Motor unit firing pattern in the trapezius muscle during long-term computer work

INTRODUCTION Muscle pain in the shoulder/neck area and the upper extremities is a common and increasing problem among computer workers (Öberg and Åström, 2000). Hägg (1991) formulated the “Cinderella” hypothesis, proposing that the pain is caused by an overuse of low-threshold motor units (MUs). While this hypothesis is supported by laboratory studies that showed continuous activity of single MUs during e.g. long-term (60-minute) static contractions (Thorn et al., 2002), MU substitution (a shift from previously active MUs to newly recruited ones) has also been observed. It is unknown if the MU behaviour in an experiment more similar to long-term computer work may differ from that during a solely static muscle contraction. The purpose of this study was to investigate MU firing patterns in the trapezius muscle during a long-term computer work task. Specifically, are MUs continuously active during such work?