Limb Mass And Passive Muscle Tension Contributions To Knee Flexion Torque At Long Muscle Lengths

Abstract

PURPOSE: During isokinetic and isometric strength testing it is important to correct torque for the contribution of limb mass and passive muscle tension. This is particularly important in testing hamstring strength since weakness at long muscle lengths is a factor in recurrent hamstring strains. The purpose of this study was to quantify the relative contributions of passive muscle tension and limb mass during strength testing with the hamstrings in a stretched position. METHODS: Isometric strength tests were performed on 50 athletes (30 men, 20 women) prior to return to sport after a hamstring strain. Strength was assessed at 80o, 60o, 40o, and 20o knee flexion in sitting with the test thigh flexed 40o above horizontal and the seat back at 90o to the horizontal. In this set up the leg was horizontal at 50o knee flexion. Passive torque (limb mass + passive hamstring tension) was recorded at each angle. Passive torque at 80o was defined as limb mass with no muscle tension; limb mass at 80o and 20 o were equal (30o below/above horizontal); limb mass at 60o and 40o were calculated [(passive torque at 80o/cos 30o) *cos 10o]. Torque due to passive muscle tension = measured passive torque - limb mass. RESULTS: Passive muscle tension contributed 21%, of total torque at 20o with limb mass contributing 16% and contractile torque accounting for only 63% of total torque. Angle of peak torque occurred at a significantly shorter muscle length (P<0.01) when calculated from contractile torque (62o) versus total torque (49o).Table: No title available.Conclusions: Currently “gravity correction” functions for isokinetic testing measure passive torque at only one joint angle and therefore cannot account for changing passive muscle tension from short to long muscle lengths. This invalidates isokinetic assessment of hamstring angle of peak torque or weakness at long muscle lengths where passive muscle tension is considerable.