Shoulder muscle co-ordination during chronic and acute experimental neck-shoulder pain. An occupational pain study

Little is known about the mechanisms leading to chronic neck-shoulder muscu loskeletal disorders (MSD). The aim of the present study was to investigate and compare motor function during controlled, low load, repetitive work to gether with chronic or acute experimental neck-shoulder pain. The clinical study was performed on workers with (n = 12) and without (n = 6) chronic ne ck-shoulder pain. In the experimental study, experimental muscle pain was i nduced in healthy subjects by intra-muscular injection of hypertonic saline into the trapezius muscle (n = 10). The assessed parameters related to mot or performance were: work task event duration, cutting forces, surface elec tromyogram (EMG) activity in four shoulder muscles, displacement of the cen tre of pressure, and arm and trunk 3D movements. For controlled cutting for ce levels, chronic and acute experimental pain provoked a series of changes : a decreased working rhythm and a protective reorganisation of muscle syne rgy (experimental study), higher EMG frequency contents which may indicate altered motor unit recruitment, and greater postural activity and a tendenc y towards increased arm and trunk movements. These pain-related changes can play a role in the development of MSD. The present clinical and experiment al study demonstrated similar interactions between motor co-ordination and neck-shoulder pain in occupational settings. We therefore suggest that this experimental model can be used to study mechanisms related to MSD. Informa tion on such modulatory processes may help in the design of new strategies aimed at reducing the development of MSD.