Direction Of Force Changes Research Articles

Contraction Phase and Force Differentially Change Motor Evoked Potential Recruitment Slope and Interhemispheric Inhibition in Young Versus Old.

Interhemispheric interactions are important for arm coordination and hemispheric specialization. Unilateral voluntary static contraction is known to increase bilateral corticospinal motor evoked potential (MEP) amplitude. It is unknown how increasing and decreasing contraction affect the opposite limb. Since dynamic muscle contraction is more ecologically relevant to daily activities, we studied MEP recruitment using a novel method and short interval interhemispheric inhibition (IHI) from active to resting hemisphere at 4 phases of contralateral ECR contraction: Rest, Ramp Up [increasing at 25% of maximum voluntary contraction (MVC)], Execution (tonic at 50% MVC), and Ramp Down (relaxation at 25% MVC) in 42 healthy adults. We analyzed the linear portion of resting extensor carpi radialis (ECR) MEP recruitment by stimulating at multiple intensities and comparing slopes, expressed as mV per TMS stimulation level, via linear mixed modeling. In younger participants (age ≤ 30), resting ECR MEP recruitment slopes were significantly and equally larger both at Ramp Up (slope increase = 0.047, p < 0.001) and Ramp Down (slope increase = 0.031, p < 0.001) compared to rest, despite opposite directions of force change. In contrast, Active ECR MEP recruitment slopes were larger in Ramp Down than all other phases (Rest:0.184, p < 0.001; Ramp Up:0.128, p = 0.001; Execution: p = 0.003). Older (age ≥ 60) participants’ resting MEP recruitment slope was higher than younger participants across all phases. IHI did not reduce MEP recruitment slope equally in old compared to young. In conclusion, our data indicate that MEP recruitment slope in the resting limb is affected by the homologous active limb contraction force, irrespective of the direction of force change. The active arm MEP recruitment slope, in contrast, remains relatively unaffected. Older participants had steeper MEP recruitment slopes and less interhemispheric inhibition compared to younger participants.

Force and Directional Force Modulation Effects on Accuracy and Variability in Low-Level Pinch Force Tracking

ABSTRACTThe authors investigated how varying the required low-level forces and the direction of force change affect accuracy and variability of force production in a cyclic isometric pinch force tracking task. Eighteen healthy right-handed adult volunteers performed the tracking task over 3 different force ranges. Root mean square error and coefficient of variation were higher at lower force levels and during minimum reversals compared with maximum reversals. Overall, the thumb showed greater root mean square error and coefficient of variation scores than did the index finger during maximum reversals, but not during minimum reversals. The observed impaired performance during minimum reversals might originate from history-dependent mechanisms of force production and highly coupled 2-digit performance.

Direct Corticospinal Control of Force Derivative

During simultaneous generation of static and dynamic forces, motor cortical signals only predict the dynamic components, suggesting a key role in the coding of force changes. However, such a role is obscured by uncertainties regarding the representation of dynamic force signals in corticospinal outputs. We used transcranial magnetic stimulation (TMS) of the motor cortex in humans during a task that dissociated the direction of instantaneous net force and that of force derivative. The direction of TMS-evoked force outputs was closely associated with that of the force derivative, and had no relationship with that of the net force generated simultaneously, even though the magnitude of the instantaneous net force largely exceeded that of the force derivative. This observation supports the hypothesis that during dynamic force generation, the motor cortex and the corticospinal system assume a pivotal role in coding the direction of force changes, through selective recruitment of spinal motoneurons.

Task-dependent organization of pinch grip forces

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb (F(t)) and index finger (F(i)) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant's maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F(t) = F(i) was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F(t) = F(i) coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.

Development of externally guided grip force modulation in man

We examined the development of externally guided changes of grip force with respect to force rate and direction of force change. Sixty-nine children, 3–6 years of age, and 17 adults produced increasing or decreasing isometric forces on a small cylindrical sensor using a pinch grip with visual feedback. Force changes were instructed with a visual tracking task. Ramp-like paradigms with both force increase and force decrease and two different target force rates were used (0.5 N/s, 1.25 N/s). Precision of force tracking showed clear age effects and was influenced by the required force rates and directions of force change. In adults, tracking errors were much more dependent on target force rate and direction than in children. Up to four years of age, the children tended to overshoot the target force change in a ‘jump and wait’ manner in all conditions except for fast target force decreases. Older children tended to overshoot only in the condition with slow target force decrease. Adults showed close undershooting in all conditions when following the target. Adults used either a continuous ‘following’ strategy or a ‘see and catch-up’ strategy. The distinct effects on tracking errors suggest an age-related change of strategies from a feedforward strategy with intermittent use of sensorimotor feedback towards a fairly parallel and well-integrated feedback and feedforward processing. A critical age appears to be around five to six years. We suggest that these age effects may reflect distinct developmental velocities of neuronal subpopulations of the cortex and of the cerebello-cortical connections.

The motor cortex and the coding of force.

The relation of cellular activity in the motor cortex to the direction of two-dimensional isometric force was investigated under dynamic conditions in monkeys. A task was designed so that three force variables were dissociated: the force exerted by the subject, the net force, and the change in force. Recordings of neuronal activity in the motor cortex revealed that the activity of single cells was directionally tuned and that this tuning was invariant across different directions of a bias force. Cell activity was not related to the direction of force exerted by the subject, which changed drastically as the bias force changed. In contrast, the direction of net force, the direction of force change, and the visually instructed direction all remained quite invariant and congruent and could be the directional variables, alone or in combination, to which cell activity might relate.

Influence of a diagonal postural pattern on parameters of movement and associated postural adjustment in the cat.

The aim of this study was to investigate the influence of initial posture on the latency and amplitude of a movement and the postural adjustment associated with it. The movement studied was a limb flexion induced by stimulation of the motor cortex in chronically implanted cats. Only weak stimulations were used and the motor activity was mainly isometric, which facilitated the quantification of responses, analysed by measuring vertical forces exerted at the level of each limb. The initial posture consisted of the diagonal force pattern induced by stimulation of another cortical site corresponding to a different limb. The choice of this cortical site depended on the desired direction of force changes for the initial posture, i.e. either in the same direction as those accompanying the movement or in the opposite direction. The conditioning stimulation leading to the initial posture was triggered only when the computer analysing postural parameters showed a stable and balanced posture. Within the limits of the experimental method utilized, the following results have been established: 1. The latency of onset of the test movement and its associated postural adjustment were independent of initial posture. 2. The peak forces recorded were constant for a given stimulation intensity, and independent of initial force values. This shows that the amplitudes of force variations depend on the initial values: in the case of a low initial value the force variation must be greater to reach the same extreme value as in the case of a high initial value. Thus, the same stimulus intensity can induce different motor activities depending on the initial conditions.