PAVOL, M. J., and M. D. GRABINER. Knee strength variability between individuals across ranges of motion and hip angles. Med. Sci. Sports Exerc., Vol. 32, No. 5, pp. 985–992, 2000.
Purpose: Isokinetic strength is normally measured for a single range of motion and body position. This study quantified the variability, between individuals, in the relationships between a single peak knee extension moment and the isokinetic extension moments measured for different hip angles and ranges of knee motion. Effects of hip angle, and of the starting knee angle of the range of motion, on isokinetic knee extension strength were also determined.
Methods: The isokinetic knee extension strength of 10 subjects was measured at 30°·s−1 to a knee flexion angle of 10° from starting knee angles of 90, 75, 60, 45, and 30°, in both the seated and supine positions. Moments were normalized to the peak moment from a reference contraction.
Results: Peak moments and moments at larger knee flexion angles were greater in the seated than in the supine position. The starting knee angle affected the peak moment, the angle of peak moment, and the moments over the initial and final portions of the range of motion. Peak moments were highly correlated between all hip angle-starting knee angle combinations. However, the normalized peak moments, the angles of peak moment, and the normalized angle-specific moments all varied considerably between subjects. The pooled standard deviation and average coefficient of variation of the normalized angle-specific moments between subjects were 10.5% of the normalizing moment and 15.7%, respectively. Excluding the reference contraction, between-subject variability was unaffected by hip angle or starting knee angle.
Conclusions: 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.
Peak moments measured during isokinetic contractions are the most commonly used quantitative descriptors in comparing strength between and within individuals (18). In addition, angle-specific moments measured during such isokinetic contractions may be of interest in estimating the percentage of an individual’s strength capabilities that are employed during a task (e.g., 10,25). In most applications of isokinetic strength testing, strength is measured over only a single tested range of joint motion and in a single body position. It is, therefore, implicitly assumed that the isokinetic moments so measured fully characterize the strength capacity of the individual at the tested isokinetic velocity.
However, focusing on the task of knee extension, there is evidence that the moments measured during an isokinetic maximal voluntary contraction (MVC) are affected by the range of knee motion being tested and by the movement threshold moment of the dynamometer (1,11,12,22). One would expect body position to affect isokinetic knee extension moments, as changes in hip angle affect the operating length, thus the strength, of the rectus femoris. Curiously, significant effects of hip angle on isokinetic knee extension strength have not been observed (3), although such effects have been shown theoretically (15) and observed, selectively, under isometric conditions (5,8).
The practical consequences of the aforementioned factors on isokinetic knee extension strength measurement depend on the amount of variability that they introduce into the strength measures, that is, on the degree to which the effects of these factors are inconsistent and unpredictable between individuals. A few studies to date have looked at between-subject variability in the relationships between peak and angle-specific moments for isokinetic knee extension over a single range of knee motion at a single hip angle (19–21). However, the inherent variability between individuals in the relationships between isokinetic knee extension moments measured over differing ranges of knee motion or at differing hip angles is not known. Such information is needed to evaluate the degree to which isokinetic moments measured over a single range of motion in a single body position characterize the general strength capabilities of an individual in a given direction of exertion at a joint. The greater the variability between individuals, the lesser the confidence that can be placed in a single measure of strength.
Thus, the primary purpose of this study was to quantify the between-subject variability in the relationships between a single measured peak knee extension moment and the knee extension moments measured isokinetically from different starting knee angles and at different hip angles. However, such measures of variability are only of interest if knee extension strength characteristics change with hip angle and starting knee angle. Therefore, we also characterized whether, and in what manner, the measured isokinetic knee extension moments were affected by the tested hip angle and starting knee angle.