual’s overall knee extension strength characteristics at the tested velocity to within an overall, baseline level of uncertainty. However, beyond this baseline, a given measure only allows a more accurate characterization of strength for conditions near-similar to those of the measurement. As a result, improving on the between-subject variability of this study would likely require individual-specific models of knee extension strength based on moment-angle relationships measured over a number of hip angle-starting knee angle combinations. Where practical considerations limit such an approach, a general model of knee extension strength will likely exhibit average errors of approximation of at least the magnitude of the variability observed in this study.
Finally, the dependence of knee extension strength on hip angle and starting knee angle cautions against using moment-angle-velocity relationships determined from isokinetic strength testing over a single range of motion in a single body position (9,23) to estimate the percentage of an individual’s strength capabilities that are employed during another motor task. Isokinetic moments measured under steady-state voluntary maximal activation will significantly overestimate functional moment-generating capacity during transient changes in muscle activation or in the latter portion of sustained contractions. The converse is also true. In addition, strength relationships determined for one body position may significantly misrepresent the strength capacity in a different position.
In conclusion, the results of the present study indicate that the common practice of characterizing knee extension strength by the peak moment measured over a single range of isokinetic motion in a single body position is justified if one is willing to accept a considerable degree of uncertainty in inferring strength capabilities over ranges of motion and hip angles other than those tested. Significant influences of hip angle, starting knee angle, and individual differences on isokinetic knee extension strength must be considered to ensure that the moments obtained from isokinetic testing adequately reflect the general strength capabilities of an individual.
The authors thank A. Al-Johar for assisting in data collection. This study was partially funded by NIH R01AG10557 (to MDG).
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Keywords:© 2000 Lippincott Williams & Wilkins, Inc.
KNEE EXTENSION; ISOKINETIC TESTING; MOMENT-ANGLE RELATIONSHIPS