Control Of Force Output Research Articles

The acute effects of spinal manipulation on neuromuscular function in asymptomatic individuals: A preliminary study

ObjectivesTo analyse the acute effects of spinal manipulation on neuromuscular function in asymptomatic individuals. DesignRandomised controlled, cross-over trial. SettingsSpinal manipulation (SM) is used as therapeutic modality for various neuromuscular disorders and also in sport with asymptomatic individuals to improve range of motion and/or facilitate motor control. Experimental evidence of its effectiveness is lacking. Participants27 asymptomatic participants (15 males and 12 females) [age (mean ± standard deviation) 24 ± 3 years] were exposed to three separate treatments in random order: 1) Spinal Manipulation of the lumbar spine (MAN); 2) Stretching of the Lumbar spine (STR); 3) sham manipulation (SHA). Main outcome measuresBefore (PRE), after (POST) and 15 min after (15_MIN) each treatment, the participants were asked to perform three tasks always in the same order: 1) force fluctuation task; 2) Modified Sörensen’s test; 3) sit and reach. Surface EMG was recorded from Gastrocnemius medialis and Erector Spinae muscles using linear arrays during task 1 and 2. ResultsMAN was not shown to determine improvements superior to other treatments in the control of force output and sEMG parameters. ConclusionsStudies with larger populations are needed in order to ascertain the effectiveness of SM on neuromuscular function.

Cutaneous mechanisms of isometric ankle force control

The sense of force is critical in the control of movement and posture. Multiple factors influence our perception of exerted force, including inputs from cutaneous afferents, muscle afferents and central commands. Here, we studied the influence of cutaneous feedback on the control of ankle force output. We used repetitive electrical stimulation of the superficial peroneal (foot dorsum) and medial plantar nerves (foot sole) to disrupt cutaneous afferent input in 8 healthy subjects. We measured the effects of repetitive nerve stimulation on (1) tactile thresholds, (2) performance in an ankle force-matching and (3) an ankle position-matching task. Additional force-matching experiments were done to compare the effects of transient versus continuous stimulation in 6 subjects and to determine the effects of foot anesthesia using lidocaine in another 6 subjects. The results showed that stimulation decreased cutaneous sensory function as evidenced by increased touch threshold. Absolute dorsiflexion force error increased without visual feedback during peroneal nerve stimulation. This was not a general effect of stimulation because force error did not increase during plantar nerve stimulation. The effects of transient stimulation on force error were greater when compared to continuous stimulation and lidocaine injection. Position-matching performance was unaffected by peroneal nerve or plantar nerve stimulation. Our results show that cutaneous feedback plays a role in the control of force output at the ankle joint. Understanding how the nervous system normally uses cutaneous feedback in motor control will help us identify which functional aspects are impaired in aging and neurological diseases.

Force and time gain interact to nonlinearly scale adaptive visual-motor isometric force control

This study examined the influence of force-time gain on the visual-motor control of isometric force. The spatial lengths on the computer screen representing the unit of elapsed time (time gain) and force (force gain) of the force output were compressed or extended in a crossed fashion while subjects produced index finger abduction force to a sinewave and constant force target that was 20% of maximal voluntary contraction. The results revealed a U-shaped interactive influence of force-time gain on force performance, namely a particular combination of moderate force-time gains leads to optimal force performance. The nature of the interaction between the force and time gains also differed depending on the task demand. During constant force production, the best gain at one dimension (force or time) was invariant across the other dimension (time or force), whereas during sinewave force production, the best gain at one dimension varied with the gain at the other dimension. The results support the proposition that the control of force output is organized by the interactive influence of different categories of constraints where the influence of visual information gain depends on the dynamics of the force control and the task demand. The findings also provide implications for visual gain parameter settings for adaptive force control.

Age-related differences in force variability and visual display

It is well established that older adults are more variable in their force output and that this age-related decrement is mediated by visuomotor processing. The purpose of this investigation was to examine whether the type of visual display impacts age-related differences in the control of force output. In order to address this question, young and old participants produced constant isometric force via index finger abduction to 3 force levels [5, 10, and 20% of their maximal voluntary contraction (MVC)]. Visual feedback was presented with either a compensatory or a pursuit display. A compensatory display provides visual feedback about force amplitude in relation to the criterion target whereas a pursuit display provides visual feedback about the force trajectory in relation to the criterion target and preview of the target path of the force trajectory. The magnitude of force variability was indexed with standard deviation and coefficient of variation. The structure of force output was indexed with spectral slope and approximate entropy. As expected, older adults were more variable and had more structured force output compared to younger adults. Moreover, this age-related difference in force control was paramount in pursuit displays. Overall, the findings suggest that age-related differences in force control are centrally mediated. It is proposed that older adults have deficits in visuomotor processing and this may be partly related to age-related decrements in the control of eye movements.

Are Visual Feedback Delays Responsible for Aging-Related Increases in Force Variability?

The current investigation examined if age-related differences in the control of isometric force production are related to an increase in visual motor processing time. Young and old adults produced isometric force production to a visually displayed target while visual feedback delay was manipulated over a broad range (50 to 3200 ms). The force output of the oldest age group was more variable across the range of delays, but only demonstrated enhanced time dependent structure at short delays. It is concluded that age differences in visual motor processing time contribute to decrements in both the feedback and feedforward control of force output.

Coordination and Control of Forces during Multifingered Grasping in Parkinson's Disease

In this study, we focused on how subjects with Parkinson's disease (PD) grasp and lift with five-digits of the hand. This task provided the opportunity to simultaneously examine (a) the coordination of multiple segments (i.e., digits), (b) the sequencing of multiple tasks (i.e., force development, object lift, and hold), and (c) the control of force output. We found that PD patients coordinated and controlled five-digit forces comparable to that of age-matched controls. Specifically, these groups developed and maintained similar force amplitudes and force sharing patterns across all grasping phases. In addition, PD patients demonstrated similar levels of variability both within and across trials. In the frequency domain, however, some differences were observed across groups, especially in PD patients exhibiting obvious action tremor (AT) at a single modal frequency. In these subjects (four of nine PD patients), there was a systematic disruption, i.e., a phase-shifting away from ∼0°, in-phase force synchronization patterns normally observed between digits. This disruption typically occurred at and around the AT frequency, while at many other frequencies synchronization patterns were maintained. The composite of these findings implies that although global features observed in five-digit grasping in PD patients are preserved, more subtle aspects of the coordination between digits, as revealed by frequency domain analysis, are not. These results are discussed in relation to the neural mechanisms that might underlie physiological synchronization of forces and its pathological disruption.

Cerebral activation during a simple force production task: changes in the time course of the haemodynamic response.

An event-related paradigm was used to investigate the fMRI signal from the primary motor cortex (M1) and the supplementary motor area (SMA) during isolated isometric wrist extension at five different force levels. There was only a weak trend towards increased area of activation with increased force output, but there was a force-related increase in percentage change of signal within voxels in M1 (Kendall Tc = 0.48, p < 0.01), which may indicate control of force output by variation of neural firing rate. In SMA there was a correlation between peak force output and time-to-peak of the haemodynamic response in SMA (Kendall Tc = 0.74, p < 0.0001). This unexpected finding of a task-related change in the shape of the haemodynamic response within a single brain area requires further investigation. It may indicate a slower rise time at lower perfusion rates, or may be the result of inhibitory processes in motor control.

Influence of cold shivering on fine motor control in the upper limb.

The aim of the investigation was to determine the effects of cold shivering on the accuracy of force output in distal, middle and proximal muscles of the upper limb. Test of hand grip strength, elbow flexion and shoulder flexion (each done at 10% maximal voluntary contraction for 15 s) were done under three conditions: (1) thermoneutral air (27 degrees C), a condition of thermal comfort; (2) cold air (10 degrees C), a condition eliciting an increase in tonic muscle activity; (3) and cold air (10 degrees C) with a cold drink (8 degrees C), a condition that causes visible shivering. The averaged (root mean square) electromyogram (AEMG) and mean power frequency (MPF) were measured from proximal, middle and distal arm muscles during the tests and compared. The control of force output was highly effective at thermoneutral condition for all motor tasks. During the cold air condition, all muscles were tonically active but there was no effect on accuracy of test performance. However, AEMG increased approximately 20% (P < 0.05) with respect to test performance in thermoneutral condition. During the cold air/cold drink condition, all muscles were shivering to a different extend. AEMG during test performance increased 30-150% in comparison to thermoneutral condition (P < 0.05). In this case, hand grip and elbow flexion were not adversely affected (these tests require middle and distal muscles) by cold shivering. However, the accuracy of performance of shoulder flexion was adversely affected. This is consistent with the fact that proximal muscles are more active during cold shivering.