Knee strength variability between individuals across ranges of motion and hip angles

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.

The isokinetic knee extension strength of 10 subjects was measured at 30 degrees x s(-1) to a knee flexion angle of 10 degrees from starting knee angles of 90, 75, 60, 45, and 30 degrees, in both the seated and supine positions. Moments were normalized to the peak moment from a reference contraction.

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.

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.