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

Purpose: Isokinetic strength is normally measured for a single range of mot ion and body position. This study quantified the variability, between indiv iduals, in the relationships between a single peak knee extension moment an d the isokinetic extension moments measured for different hip angles and ra nges of knee motion. Effects of hip angle, and of the starting knee angle o f the range of motion, on isokinetic knee extension strength were also dete rmined. Methods: The isokinetic knee extension strength of 10 subjects was measured at 30 degrees.s(-1) to a knee flexion angle of 10 degrees from sta rting 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 refere nce contraction. Results: Peak moments and moments at larger knee flexion a ngles 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 mome nts over the initial and final portions of the range of motion. Peak moment s were highly correlated between all hip angle-starting knee angle combinat ions. However, the normalized peak moments, the angles of peak moment, and the normalized angle-specific moments all varied considerably between subje cts. The pooled standard deviation and average coefficient of variation of the normalized angle-specific moments between subjects were 10.5% of the no rmalizing moment and 15.7%, respectively. Excluding me reference contractio n, between-subject variability was unaffected by hip angle or starting knee angle. Conclusions: Influences of hip angle, starting knee angle, and indi vidual differences on isokinetic knee extension strength must be considered to ensure that the moments obtained from isokinetic testing adequately ref lect the general strength capabilities of an individual.