Perception of finger forces within the hand after index finger fatiguing exercise

Impact of repetitive mouse clicking on forearm muscle fatigue and mouse aiming performance

Exercise induced performance fatigue has been shown to impair many aspects of fine motor function in the distal upper limb. However, most fatiguing protocols do not reflect the conditions experienced with computer use. The purpose of this study was to determine how a prolonged, low-force mouse clicking fatigue protocol impacts performance fatigue of the distal upper limb for gamers and non-gamers. Participants completed a total of 1 h of mouse clicking at 5 clicks per second. Muscle fatigue and performance were intermittently assessed. RMS amplitude increased for the forearm flexors throughout the fatigue protocol. Accuracy decreased following the first bout of clicking and returned to baseline values after 40-min. EDC and ECU displayed the greatest muscle activity while aiming, producing 11.4% and 12.9% of MVC, respectively. These findings indicate that mouse clicking may not result in performance fatigue, however, high levels of extensor activity may explain common injuries among gamers.

Recovery of pinch force sense after short-term fatigue

The aim of this study was to identify the exact origin of force sense and identify whether it arises centrally or peripherally. The present study was designed to analyze the effects of short-term fatigue on pinch force sense and the duration of these effects. During the fatigue protocol, twenty (10 men and 10 women; Mage = 22.0 years old) young Chinese participants were asked to squeeze maximally until the pinch grip force decreased to 50% of its maximal due to fatigue. Participants were instructed to produce the target force (10% of maximal voluntary isometric contraction) using the same hand before and after fatigue (immediately, 10, 30, 60, 180, 300 s). The results showed significantly higher absolute error immediately after fatigue (1.22 ± 1.06 N) than before fatigue (0.68 ± 0.34 N), and 60 s (0.76 ± 0.69 N), 180 s (0.67 ± 0.42 N), and 300 s (0.75 ± 0.37 N) after fatigue (all P < 0.05) but with no effect on the variable error (P > 0.05). It was also revealed that there was a significant overestimate of the constant error values before (0.32 ± 0.61 N) and immediately after fatigue (0.80 ± 1.38 N, all P < 0.05), while no significant overestimation or underestimation exceeded 300 s after fatigue (P > 0.05). Our study results revealed that short-term fatigue resulted in a significant decrease in force sense accuracy, but it did not affect force sense consistently; however, force sense accuracy recovered to a certain extent within 10 s and 30 s, whereas it recovered fully within 60 s, and force sense directivity improvement exceeded 300 s after fatigue. The present study shows that the sense of tension (peripherally) is also an important factor affecting force sense. Our study supports the view that the periphery is part of the origin of force sense.

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Fatigue induces long-lasting detrimental changes in motor-skill learning

Fatigue due to physical exertion is a ubiquitous phenomenon in everyday life and especially common in a range of neurological diseases. While the effect of fatigue on limiting skill execution are well known, its influence on learning new skills is unclear. This is of particular interest as it is common practice to train athletes, musicians or perform rehabilitation exercises up to and beyond a point of fatigue. In a series of experiments, we describe how muscle fatigue, defined as degradation of maximum force after exertion, impairs motor-skill learning beyond its effects on task execution. The negative effects on learning are evidenced by impaired task acquisition on subsequent practice days even in the absence of fatigue. Further, we found that this effect is in part mediated centrally and can be alleviated by altering motor cortex function. Thus, the common practice of training while, or beyond, fatigue levels should be carefully reconsidered, since this affects overall long-term skill learning.

Mastering a new movement requires practice. Intensive and repetitive training is essential for musicians, athletes, or surgeons. It is also important for people undergoing rehabilitation to help them regain normal movements after an illness or injury. Although practice is said to make perfect, there comes the point when it also causes physical fatigue. Fatigue can impair how well a person performs a movement, but its effects on learning a task are less clear.

Now, Branscheidt et al. show that being physically fatigued interferes with learning a new movement skill. In the experiments, volunteers were divided in two groups: the first group had to learn a new motor skill after their hand muscles were physically fatigued, the second group learned the same task without being worn out. The fatigued volunteers had a harder time learning a new motor task both on the day of the task and on the following days, even after they had recovered from the fatigue. The same experiment was repeated, but instead of learning a motor task, the volunteers were asked to learn a sequence of keystrokes. The volunteers in both groups learned this new thinking task easily. This suggests that learning new thinking tasks is not affected by physical fatigue.

Branscheidt et al. also disrupted memory formation in part of the brain that controls movement after volunteers finished learning the motor task using a technique called repetitive transcranial magnetic stimulation. This eliminated the motor learning deficit in the fatigued group. This may suggest that memories formed after fatigue may impair later motor learning and that physical training or rehabilitation that pushes people to work past fatigue may be counterproductive. Further study of these processes may help to develop better training regimens and rehabilitation methods.

Quantification of perceived exertion during isometric force production with the Borg scale in healthy individuals and patients with chronic stroke

BACKGROUND

The Borg Rating of Perceived Exertion (RPE) scale is widely used for many patient populations, but the ability of patients to accurately report their RPE without visual feedback has not been studied.

OBJECTIVE

The objective was to determine whether the Borg scale could be used as a measure of perceived isometric exertion in healthy subjects and patients with stroke.

METHODS

In experiment 1, young healthy subjects (n = 15) were instructed to perform spontaneous pushes (ie, self-selected effort) without visual feedback and to produce and hold self-determined isometric finger flexion at 5 Borg levels without visual feedback. In experiment 2, subjects with chronic stroke (n = 10) were instructed to produce self-determined isometric elbow flexion at 4 perceptual levels on the impaired and nonimpaired sides.

RESULTS

In experiment 1, young healthy subjects, asked to self-select without visual feedback (spontaneous push), tended to exert at a "somewhat hard" level, about 12% of maximal voluntary contraction (MVC). Self-selection of forces ranged from 2% of MVC (ie, very light) to 39% of MVC (ie, very hard). In experiment 2, subjects with stroke were able to distinguish different levels of perceived exertion among light (19% MVC), somewhat hard (33% MVC), and hard (63% MVC) levels; this ability was not different for the impaired and nonimpaired limbs.

CONCLUSION

Both healthy subjects and subjects with stroke are able to differentiate distinct levels of perceived exertion during isometric force when prompted with the Borg scale. Efforts at lower percentages of MVC are perceived by subjects with stroke as greater-than-normal Borg RPE levels.

Fatigue effect on low-frequency force fluctuations and muscular oscillations during rhythmic isometric contraction

Continuous force output containing numerous intermittent force pulses is not completely smooth. By characterizing force fluctuation properties and force pulse metrics, this study investigated adaptive changes in trajectory control, both force-generating capacity and force fluctuations, as fatigue progresses. Sixteen healthy subjects (20-24 years old) completed rhythmic isometric gripping with the non-dominant hand to volitional failure. Before and immediately following the fatigue intervention, we measured the gripping force to couple a 0.5 Hz sinusoidal target in the range of 50-100% maximal voluntary contraction. Dynamic force output was off-line decomposed into 1) an ideal force trajectory spectrally identical to the target rate; and 2) a force pulse trace pertaining to force fluctuations and error-correction attempts. The amplitude of ideal force trajectory regarding to force-generating capacity was more suppressed than that of the force pulse trace with increasing fatigue, which also shifted the force pulse trace to lower frequency bands. Multi-scale entropy analysis revealed that the complexity of the force pulse trace at high time scales increased with fatigue, contrary to the decrease in complexity of the force pulse trace at low time scales. Statistical properties of individual force pulses in the spatial and temporal domains varied with muscular fatigue, concurrent with marked suppression of gamma muscular oscillations (40-60 Hz) in the post-fatigue test. In conclusion, this study first reveals that muscular fatigue impairs the amplitude modulation of force pattern generation more than it affects the amplitude responsiveness of fine-tuning a force trajectory. Besides, motor fatigue results disadvantageously in enhancement of motor noises, simplification of short-term force-tuning strategy, and slow responsiveness to force errors, pertaining to dimensional changes in force fluctuations, scaling properties of force pulse, and muscular oscillation.

Asymmetry in grasp force matching and sense of effort

While asymmetries in upper limb force matching have been observed, the mechanisms underlying asymmetry in the sense of effort have not been conceptualized. The aim of this study was to investigate asymmetries in the perception and reproduction of grasp force. Forty-two young adults, 22 right-handed (RH) and 20 left-handed (LH), were, respectively, divided into three groups according to differences between their right and left-hand strength (left stronger than right, right stronger than left and right & left equivalent). A reference force, representing 20% of the maximal voluntary contraction (MVC) produced by the right or left hand, was matched with same hand (Ipsilateral Remembered--IR) or opposite (Contralateral Remembered--CR) hand. The matching relative error was 92% (for RH) and 46% (for LH) greater in the CR than IR condition. Asymmetries in matching were significant for RH participants only in the CR condition and were dependent on right/left differences in hand strength as shown by the constant error (CE). For this RH population, right-hand overshoot of the left-hand reference and left-hand undershoot of the right-hand reference were significant when the right hand was stronger than the left. Asymmetry remained significant when CE was normalized (%MVC). Asymmetry was reduced when the strength of each hand was equivalent or when the left hand was stronger than the right. These findings suggest that effort perception is asymmetric in RH but not in LH individuals. The hand x strength interaction indicates that asymmetry in force matching is a consequence of both a difference in the respective cortical representations and motor components, which confer a different "gain" (input-output relationship) to each system. The similarity with position sense asymmetry suggests that the gain concept may be generalized to describe some functional/performance differences between the two hand/hemisphere systems. The more symmetrical performance of the LH than RH group underlines that context specific influence of handedness, hemisphere dominance and hemispheric interactions modulate performance symmetries/asymmetries.

Finger force perception during ipsilateral and contralateral force matching tasks

The aims of the present study were to compare matching performance between ipsilateral and contralateral finger force matching tasks and to examine the effect of handedness on finger force perception. Eleven subjects were instructed to produce reference forces by an instructed finger (index-I or little-L finger) and to reproduce the same amount force by the same or a different finger within the hand (i.e., ipsilateral matching task), or by a finger of the other hand (i.e., contralateral matching task). The results of the ipsilateral and contralateral tasks in the present study commonly showed that (1) the reference and matching forces were matched closely when the two forces were produced by the same or homologous finger(s) such as I/I task; (2) the weaker little finger underestimated the magnitude of reference force of the index finger (I/L task), even with the higher level of effort (relative force), but the two forces were matched when considering total finger forces; (3) the stronger index finger closely matched the reference force of the little finger with the lower level of relative force (i.e., L/I task); (4) when considering the constant errors, I/L tasks showed an underestimation and L/I tasks showed an overestimation compared to I/I tasks. There was no handedness effect during ipsilateral tasks. During the contralateral task, the dominant hand overestimated the force of the non-dominant hand, while the non-dominant hand attempted to match the absolute force of the dominant hand. The overall results support the notion that the absolute, rather than relative, finger force is perceived and reproduced during ipsilateral and contralateral finger force matching tasks, indicating the uniqueness of finger force perception.

The effect of intervening forces on finger force perception

The purpose of the present study was to investigate the effect of intervening forces on the estimation of finger forces. To do this, we introduced intervening forces during a delayed force matching task. The basic idea in the present study was that when a reference force (or to-be-remembered force) is followed by another force, this second force (i.e., intervening force) will interfere with the estimation of the reference force. Subjects performed a modified delayed force matching task using the index finger of their dominant hand. This study consisted of eight experimental conditions which combined two reference forces (i.e., 10 and 30% MVCs) with four intervening forces (i.e., No, Half, Same and Double the reference force). The main finding of the present study was that the matching performance was systematically affected by intervening forces. The results showed that the reference force was underestimated in the condition where the intervening force was half the reference force, and overestimated in the condition where the intervening force was double the reference force. When the reference and intervening forces were the same, no intervening force effect was found. The effect of intervening force was explained by a distortion of force memory.