The measurement of muscle stiffness in anterior cruciate injuries—an experiment revisited

Objective. To investigate the reported changes in angular stiffness of the knee due to the hamstrings muscles in the anterior cruciate ligament (ACL) deficient knee. Design. The flexors of the knee were modelled as springs and the model tested on patients and controls. Background. A previous study had used a similar model but showed no difference between an ACl deficient and an uninjured knee. Methods. A modification of a recently described method for measuring the stiffness of the hamstrings was used. ACL deficient patients were investigated and also normal controls. Results. We found that stiffness not only varied between patients, but also between legs, with the injured leg having significantly stiffer flexor muscles (P<0.001). We also showed that the stiffness of the knee due to the flexors is the same in normal subjects as that in the uninjured contralateral knee of the ACL injured patient. Conclusions. The measurement of the stiffness of the hamstrings following ACL injury may be a valid method of assessing patients with such injuries. Further investigation is required. Relevance Muscie stiffness may be an important factor in rehabilitation of patients with ACI injuries. Modification of muscle stiffness of the flexors of the knee following injury may speed up recovery.