THE ROLE OF MECHANICAL AND NEURAL RESTRAINTS TO JOINT RANGE OF MOTIONDURING PASSIVE STRETCH

Musculoskeletal flexibility is typically characterized by maximum rang e of motion (ROM) in a joint or series of joints. Resistance to passiv e stretch in the mid-range of motion is a function of the passive mech anical restraints to motion. However, an active contractile response m ay contribute resistance at terminal ROM. Purpose: The purpose of this study was to examine whether maximum straight leg raise (SLR) ROM was limited by passive mechanical forces or stretch-induced contractile r esponses to stretch. Methods: An instrumented SLR stretch was applied to the right leg of 16 subjects ending at the point of discomfort. Tor que was measured with a load cell attached to the ankle. An electrogon iometer was placed on the hip, and the knee was braced in extension. S urface electrodes were placed over the rectus and biceps femoris muscl es. Following the instrumented SLR test, maximum ROM was measured goni ometrically by a physical therapist using the standard SLR test (PT SL R ROM). Torque/ROM curves were plotted for each subject. Results: PT S LR ROM was positively related to total energy absorbed (area under the curve) (r = 0.49, P = 0.044), negatively related to the increase in t orque from 20 to 50 degrees (r = -0.81, P < 0.0001) and negatively rel ated to energy absorbed from 20 to 50 degrees (r = -0.73, P < 0.001). Minimal stretch-induced hamstring activity was elicited (3 +/- 1% MVC) , and the EMG activity was unrelated to PT SLR ROM (r = -0.06, P = 0.8 ). A combination of the increase in torque from 20 to 50 degrees and t otal energy absorbed improved the relationship to PT SLR ROM (r = 0.89 . P = 0.001). Seventy-nine percent of the variability in maximum SLR R OM could be explained by the passive mechanical response to stretch. C onclusions: These data lend support to the concept that musculoskeleta l flexibility can be explained in mechanical terms rather than by neur al theories.