Low-threshold motor unit activity during a 1-h static contraction in the trapezius muscle

Many workers, including computer workers, with repetitive or monotonous work tasks develop work-related shoulder myalgia, even from work with low muscle activity. The Cinderella hypothesis postulates that low-threshold motor units (MUs) which are first recruited also are the first ones to be at risk for selective fibre injuries in a sustained activation of the muscle due to long-duration activation. The validity of the Cinderella hypothesis demands that there exist MUs which can be active for long times. MU firing patterns were examined in the trapezius muscle on 8 healthy subjects during 60min of static isometric contraction corresponding to approximately 5% of maximal voluntary electrical activity. In 3 out of 8 subjects, MUs were found that were continuously active throughout the 60-min measurement, with the exception of a few 1-3 s interruptions coinciding with surface electromyographic dips in two of these subjects. Indications of MU substitution were also found in all subjects. Thus, it seems to be possible for some low-threshold MUs to be continuously active, while substitution occur among others. The results support the Cinderella hypothesis for fibre injuries in low-threshold MUs exposed to long-term muscle activations.