The following article is taken from Medicine & Science in Sports & Exercise, June 1998;30(6), pp928-32, 1998 (Vol. 01, Issue 06)
Musculoskeletal flexibility is typically characterized by maximum range of motion (ROM) in a joint or series of joints.
Resistance to passive stretch in the mid-range of motion is a function of passive mechanical restraints. However, an active contractile response may contribute to resistance at terminal ROM.
This study investigated whether maximum straight leg raise (SLR) ROM is limited by passive mechanical forces, or stretch-induced contractile responses to stretch. An instrumented hip flexion stretch was applied to the right leg of 16 subjects, ending at the point of discomfort. Torque was measured with a load cell attached to the ankle. An electrogoniometer was placed on the hip, and the knee was braced in extension. Surface electrodes were placed over the rectus and biceps femoris muscles. Straight-leg ROM was positively related to total energy absorbed and negatively related to both the increase in torque and the energy absorbed from 20-50 degrees.
Joint torque measured during passive stretch seemed to be a function of the passive mechanical restraints to joint motion, rather than a contractile response to the stretch. These findings suggest that flexibility can be explained in mechanical terms rather than by a reflex response which facilitates active contractile resistance to a slow passive stretch.
McHugh MP, Kremenic IJ, et al. (1998)