This study analyzed forces in the tibiofemoral and patellofemoral joints during isokinetic exercise using an analytical biomechanical model. The results show that isokinetic exercise can produce large loads on these joints, especially during extension exercises. The tibiofemoral compressive force (4.0 body weight) is approximately equal to that obtained during walking but it occurs at 55 degrees of knee flexion. Anterior shear forces (resisting force to anterior drawer) exist during extension exercise at less than 40 degrees of knee flexion, with a maximum of 0.3 body weight. Posterior shear forces (resisting force to posterior drawer) exist during extension exercise at knee joint angles greater than 40 degrees and during the flexion portion of isokinetic exercise. The maximum posterior shear force is 1.7 body weight. The patellofemoral joint can encounter loads as high as 5.1 body weight which are 10 times higher than during straight leg raises. These results suggest that isokinetic exercise should be used cautiously in patients with knee lesions.





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