Decreased EMG inhibition following electrical stimulation over muscle tendons in myopathies

Objective: To assess the central EMG inhibitory action of tendon afferent input in muscle diseases. Methods: The EMG inhibition elicited by electrical stimulation over muscle tendons was tested in 13 healthy voluntary subjects and 8 patients who had a primary muscle disease with a mild force deficit. Electrical stimuli were delivered to the tendon of the extensor carpi radialis muscle at the wrist during tonic voluntary isometric contraction at 50% of the maximum EMG level. The EMG signal was recorded by surface electrodes over the extensor carpi radialis muscle. Results: The prestimulus background EMG level was reduced in 7 out 8 of the patients. Both groups had the same phases of EMG modulation following tendon stimulation (TE1, TI1, TE2) and their latency and amplitude did not differ significantly. Conversely, the area of TI1 was significantly larger (i.e. the inhibition decreased) in patients ([mean±SD] absolute area: CONTROLS=4.1±1.6 mVms, PATIENTS=6.9±2.9 mVms, P<0.05). Conclusions: In muscle dysfunction there are serial ‘upstream’ changes of central inhibitory systems, probably to maximize the residual muscle power of the affected muscle.