Aim: 

Upper extremities muscle-skeletal disorders are an emerging occupational syndrome with high social an economical impact, the underlying pathophysiological mechanisms being still largely unknown. Repetitive movements are a recognized risk factor. By means of non invasive techniques muscle function was monitored during a controlled low-load repetitive flexo-extension task, aimed at detecting early changes in different fatigue indices and their time course during recovery. Methods:

Sixteen subjects performed a wrist flexo-extension task: +/­ 30 deg, 1 cycle/s, peak torque=10% maximum voluntary contraction (MVC), 1-h duration divided in 10 slots of 5.5-min work + 0.5 min rest. Surface electromyogram was measured from wrist flexor and extensor muscles during the task and during static contractions (5% MVC, 20 s) performed before and after the test. Amplitude (RMS) and spectral (MNF) indices of myoelectric fatigue were computed in both conditions.

Results and conclusion: 

Wrist torque response to electrical stimulation of extensor muscles yielded the low frequency fatigue (LFF=torque at 20Hz/torque at 80Hz). LFF decreased by 29% and did not recover in the following 50 min. LFF was not correlated with changes in MNF and RMS computed in static and dynamic conditions. Low-load repetitive working tasks induce distinct and long lasting changes in both myoelectric fatigue indices and in LFF; the fact that they are not correlated reflects the occurrence of different fatiguing processes in working muscles.