Abstract
Summary Statement
Using a weight-bearing force control task, age-related changes in muscle action were observed in osteoarthritic subjects, however, greater activation of rectus femoris and medial hamstring muscles in the OA group compared to control indicates greater cocontraction and varied stabilisation strategies.
Introduction
Osteoarthritis (OA) is the most debilitating condition among older adults. OA is thought to be mechanically driven by altering the stabilising integrity of the joint. The main contributor to knee joint stability is that of muscular contraction. In cases where the history of a traumatic knee joint injury is not a causal factor, a change in muscle function, resulting in reduced strength and force control in believed to induce OA development and progression. Since age is also a determining factor of OA, the purpose of this study was to investigate the muscle activation patterns of young healthy adults (YC), older healthy adults (OC), and adults with OA during a standing isometric force control task.
Patients & Methods
A force matching protocol was used to evaluate muscle activation patterns of 41 YC (23.1±1.9 years of age) 18 OC (59.7±5.14 years), and 19 OA (63.5±8.1 years). Subjects stood with their leg of interest fixed to a force platform and modulated ground reaction forces while exposing equal body weight to each leg. Surface electromyography (EMG) of 8 muscles that cross the knee joint, kinetics and kinematics were recorded while subjects generated 30% of their maximal force in 12 different directions, corresponding to various combinations of medial-lateral-anterior-posterior ground reaction forces. Processed EMG was normalised to previously recorded maximum voluntary isometric contraction (MVIC) and ensemble averaged into group means for each loading direction. Muscle activation patterns were displayed in EMG polar plots and were quantified with symmetry analyses, mean activation levels (XEMG), directions (Φ), and specificity indices (SI). Group differences were tested with independent T-tests at the p<0.05 level.
Results
Muscle activation patterns were similar between groups (i.e. symmetry and Φ). However, XEMG of 7 muscles was significantly greater in both the OA and OC groups compared to YC. OA group also demonstrated significantly greater XEMG in the rectus femoris and tensor fascia lata as well as lower SI in semitendinosus hamstrings compared to OC.
Discussion/Conclusion
Our results indicate that regardless of loading direction, both OC and OA groups have greater levels of muscle co-contraction than YC. This is suggested to be an adaptive response to age-related changes in muscle strength and force control. Since individuals with OA have reduced muscle strength and force control compared to age-matched controls, our results suggest that the OA group's greater, less specific activation of knee joint muscles relative to the OC is this “stiffening” response adapted by the OA group, however, to an extent that may expose the joint to detrimental loading conditions, contributing to the progression of OA. Further investigation regarding age-related neuromuscular changes and their influence on joint loading conditions and development of OA is warranted.
