Effect of object weight on self-selected lifting pattern for normal adults

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Effect of object weight on self-selected lifting pattern for normal adults

 

Author: Tong, Wai-kuen
Title: Effect of object weight on self-selected lifting pattern for normal adults
Degree: M.Sc.
Year: 1999
Subject: Lifting and carrying -- Physiological aspects
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Jockey Club Rehabilitation Engineering Centre
Pages: xii, 95 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1483496
URI: http://theses.lib.polyu.edu.hk/handle/200/2004
Abstract: Low back pain (LBP) is one of the most serious and persistent problems in occupational medicine. Epidemiological studies have shown that about 60 to 80 % of the population experience LBP at some time during their active life. Among those sufferers, quite a lot of them were not heavy material handling workers. It is hypothesized that they lift with "bad" posture repetitively. Although "correct" posture as keeping the back straight and bent the knees has been widely taught in back care workshops, there seems to have no evidence of any decrease in LBP incidence. From our daily experience, it is apparent that people prefer to flex the trunk and keep the knee straight in lifting light object while prefer to keep the trunk straight in lifting heavy object. The current study showed that "bad" lifting habit occurs when the subject lifts up an object that is neither "light" nor "heavy". It may be because people adopt the movement strategies that they commonly use for lifting light object and they may "wrongly" apply these adopted strategies in lifting objects that are neither "light" nor "heavy". It is proposed that there is an optimal lifting strategy for each object. It is also hypothesized that there exists an "inconsistent lifting pattern zone". If one tries to lift an object of weight within this zone, he will not be certain which lifting strategy should be used. If so, rapid posture adjustment will be required. Moreover, if "improper" movement strategies are frequently used, the cumulative adverse effects may lead to injury or pain. The objective of this study is to examine the effect of object weight on the self-selected lifting pattern and to document the range of "inconsistent lifting pattern zone" if it exists. In the current study, 4 dumbbells of weight 20, 40, 60, 80 N and a coin were used. 153 subjects were randomly asked to lift up one of the weights from the floor with their "preferred" lifting strategy. At the moment that the object was lifted off from the ground, the subject's posture was recorded. Parameters, including trunk inclination to vertical, bilateral knee angles, anterior bilateral shoulder and knee inclination, were measured using a video-based motion analysis system (PEAK, Peak Performance, Inc., USA). Additionally, the base of support of the initial lifting posture was classified into 4 types, namely, pelvis width, diagonal, wide base and long base in order to compare the effects of the object weight on the lifting pattern. The results showed that the subjects lifted the 0 N load with more stoop and asymmetric posture (i.e. lesser knee flexion, trunk more close to horizontal, larger bilateral shoulder and knee inclinations, and stand with more "diagonal" and "long base"). With increased object weight, the posture changes to be more towards to squat and symmetric posture (i.e. larger knee flexion, trunk more close to vertical, lesser bilateral shoulder and knee inclinations, and stand with more "pelvis width' and "wide base"). The trend of the bilateral shoulder and knee inclinations showed that the subjects tended to lift with the shortened body side with increased object weight. Furthermore, more subjects tended to use double hand to lift heavier weight. The parameters were found to become plateau when the object weight was beyond 40 N. One-way ANOVA showed that the effect of object weight on both the left knee angle and the anterior bilateral shoulder inclination were statistically significant (p < 0.05). Post hoc Tukey HSD test showed that the left knee angles of subject groups lifted 0 N and 40 N were statistically significant difference (p = 0.017). Post hoc Tukey HSD showed that the bilateral shoulder inclination of the subject groups were no statistically significant difference. (Subject groups lifted 0 N and 40 N, p = 0.055). The effect of object weight on the right knee angle (p = 0.086), trunk inclination to vertical (p = 0.062) and bilateral knee inclination (p = 0.064) were not statistically significant at 0.05 level. Chi-square analysis showed that both the number of hand used (p = 0.001) and the type of base of support (p = 0.005) were dependent on the object weight. In determining the range of the "inconsistent lifting pattern zone", there was evidence that the subjects tended to lift the relatively "heavy object" (i.e. above 40N) with strategy adopted in lifting "light object" (i.e. below 40 N). This suggests that the lower boundary of the inconsistent lifting pattern zone should be around 20 N to 40 N. However, as the maximum object weight used was only up to 80N, the upper boundary of the inconsistent lifting pattern zone is still not clear. Finally, it is suggested that there is no single universal lifting posture that is suitable for all kinds of objects. It is recommended that subjects should be more conscious for the whole lifting process when lifting object weighted within this zone, the zone is not fully documented yet and lifting process should be kept at slower speed to avoid possible cumulative adverse effect.

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