Postural fatigue of the shoulder : relationships between maximum endurance, subjective perception and electromyographic responses

The present study was undertaken to investigate issues concerned with the endurance to muscular loads created by the holding of static postures, without the presence of any other form of muscular effort. Its main aim was to explore the possibilities for the development of models which are expected to account for the capacity to endure such kind of exertion. Upright standing postures, with both arms abducted, were held by young male and female subjects (age 18-24 yr.) for as long as they could, until sensations of physical discomfort, rated on a scale with marks between 0 and 10 (Borg, 1982), became unbearable and forced them to abandon the posture. The study was constituted by two main experimental stages. In the first one, a posture as described above, with arms abducted at 60°, was used to submit to the test a model developed in 1985 by N. P. Milner (Milner's model). Although this model was originally proposed as a means to predict the remaining proportion of the maximum endurance (or 'recovery') left to the subject after a single sequence of work and rest, where work consisted of the holding of a stooped posture, its author affirmed it could be applied to any posture. The results of the testing, performed on six female subjects, demonstrated that Milner's model cannot predict with reasonable accuracy the 'recovery' for the upright standing posture with abduction of both arms. Apparently, the assumptions made by Milner concerning the relationships between the endurance capacity and the length of work and rest in a stooped posture did not apply to the test posture. The second experimental stage had three aims. The first was to test the repeatability of the endurance to standing postures with abduction of both arms. The maximum holding time for postures with arms abducted at 30°, 60° and 90° was measured on three occasions on a sample of five male and five female subjects. The maximum holding time for each of the three postures exhibited a wide variability between subjects, but when compared between the repeated measurements, the average value for the whole sample did not exhibit a significant difference. Also, male subjects had, on average, longer holding times than females, but there was a substantial overlap of the individual values. The second aim was to investigate the pattern of growth of the discomfort ratings over the length of the maximum holding time. This was found to be of a very strong linear nature, evident in all three postures studied and very similar for men and women. The strength and consistency of this relationship suggest that it may be used as a model to predict either the endurance capacity in function of the rate of growth of discomfort ratings, or the degree of discomfort that a certain length of holding will provoke. The third aim was to establish whether pure postural loads will provoke changes in the myoelectric activity which indicate the presence of heavy localised muscular fatigue. Mean power frequency (MPF) and RMS amplitude of the EMG signal were monitored throughout the 90 trials of posture holding. Significant changes were evident, with MPF decreasing and RMS amplitude increasing in most of the trials. This means that the posture, even at the lowest angle did provoke muscular fatigue. Another finding, rarely documented, was the presence of electromyographic changes that went in the reversed direction, i. e. , MPF increased whilst RMS amplitude decreased. Finally, no well-defined pattern could be established for the time course of those changes or for their relationship with the discomfort ratings.