Variation of Force Amplitude and its Effects on Muscle Fatigue
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Current trends in industry are leaning towards specialized production systems and sedentary computer work tasks that are associated with low and less varying mechanical exposures. It has been suggested that physical variation is an effective intervention to reduce local fatigue and potentially musculoskeletal disorders. However, little is known in how the differences between physical variation patterns affect physiological and psychophysical responses. The general purpose of this thesis was to explore the biophysical effects of varying force amplitudes using forces, cycle times, and duty cycles that are relevant to occupation and longer-term health outcomes. Fifteen healthy males performed an elbow extensor sustained isometric exertion at 15% Maximum Voluntary Force (MVF), an intermittent contraction between 0% MVF and 30% MVF (On/Off), an intermittent contraction between 7.5% MVF and 22.5% MVF (MinMax), an intermittent contraction between 1% MVF and 29% MVF (1 Percent), and a sinusoidal contraction between 0% and 30% MVF (Sinusoidal). Eight commonly used measurement tools recorded biophysical responses as participants performed each condition for up to 60 minutes or until exhaustion, during 60 minutes recovery, and at 24 hours post-exercise. Measures included electromyography of the triceps muscles, mechanomyography, blood flow, heart rate, stimulated tetani and twitch responses, maximum exertions, and perceived exertion. The rate of response during exercise and comparisons between baseline, cessation, and recovery values were used to assess fatigue responses. This research shows that implementing physical variation, at the same mean amplitude, may provide reduced fatigue rate and that the magnitude and shape of the intermittent force variations affect exercise and recovery measures. Time varying forces may therefore provide the necessary mechanism to encourage beneficial physiological responses that would improve long-term health and well being of workers at low-load jobs.