Stiffness of the hamstring muscles and its relationship to function in anterior cruciate ligament deficient individuals.

The relationship between hamstring muscle stiffness and the functional ability level of 17 subjects with complete anterior cruciate ligament rupture confirmed at arthroscopy was examined. The hamstring muscles were modelled as a single degree of freedom mass spring system with a damping component. Using this model the stiffness of these muscles was examined at 30, 45, and 60% of a maximum voluntary isometric muscle action. The functional ability of the subjects, attained using the Noyes knee rating system, was then correlated to muscle stiffness measures. Positive correlations of 0.71, 0.72, and 0.62 at the three respective muscle loading levels were observed. These findings suggested that hamstring muscle stiffness may have an important role to play in the functional ability of subjects with anterior cruciate ligament deficiency. At this time there is no single effective treatment for all individuals with anterior cruciate ligament deficiency. Those individuals who undergo a conservative management programme are usually treated with hamstring muscle exercises for improving knee flexion strength. The current study provides evidence that hamstring exercises may alter other properties of muscles, such as their active stiffness, which in turn may influence the functional ability of the anterior cruciate ligament deficient individual.