Effect of hamstring flexibility on isometric knee flexion angle-torque relationship

The purpose of this study was to examine the relationship between hamstring flexibility and knee flexion angle-torque relationship. Hamstring flexibility was assessed in 20 subjects (10 men, 10 women) using the straight leg raise (SLR) and active knee extension (AKE) tests. Isometric knee flexion strength was measured at five knee flexion angles while subjects were seated with the test thigh flexed 40 degrees and the trunk flexed 80 degrees. Lower extremities were classified as tight or normal based on the SLR and AKE tests. Peak knee flexion torque, angle of peak torque, and angle-torque relationship were compared between flexibility groups. Peak knee flexion torque was not different between tight and normal groups (SLR P=0.82; AKE P=0.68) but occurred in greater knee flexion (shorter muscle length) in the tight group compared with the normal group (SLR P < 0.01; AKE P < 0.05). The tight group had higher torque than the normal group at the shortest muscle length tested but lower torque at longer muscle lengths (SLR P < 0.001; AKE P < 0.001). In conclusion, the angle-torque relationship was shifted to the left in less flexible hamstrings such that knee flexion torque was increased at short muscle lengths and decreased at long muscle lengths when compared with more flexible hamstrings.