Author: Man, Wai-kei
Title: Effects of muscle fatigue and load on unexpected sudden release during lifting
Degree: M.Sc.
Year: 2002
Subject: Hong Kong Polytechnic University -- Dissertations
Muscles -- Physiology
Lifting and carrying -- Physiological aspects
Department: Multi-disciplinary Studies
Jockey Club Rehabilitation Engineering Centre
Pages: xii, 99 leaves : ill. (some col.) ; 30 cm
Language: English
Abstract: The effects of sudden release and load on the response of the trunk and leg muscles during lifting were investigated in this study. Ten normal males without history of back injuries or back pain in the past 6 months were recruited. Each subject was asked to stand on a force plate and perform a series of stoop lifting trials with different loads (20, 40, 60 and 80N). The lifting height was set at the level of the individual's lateral epicondyle of femur. For each lifting trial, the subject was asked to perform six repetitions.A sudden release of load was randomly triggered at one-third of the total lifting distance of either the 3rd 4th 5th repetition using a specially designed pulley mechanism. An accelerometer was used to register the onset of the release. Electromyoelectric activities of the subjects' right latissimus dorsi, erector spinae, rectus abdominis, external oblique, internal oblique, bicep femoris, rectus femoris, gastronemius and tibialis anterior were monitored. Response times of the trunk and leg muscles due to the sudden release of load were determined and compared. It was shown that latissimus dorsi, erector spinae, biceps femoris, gastronemius and tibialis anterior mainly responded by relaxation while the rectus femoris mainly responded by contraction. Inconsistent response of the abdominal muscles: rectus abdominis, external and internal obliques were found. They were approximately 50% silent in low lifting load 20N and increase in respond frequency by either contraction of relaxation as the loading was increased. They were found to act as either to restore the disturbance of the postural stability as they contracted or enhance spinal stabilization as they relaxed after the sudden release. The muscle responses were more exaggerated by the presence of fatigue. The co-contraction period of the five pairs of agonist and antagonist were found to be more significant in high loading 80N and in fatigued condition in order to stabilize the spine. It is proposed that this transient co-contraction may constitute an overload to the lumbar spine due to the sudden release. Moreover, it was found that the excursion of centre of pressure was more proximal to the heels with increased sudden release load especially in higher loading. This suggested that the tendency of fall was more prone to occur in higher lifting loading. By means of extrapolation method, the maximum allowable limit of sudden release load was predicted to be 127.6N and 195.9N in the non-fatigue and fatigue condition. However, the larger load limit in fatigue condition was hypothesized by a self-protective mechanism of slow lifting speed to generate larger force and increasing the co-contraction of muscles. This had the adverse effect of high spinal loading. Care should be taken on the values of the load limit. It is also recommended that sudden release condition should be taken into consideration in any program of preventive measures for industrial injury.
Rights: All rights reserved
Access: restricted access

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