A study was conducted to investigate the effects of object weight on the regulation of grip force during holding tasks using a precision grip. In addition, variations in grip force among individuals were examined. Using a force transducer-equipped grip apparatus, grip force, load force and the vertical position of the object were measured continuously while using load weights of 0.98N, 1.96N, and 2.94N under sandpaper grip surface conditions. From the recorded data, peak grip force, average static grip force, slip force, safety margin force (average static grip force-slip force), and time to stabilize the grip force from the peak grip force were evaluated.
It was found that both the slip force and safety margin force increased with object weight. The static friction coefficient, estimated from the slip force and load force, deviated from Amonton's law at a lower load force. The deviation was believed to be due to the influence of the viscoelastic nature of finger skin. An increase in safety margin force with object weight was considered to be related to the psychological reaction to the increased heaviness of the object. Indeed, in trials that included unexpected changes in object weight, the safety margin force was increased, which also seemed to be associated with the psychological reaction to uncertainty about the object's weight. A relatively large inter-subject variation was revealed for both the slip force and safety margin force.

The effects of visual information about object size on grip force programming were investigated. Fifteen subjects (26.1±7.6 yrs) repeated lifts of a cube-like grip apparatus (30×30×30 mm, 30g) using a thumb and index finger, while three boxes of different sizes but equal weight (small : 10×10×60 mm, medium : 30×30×60 mm, large : 60×60×60 mm, 25g) were pseudorandomly presented by attaching beneath the grip apparatus. Lifting tasks were performed in two visual conditions. In the full-vision condition, subjects could perceive the box size prior to the lift-off of the grip apparatus, similar to normal everyday conditions. In the obstructed-vision condition, subjects could not perceive the box size due to the placement of a screen during the initial lifting phase, and only the grip apparatus were visible over the screen. The grip apparatus measured grip and load forces during the trial and we found that the grip and load force applied to the grip apparatus in the full-vision condition significantly increased with box size regardless of equal weight. In contrast, when the size information was removed in the obstructed-vision condition, the force applied for a given box of any size was always that adequate for the largest box, suggesting that the scaling of fingertip force by utilizing size information may be achieved by reducing forces for the smaller boxes on the basis of the force output applied for the largest box, but not by increasing forces on the basis of the force output applied for the smallest box.

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.

The effects of the surface friction of a grasped object on the regulation of grip force during holding tasks using a precision grip were investigated. Using a force transducer-equipped grip apparatus, the grip force and load force acting on the object were measured continuously while surface materials (silk, wood, suede and sandpaper) and load weights (0.98N, 1.96N, 2.94N, 4.90N and 9.81N) were varied. From the recorded data, the average static grip force, slip force, safety margin force and static friction coefficient were evaluated.
It was found that both the slip force and safety margin force increased as the slipperiness of the object surface increased. Significant interactions between surface type and weight were observed in the slip force and static friction coefficient. The interaction effect resulted from the fact that the frictional relationships with the fingers changed according to both weight and surface conditions. This was considered due to the viscoelastic nature of finger skin. An increase in the safety margin force with surface slipperiness was considered due to psychological reaction, probably fear of dropping the object. Unexpected changes in surface conditions caused a greater safety margin force than trials without a surface change, which might also have been associated with psychological reaction to uncertainty of the new surface condition. A relatively large inter-subject variation was found in the slip force and safety margin force relative to slippery surfaces.

The effects of aging on adaptive force control of precision grip while manipulating a small object were compared between older (84.2±8.9 yrs, n=33) and young adults (19.1±0.24 yrs, n=18) from the following perspectives: (1) adaptation to an unfamiliar object with uncertain physical properties during 16 consecutive lifts ; (2) adaptation to an object with a non-slippery (sandpaper) surface during 12 consecutive lifts, followed by 12 consecutive lifts with a slippery (silk) surface ; and (3) adaptation to objects with different weights (0.49, 0.98, 1.96 and 2.94 N) during 24 lifts (6 consecutive lifts for each weight) .During each trial, grip and load forces were monitored. Safety margin force and slip force were evaluated from the data obtained.
The majority of older adults employed a considerably greater safety margin for an unfamiliar object in the initial trials than did young adults, while the minority of the older adults were able to adapt their safety margin force with a few trials, like the young adults. The older adults who overestimated the safety margin force, however, successfully adjusted their grip force to more optimal levels with repeated lifts, suggesting that the adaptive capability of grip force remained even at 90 years of age. The adaptation of older adults, however, was found to be slower (i. e., required more trials) than that of young adults. Upon encountering surface friction change, the safety margin forces in older adults were more strongly affected by the previous surface condition than those in the young adults. In addition, adaptation to a non-slippery surface seemed more difficult than that to a slippery surface with aging. Upon encountering weight change, older adults showed more difficulties in scaling their safety margin forces according to object weights.
Slower adaptation and difficulty in adaptation to the friction or weight change in older adults may reflect the agerelated decline of tactile sensitivity which impaired the signaling of frictional conditions and various discrete events in the hand. In addition, the lift repetition for force adaptation may possibly reflect the age-related deficit or slowing of central processing capacities related to grip force production.