Cruciate ligament forces in the human knee during rehabilitation exercises

Objective. To determine the cruciate ligament forces occurring during typical rehabilitation exercises. Design. A combination of non-invasive measurements with mathematical modelling of the lower limb. Background. Direct measurement of ligament forces has not yet been successful in vivo in humans. A promising alternative is to calculate the forces mathematically. Methods. Sixteen subjects performed isometric and isokinetic or squat exercises while the external forces and limb kinematics were measured. Internal forces were calculated using a geometrical model of the lower limb and the “dynamically determinate one-sided constraint” analysis procedure. Results. During isokinetic/isometric extension, peak anterior cruciate ligament forces, occurring at knee angles of 35–40°, may reach 0.55× body-weight. Peak posterior cruciate ligament forces are lower and occur around 90°. During isokinetic/isometric flexion, peak posterior cruciate forces, which occur around 90°, may exceed 4× body-weight; the anterior cruciate is not loaded. During squats, the anterior cruciate is lightly loaded at knee angles up to 50°, after which the posterior cruciate is loaded. Peak posterior cruciate forces occur near the lowest point of the squat and may reach 3.5× body-weight. Conclusions. For anterior cruciate injuries, squats should be safer than isokinetic or isometric extension for quadriceps strengthening, though isokinetic or isometric flexion may safely be used for hamstrings strengthening. For posterior cruciate injuries, isokinetic extension at knee angles less than 70° should be safe but isokinetic flexion and deep squats should be avoided until healing is well-advanced.