Grip strength, forearm muscle fatigue and the response to handgrip exercise in rheumatoid arthritis
Weakness and subjective fatigue are common features of rheumatoid arthritis (RA). However, whether there is a true increase in the fatigability of rheumatoid skeletal muscle, in which fibre atrophy has been frequently reported, is unclear. Such factors may influence the ability to respond to exercise programmes. In this work, a reliable and sensitive technique for the objective measurement of forearm muscle fatigue during sustained grip was developed, using power spectral analysis of the surface myoelectric signal (SMES).The inter-relationships between grip force (hand function) and the activity and severity of the rheumatoid disease process with muscle fatigue (defined as the decline in the median frequency of the SMES with work, (MDFG)) and the initial median frequency of the SMES (IMF) were examined. It has been previously suggested that the IMF of the SMES may reflect the fibre type of the underlying muscle. The response to a 12-week progressive right hand grip strengthening programme in healthy females and those with RA was also evaluated. Potential predictors of outcome and the mechanisms of strength gain were examined. Forearm muscle fatigue in RA was not significantly greater than in healthy controls. However, higher levels of fatigue were associated with greater systemic disease activity and greater disease severity. The IMF of the SMES was shown to be stable over a wide range of grip forces for a given individual. It was significantly elevated in rheumatoid subjects, and showed a direct association with greater disease severity. Handgrip exercise was highly effective in improving hand function in females with RA. Strength gains were also demonstrated in healthy controls. Subjects with more severe disease and greater IMF of the SMES showed the greatest improvement in hand function. Greater systemic and local disease activity during the 12-week programme were limiting factors to improvement in grip. Local (right hand) disease activity remained stable or improved in the RA group overall, in spite of a trend towards deteriorating systemic and left handed disease activity. The two main potential mechanisms of strength gain (neural adaptation and gains in muscle mass) were assessed in both rheumatoid and healthy groups. The former was assessed by evaluation of the neuromuscular efficiency, derived from the relationship of the root mean square of the SMES at a given grip force. Gains in muscle mass were also assessed using this technique and by volumetric analysis of forearm musculature using magnetic resonance imaging. Although significant gains in muscle mass were demonstrated in the control group, no such gains were seen in the rheumatoid subjects. This indicates that neural adaptation was an effective method of strength gain in the rheumatoid group.