Background Current rehabilitation programs place a significant emphasis on strength training, with minimal attention paid to specifically developing muscle endurance. Yet resistance to fatigue may be of importance, as the successful performance of many functional activities requires individuals to sustain muscle activity over long periods. To date, there has been limited research into the fatigue characteristics of the quadriceps in those with OA and findings are contrasting.
Objectives The objective of this study was to investigate differences in the fatigue resistance of the vastus medialis quadriceps muscle of subjects with and without knee joint osteoarthritis. Endurance characteristics were investigated during a sub-maximal (50%) isometric contraction with a high-spatial resolution surface electromyography technique.
Methods Twenty-six subjects with unilateral knee osteoarthritis, and seventeen subjects with no known knee pathology were evaluated. All subjects performed initial tests to evaluate maximum voluntary contraction (MVC), voluntary activation levels, and true maximum force (TMF). Endurance time was assessed during an isometric quadriceps contraction at 50% of the true maximum force with the knee at 90 degrees flexion while sitting. Surface electromyography (sEMG) data was collected from the vastus medialis muscle of the quadriceps group during the endurance test with a Laplacian high spatial resolution electrode. The variables of interest were median frequency (MDF); mean power frequency (MPF); root mean square (RMS); muscle fibre conduction velocity (CV); the relative slopes of each sEMG parameter calculated over the initial thirty seconds (T30), and the percentage change in each parameter over the entire endurance test.
Results MVC tests showed that the affected leg of the group with osteoarthritis was significantly weaker (p<0.05) than the unaffected leg. Voluntary activation data showed that subjects with osteoarthritis presented with significant bilateral deficits (p<0.05). TMF data showed a significantly lower (p<0.05) true potential for force generation in the affected compared to the unaffected leg of the osteoarthritis group.
No significant differences were observed between the endurance times of the groups: affected legs mean: 83 sec, SD: 31; unaffected legs mean: 79 sec, SD: 33; control legs 67s, SD: 23. Irrespective of group, all parameters were significantly different at the beginning of the contraction compared to the end of the test. MDF, MPF and CV decreased with time, while RMS increased with time. In comparison to the unaffected leg, the affected leg showed significantly lower (p<0.05) initial values for MDF, MPF and CV. No significant differences were seen between the initial values of the unaffected legs and controls. From initial to end values, across groups, a significant difference (p<0.05) was detected in the conduction velocity between the affected (-14.3%) and un-affected (-28.3%) legs of the group with osteoarthritis.
Conclusions The endurance times and the behaviour of the EMG signals indicate an improvement in the relative fatigue resistance of the affected leg in relation to the unaffected leg of the group with knee osteoarthritis. Such findings are consistent with other literature showing increased resistance to fatigue during isometric contractions in muscles with low proportions of type-2 muscle fibres, as well as studies indicating improved relative fatigue resistance following knee injury.
Disclosure of Interest None declared