ABSTRACT
The electromyographic (EMG) activities of the back and thigh muscles while pedaling a bicycle ergometer at different load levels (300, 450, 600 and 750 kpm/min) and during walking and running at top speed up and down a staircase were investigated in children classified as physically less and more active than average. Each child underwent a battery of physical fitness tests to determine his physical fitness level relative to the national standard. Although the physiques of the inactive and active children did not differ, there were considerable differences between their back-lift, grip and knee-extension strengths, and the maximum anaerobic power, and 50-m dash performances of the two groups. The EMG data for each of the different tasks over selected periods (bicycle pedaling: 5 complete revolutions, staircase task: 5 stepping cycles) under different workload conditions were full-wave rectified and integrated (IEMG) . Under low workload conditions (ergometer tasks at 300 and 450 kpm/min and walking up and down stairs), the mean IEMG values (mIEMG) of all the muscles tested did not differ significantly in the inactive and active children. However, for all the higher workload tasks (pedaling at 600 kpm/min and running up and down stairs), the mIEMG values of the erector spinae muscles in the inactive children were significantly lower than those of the active children, and the difference increased gradually as the workload increased. This trend was even more marked when normalized mIEMG values were used. When the children ran up and down stairs at top speed, the inactive group had lower thigh, gluteus maximus and erector spinae muscle mIEMG values than the active group, and the difference between the normalized mIEMGs of the erector spinae muscles of the two groups showed a particularly strong statistical significance (P<0.01) when running both up and down stairs. As a similar trend was observed when the workload was maintained at a high level for the bicycle pedaling task, we concluded that at least part of the difference between the muscular activities of the two groups of children demonstrated when they carried out the running task was attributable to differences in the development of the muscle fibers and neuronal mechanisms of the erector spinae muscles.
ABSTRACT
The effects of“lifting height (2 cm, 6 cm, and 10 cm) ”and“lifting velocity (natural and fast) ”, and influence of reaching action on timing and manipulative forces of the precision grip were examined while lifting a test object. Five adult males volunteered as subjects for the study. Grip forces, load force (the vertical lifting force) and vertical position of the test object were measured and time derivatives of them were computed using laboratory software. It was found that lifting height and velocity exerted significant interaction effects in movement time of the object during lifting phase (T3), peak load force (PLF) and peak velocity (PVel) . Significant main effects of height and velocity were as follows. The higher the lifting height (2 cm, 6 cm, 10 cm) was, the longer T 3 as well as larger PGF, PLF, PdGF/dt, and PVeI were. For the fast velocity trials, T 2 and T 3 were shorter, and PGF, PLF, PdGF/dt, PdLF/dt, and PVeI were larger than the natural velocity condition. The influence of the lifting velocity was stronger in the height of 6, 10 cm than in the height of 2 cm. For all trials with different heights and different velocity, the force rate profiles (dGF/dt and dLF/dt) were continuous, bell-shaped and single-peaked during the loading phase. There was a parallel increase of grip force in relation to an increase of load force. To reveal the height effect further, the target height was unexpectedly changed in some trials at the moment of finger contact with the grip surface. It was found that both timing and force actions were similar to the initial target height condition followed by an apparent correction of gripping and lifting actions to reach the secondary target height.<BR>Results of this study suggest that both lifting height and velocity were important factors to determine the force actions when manipulating an object. Both grip force and load force seem to be well-programmed according to intended height and velocity conditions.<BR>When the lifting action was made following the reaching action, T 3 became shorter, PGF became less, and PVeI became longer than those without the reaching action. The reaching action thus seems to facilitate the force coordination during the lifting action.