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Fitts’s law states that movement time (MT) is related to precision (index of difficulty, ID). The introduction of a new effector in a Fitts’s task induces an increase of the MT/ID slopes (Langolf et al., 1976). It is unclear, however, whether kinetics or pure coordinative constraints based on the introduction of new degrees of freedom (at the joint or muscular level) are responsible for this effect. To determine the influence of body kinetics on the MT/ID slope when pointing beyond reach, we compared pointing movements executed within (control) or beyond reach, and in this latter case with (loaded) and without (distant) an additional mass applied to the trunk. Eight subjects were required to point as fast and as accurately as possible to a target (width: 0.5, 1.0, or 2.5 cm; amplitude: 30 cm). The MT/ID slope increased when pointing beyond reach (control versus distant or loaded). This slope did not increase with an additional load applied to the trunk (distant versus loaded). Therefore, we conclude that the MT/ID slope is more likely a function of the number of degrees of freedom introduced in the task rather than a function of the kinetics constraints.

Various studies have suggested that postural sway is controlled by at least two subsystems. Rambling-Trembling analysis is a widely accepted methodology to dissociate the signals generated by these two hypothetical subsystems. The core assumption of this method is based on the equilibrium point hypothesis which suggests that the central nervous system preserves upright standing by transiently shifting the center of pressure (COP) from one equilibrium point to another. The trajectory generated by this shifting is referred to as rambling and its difference from the original COP signal is referred to as trembling. In this study we showed that these two components of COP are differentially affected when standing with external loads. Using Detrended Fluctuation analysis, we compared the pattern of these two signals in different configurations of body loading. Our findings suggest that by applying an external load, the dynamics of the trembling component is altered independently of the area of postural sway and also independently of the rambling component. The dynamics of rambling changed only during the backloading condition in which the postural sway area also substantially increased. It can be suggested that during loaded standing, the trembling mechanism (which is suggested to be activated by peripheral mechanisms and reflexes) is altered without affecting the central influence on the shifts of the equilibrium point.

Previous studies of fine motor control have focused on the ability of participants to match their grip force production to a visually provided template. We investigated differences exhibited in pinch force control during variable force production templates, including sine-, sawtooth-, and square-wave templates. Our results indicate that increased force requirements are associated with increased error rates and a noisier frequency spectrum, consistent with previous studies. Our results also indicate that visual feedback, in the form of template shape, directly affect pinch force production features and motor unit firing patterns, despite the use of consistent baseline force requirements, amplitude changes, and visual signal frequency. This suggests that CNS modulation of motor unit responses can be triggered by basic changes in visual feedback unrelated to force requirements. The potential implications of error compensation based on this study due to aging are also discussed.

The present study investigated the adaptations of specific power bursts during the combined contexts of the proximity (lead vs. trail limb) and height of an obstruction in relation to limb elevation versus progression. Ten young, adult, male subjects walked at their natural speed during unobstructed walking and the bilateral avoidance of moderate and high obstacles. Hip flexor generation power was unaffected by obstacle height for the leading limb and always delayed for the trailing limb. The knee extensor absorption power burst at toe-off was also eliminated for the trailing limb and was found to reappear in mid-swing. Few differences were seen for ankle push-off power. The results suggest that the hip joint is dedicated to limb advancement only, while the knee joint is directly involved in limb elevation and the control of multiarticular effects.

We examined the effects of unilateral muscle fatigue on the performance and coordination of grip (GF; normal component acting between the hand and object) and load force (LF; tangential component) in bimanual manipulation tasks, as well as the associated lateral differences. Eleven participants performed various symmetric bimanual tasks either without fatigue, or after fatiguing the GF producing muscles of either the nondominant or dominant hand. The GF/LF ratio of the fatigued and nonfatigued hand decreased and increased, respectively, while the neither the effects of fatigue on the task performance and GF-LF coordination, nor the lateral differences were revealed. The lack of the fatigue associated effects on most of the tested variables typically observed from unimanual tasks could be explained by bimanual assimilation. The findings also suggest that in daily life switching to bimanual tasks when one hand becomes fatigued could be beneficial regarding preserving the high level of both the manipulation performance and force coordination.

The present study investigates whether regulation of interactive torque during multijoint movements decays with advanced age as a result of declines in the motor system. Young and elderly adults repeatedly drew a circle and ovals oriented in different directions using shoulder and elbow joint movements. Each template was traced at three levels of cycling frequency with and without vision. Although vision did not affect performance, increases in cycling frequency caused distortions of movement trajectories in both groups. The pattern of distortions differed, however, between the groups. These differences were accounted for by differences in elbow control. Young adults provided regulation of elbow amplitude and timing by matching muscle torque magnitude with increased interactive torque. In contrast, elderly adults did not increase muscle torque magnitude and modulated torque timing for elbow motion regulation. This strategy is discussed as adaptation to decrements in the aging motor system.

This study evaluated the effect of context on the reaching performance of the unaffected arm and postural control while standing in patients with right cerebral vascular accidents (RCVA) and in healthy adults. Fifteen subjects with RCVA and sixteen healthy subjects performed tasks with the right hand under two conditions while standing. One condition involved moving coins forward on a table as far as possible (concrete task) and the other reaching forward without a target (abstract task). Forward reaching distance, forward displacement and lateral shift of center of pressure (CoP), and weight distribution were the dependent variables. The RCVA and control groups achieved significantly greater reaching distances in the concrete task than in the abstract one. The RCVA group showed significantly less lateral shift of the CoP and placed more weight on the affected leg in the concrete than the abstract task, whereas the control group made a greater lateral shift in the concrete task and had a similar mean ratio of weight distribution during both tasks. The results demonstrate that a functional application of task targets may favorably modulate both reaching and posture performance and exert various positive affects on postural control. Such applications may have a place in the therapeutic recovery efforts for patients afflicted with stroke.

Sensory feedback from the vestibular system and neck muscle stretch receptors is critical for the regulation of postural control. The postural relationship of the head to the trunk is a major factor determining the integration of sensory feedback and can be interfered with by varying head orientation. This study assessed how 60-s of standing with the head neutral, flexed, or extended impacted postural stability during upright stance and during forward lean in 13 healthy participants (26 ±5 years old). During both quiet upright stance and maximal forward lean, head extension increased postural center of pressure (COP) velocity and decreased the COP time-to-contact the anterior stability boundary compared with the head neutral condition. Head flexion did not differ from head neutral for either of the stance conditions. This study demonstrates that interfering with the head-trunk relationship by adopting extended, but not flexed, head orientations interferes with postural control that may impact postural stability during both quiet upright stance and maximal forward lean conditions.