The sensory origin of the sense of effort is context-dependent

Effectiveness of a home training program on improving pinch force perception in older adults

BACKGROUND

Hand function is reduced with aging which can lead to impairments in the performance of daily activities and eventually loss of independence. The ability to perceive the forces being applied to an object is an important component of hand control that also declines with age. However, the extent to which force perception can be improved through training remains largely unknown.

PURPOSE

This study evaluated the effectiveness of a home-training program focused on improving force perception in older adults.

STUDY DESIGN

Quasi-experimental - Uncontrolled trial.

METHODS

Eleven independent, healthy adults (mean age: 77.2 ± 6.8 years) participated in a home-based sensorimotor hand training program 6 days/week for 6 weeks. Force perception, the primary outcome variable, was measured as the ability to reproduce a pinch force equal to 25% maximum voluntary contraction in the absence of visual feedback using either the ipsilateral remembered or contralateral concurrent (CC) hand. We also measured hand strength, dexterity, tactile acuity, and cognition before and after training.

RESULTS

After the program was completed, participants showed a 35% reduction in absolute (p < 0.01, confidence interval (CI): [7.3, 33.2], effect sizes (ES): 0.87) and constant (p = 0.05, CI: [0.0, 34.9], ES: 0.79) force matching errors in the CC condition. Improvements in dominant hand dexterity (Purdue pegboard test) (p < 0.05, CI: [0.2, 2.4], ES: 0.60) and tactile sensitivity (JVP thresholds) (p < 0.05, CI: [-1.7, -0.1], ES: 0.94), as well as cognition (Trail Making Test B) (p < 0.05, CI: [-24,1. -1.6], ES: 0.30) were also observed post-training.

CONCLUSIONS

The results suggest that home-hand training can be an effective way to improve force perception among older adults.

Leveraging Tendon Vibration to Enhance Pseudo-Haptic Perceptions in VR

Pseudo-haptic techniques are used to modify haptic perception by appropriately changing visual feedback to body movements. Based on the knowledge that tendon vibration can affect our somatosensory perception, this article proposes a method for leveraging tendon vibration to enhance pseudo-haptics during free arm motion. Three experiments were performed to examine the impact of tendon vibration on the range and resolution of pseudo-haptics. The first experiment investigated the effect of tendon vibration on the detection threshold of the discrepancy between visual and physical motion. The results indicated that vibrations applied to the inner tendons of the wrist and elbow increased the threshold, suggesting that tendon vibration can augment the applicable visual motion gain by approximately 13% without users detecting the visual/physical discrepancy. Furthermore, the results demonstrate that tendon vibration acts as noise on haptic motion cues. The second experiment assessed the impact of tendon vibration on the resolution of pseudo-haptics by determining the just noticeable difference in pseudo-weight perception. The results suggested that the tendon vibration does not largely compromise the resolution of pseudo-haptics. The third experiment evaluated the equivalence between the weight perception triggered by tendon vibration and that by visual motion gain, that is, the point of subjective equality. The results revealed that vibration amplifies the weight perception and its effect was equivalent to that obtained using a gain of 0.64 without vibration, implying that the tendon vibration also functions as an additional haptic cue. Our results provide design guidelines and future work for enhancing pseudo-haptics with tendon vibration.

The role of clinical neurophysiology in the definition and assessment of fatigue and fatigability

Though a common symptom, fatigue is difficult to define and investigate, occurs in a wide variety of neurological and systemic disorders, with differing pathological causes. It is also often accompanied by a psychological component. As a symptom of long-term COVID-19 it has gained more attention. In this review, we begin by differentiating fatigue, a perception, from fatigability, quantifiable through biomarkers. Central and peripheral nervous system and muscle disorders associated with these are summarised. We provide a comprehensive and objective framework to help identify potential causes of fatigue and fatigability in a given disease condition. It also considers the effectiveness of neurophysiological tests as objective biomarkers for its assessment. Among these, twitch interpolation, motor cortex stimulation, electroencephalography and magnetencephalography, and readiness potentials will be described for the assessment of central fatigability, and surface and needle electromyography (EMG), single fibre EMG and nerve conduction studies for the assessment of peripheral fatigability. The purpose of this review is to guide clinicians in how to approach fatigue, and fatigability, and to suggest that neurophysiological tests may allow an understanding of their origin and interactions. In this way, their differing types and origins, and hence their possible differing treatments, may also be defined more clearly.

Influence of Proprioceptive Inputs and Force Feedback Modality on Force Reproduction Performance

The sense of force can be assessed using a force reproduction task (FRT), which consists of matching a target force with visual feedback (TARGET phase) and reproducing it without visual feedback (REPRODUCTION phase). We investigated the relevance of muscle proprioception during the TARGET phase (EXP1) and the influence of the sensory source used for the force feedback (EXP2). Accordingly, EXP1 compared the force reproduction error (RE) between trials with (LV) and without (NoLV) local tendon vibration applied on the first dorsal interosseous during the TARGET phase, while EXP2 compared RE between trials performed with visual (VISIO) or auditory (AUDIO) feedback. The FRT was performed with the index finger at 5% and 20% of the maximal force (MVC). RE was greater with LV compared with NoLV at 5% (p = 0.004) but not 20% MVC (p = 0.65). The involvement of muscle proprioception in RFT was further supported by the increase in RE with LV frequency (supplementary experiment). RE was greater for VISIO than AUDIO at 5% (p < 0.001) but not 20% MVC (p = 0.054). This study evidences the relevance of proprioceptive inputs during the target PHASE and the influence of the force feedback modality on RE, and thereby on the assessment of the sense of force.

Aging increases proprioceptive error for a broad range of movement speed and distance estimates in the upper limb

Previous work has identified age-related declines in proprioception within a narrow range of limb movements. It is unclear whether these declines are consistent across a broad range of movement characteristics that more closely represent daily living. Here we aim to characterize upper limb error in younger and older adults across a range of movement speeds and distances. The objective of this study was to determine how proprioceptive matching accuracy changes as a function of movement speed and distance, as well as understand the effects of aging on these accuracies. We used an upper limb robotic test of proprioception to vary the speed and distance of movement in two groups: younger (n = 20, 24.25 ± 3.34 years) and older adults (n = 21, 63 ± 10.74 years). The robot moved one arm and the participant was instructed to mirror-match the movement with their opposite arm. Participants matched seven different movement speeds (0.1-0.4 m/s) and five distances (7.5-17.5 cm) over 350 trials. Spatial (e.g., End Point Error) and temporal (e.g., Peak Speed Ratio) outcomes were used to quantify proprioceptive accuracy. Regardless of the speed or distance of movement, we found that older controls had significantly reduced proprioceptive matching accuracy compared to younger control participants (p ≤ 0.05). When movement speed was varied, we observed that errors in proprioceptive matching estimates of spatial and temporal measures were significantly higher for older adults for all but the slowest tested speed (0.1 m/s) for the majority of parameters. When movement distance was varied, we observed that errors in proprioceptive matching estimates were significantly higher for all distances, except for the longest distance (17.5 cm) for older adults compared to younger adults. We found that the magnitude of proprioceptive matching errors was dependent on the characteristics of the reference movement, and that these errors scaled increasingly with age. Our results suggest that aging significantly negatively impacts proprioceptive matching accuracy and that proprioceptive matching errors made by both groups lies along a continuum that depends on movement characteristics and that these errors are amplified due to the typical aging process.

What if muscle spindles were also involved in the sense of effort?

Effort perception is widely acknowledged to originate from central processes within the brain, mediated by the integration of an efference copy of motor commands in sensory areas. However, in this topical review, we aim to challenge this perspective by presenting evidence from neural mechanisms and empirical studies that suggest that reafferent signals from muscle spindles also play a significant role in effort perception. It is now imperative for future research (a) to investigate the precise mechanisms underlying the interactions between the efference copy and reafferent spindle signals in the generation of effort perception, and (b) to explore the potential for altering spindle sensitivity to affect perceived effort during ecological physical exercise and, subsequently, influence physical activity behaviours.

Pharmacological Blockade of Muscle Afferents and Perception of Effort: A Systematic Review with Meta-analysis

BACKGROUND

The perception of effort provides information on task difficulty and influences physical exercise regulation and human behavior. This perception differs from other-exercise related perceptions such as pain. There is no consensus on the role of group III/IV muscle afferents as a signal processed by the brain to generate the perception of effort.

OBJECTIVE

The aim of this meta-analysis was to investigate the effect of pharmacologically blocking muscle afferents on the perception of effort.

METHODS

Six databases were searched to identify studies measuring the ratings of perceived effort during physical exercise, with and without pharmacological blockade of muscle afferents. Articles were coded based on the operational measurement used to distinguish studies in which perception of effort was assessed specifically (effort dissociated) or as a composite experience including other exercise-related perceptions (effort not dissociated). Articles that did not provide enough information for coding were assigned to the unclear group.

RESULTS

The effort dissociated group (n = 6) demonstrated a slight increase in ratings of perceived effort with reduced muscle afferent feedback (standard mean change raw, 0.39; 95% confidence interval 0.13-0.64). The group effort not dissociated (n = 2) did not reveal conclusive results (standard mean change raw, - 0.29; 95% confidence interval - 2.39 to 1.8). The group unclear (n = 8) revealed a slight ratings of perceived effort decrease with reduced muscle afferent feedback (standard mean change raw, - 0.27; 95% confidence interval - 0.50 to - 0.04).

CONCLUSIONS

The heterogeneity in results between groups reveals that the inclusion of perceptions other than effort in its rating influences the ratings of perceived effort reported by the participants. The absence of decreased ratings of perceived effort in the effort dissociated group suggests that muscle afferent feedback is not a sensory signal for the perception of effort.

Cancer survivors post-chemotherapy exhibit unique proprioceptive deficits in proximal limbs

BACKGROUND

Oxaliplatin (OX) chemotherapy for colorectal cancer is associated with adverse neurotoxic effects that can contribute to long-term sensorimotor impairments in cancer survivors. It is often thought that the sensorimotor impairments are dominated by OX-induced dying-back sensory neuropathy that primarily affects the distal regions of the limb. Recent preclinical studies have identified encoding dysfunction of muscle proprioceptors as an alternative mechanism. Unlike the dying-back sensory neuropathy affecting distal limbs, dysfunction of muscle proprioceptors could have more widespread effects. Most investigations of chemotherapy-induced sensorimotor impairments have considered only the effects of distal changes in sensory processing; none have evaluated proximal changes or their influence on function. Our study fills this gap by evaluating the functional use of proprioception in the shoulder and elbow joints of cancer survivors post OX chemotherapy. We implemented three multidirectional sensorimotor tasks: force matching, target reaching, and postural stability tasks to evaluate various aspects of proprioception and their use. Force and kinematic data of the sensorimotor tasks were collected in 13 cancer survivors treated with OX and 13 age-matched healthy controls.

RESULTS

Cancer survivors exhibited less accuracy and precision than an age-matched control group when they had to rely only on proprioceptive information to match force, even for forces that required only torques about the shoulder. There were also small differences in the ability to maintain arm posture but no significant differences in reaching. The force deficits in cancer survivors were significantly correlated with self-reported motor dysfunction.

CONCLUSIONS

These results suggest that cancer survivors post OX chemotherapy exhibit proximal proprioceptive deficits, and that the deficits in producing accurate and precise forces are larger than those for producing unloaded movements. Current clinical assessments of chemotherapy-related sensorimotor dysfunction are largely limited to distal symptoms. Our study suggests that we also need to consider changes in proximal function. Force matching tasks similar to those used here could provide a clinically meaningful approach to quantifying OX-related movement dysfunction during and after chemotherapy.

Effects of tendon vibration and age on force reproduction task performed with wrist flexors

The sense of force is suggested to rely in part on proprioceptive inputs when assessed with a force reproduction task. The age-related alterations in proprioceptive system could, therefore, alter the sense of force. This study investigated the effects of tendon vibration on a force reproduction task performed with the wrist flexors in 18 young (20-40 year) and 18 older adults (60-90 year). Participants matched a target force (5% or 20% of their maximal force) with visual feedback of the force produced (target phase), and reproduced the target force without visual feedback (reproduction phase) after a 5-s rest period with or without vibration. The force reproduction error was expressed as the ratio between the force produced during the reproduction and the target phases. For the trials with vibration, the error was expressed as the ratio between the force produced during the reproduction phase performed with and without vibration. Tactile acuity was assessed with a two-point discrimination test. The error was greater at 5% than at 20% contraction intensity (p < 0.001), and in older [56.5 (32.2)%; mean (SD)] than in young adults [33.5 (13.6)%] at 5% (p = 0.002) but not 20% target (p = 0.46). Tendon vibration had a greater effect at 5% than 20% contraction intensity, and in older [41.7 (32.4)%, p < 0.001] than young adults [20.0 (16.1)%]. Tactile acuity was lesser in older than young adults (p < 0.001). The results support the contribution of proprioception in the sense of force, and highlight a decrease in performance with ageing restricted to low-force contractions.

Impact of Abducting at the Shoulder on Perceiving Torques about the Elbow

Literature indicates that an individual's perception of their self-generated torques is largely influenced by their descending motor commands. These studies often rely on between-limbs matching protocols, which can introduce confounding factors when interpreting results from populations with unilateral impairments. Here, we demonstrate how changes in descending motor commands impact one's perception of torques using a single-arm protocol. Thirteen participants generated and perceived 25% of their maximum voluntary torque (MVT) in elbow flexion while simultaneously abducting at their shoulder to 10%, 30%, or 50% of their MVT in shoulder abduction (MVT_SABD). Subsequently, the participants matched the elbow torque without feedback and without activating their shoulder. The accuracy in matching the elbow torque was influenced by the extent to which the shoulder abducted (p=0.002); the average error in matching elbow torques was greatest at 50% MVT_SABD (3.9 ± 4.9 Nm), followed by 30% MVT_SABD (2.1 ± 2.7 Nm), and then 10% MVT_SABD (0.0 ±2.1 Nm). These results indicate that perception of a torque about the elbow is influenced by the extent of simultaneous activation about the biomechanically-coupled shoulder. Therefore, this approach can quantify, using a single arm, the impact of changes in muscle activation on torque perception.

Sense of effort is distorted in people with chronic low back pain

BACKGROUND

Proprioceptive deficits in people with low back pain (LBP) have traditionally been attributed to altered paraspinal muscle spindle afference and its central processing. Studies conducted in the upper limb demonstrated that sense of effort is also an important source of kinaesthetic information.

OBJECTIVES

To better understand proprioceptive deficits in people with chronic LBP (cLBP), this study aimed to test whether sense of effort is affected in people with cLBP.

DESIGN

Cross-sectional study.

METHOD

Fourteen participants with cLBP and fourteen healthy participants performed a 120 s force matching task with their trunk extensor muscles at a low intensity.

RESULTS

When visual feedback of the generated force was provided, both groups performed the task accurately. Removal of visual feedback resulted in an increase in error for both groups (p < 0.0001), but the increase in error was significantly larger for the cLBP group (p = 0.023). This larger error could be attributed to undershooting of the target force (p = 0.020). The control group did not consistently undershoot or overshoot the target force (p = 0.93). Furthermore, the amount of undershooting for the cLBP group increased as the task progressed (p = 0.016), which was not observed for the control group (p = 0.80).

CONCLUSIONS

The results of this study revealed that sense of effort is affected in cLBP. People with cLBP overestimated the trunk extension force they generated, and the error increased as the trial progressed. With visual feedback however, people with cLBP were able to compensate and perform the task as accurately as people without cLBP.

Production and Perception of Intentional and Unintentional Actions

Physical approach to biological movement is based on the idea of control with referent spatial coordinates for effectors, from the whole body to single muscles. Within this framework, neural control signals induce changes in parameters of corresponding biology-specific laws of nature, and motor performance emerges as a result of interaction with the external force field. This approach is naturally compatible with the principle of abundance and the uncontrolled manifold hypothesis, which offer the framework for analysis of movement stability. The presence of two basic commands, reciprocal and co-activation, makes even single-effector tasks abundant and allows stabilizing their performance at the control level. Kinesthetic perception can be viewed as the process of estimating afferent signals within a reference system provided by the efferent process. Percepts are reflections of stable iso-perceptual manifolds in the combined afferent-efferent multi-dimensional space. This approach offers new, logical and based on laws of nature, interpretations for such phenomena as muscle co-activation, unintentional drifts in performance, and vibration-induced kinesthetic illusions. It also allows predicting new phenomena such as counter-intuitive effects of muscle co-activation of force production and perception, vibration-induced force illusions, performance drifts at two different speeds, and high variability in matching the contribution of individual elements in multi-element tasks. This approach can be developed for various subfields of movement studies including studies of athletics, movement disorders, and movement rehabilitation.

Fatiguing Neuromuscular Electrical Stimulation Decreases the Sense of Effort During Subsequent Voluntary Contractions in Men

As voluntary muscle fatigue increases, the perception of the effort required to produce a particular level of force also increases. This occurs because we produce greater neural outputs from the brain to compensate for the fatigue-induced loss of force. Muscle fatigue can also be generated following bouts of neuromuscular electrical stimulation (NMES), a technique widely used for rehabilitation and training purposes. Yet the effects of NMES-induced fatigue on the perception of effort have never been tested. In this study, we thus evaluated how electrically evoked fatigue would affect the sense of effort. For this purpose, we used two psychophysical tasks intended to assess effort perception: (i) a bilateral matching task in which subjects were asked to contract the elbow flexors of their reference and indicator arms with similar amounts of effort and (ii) a unilateral matching task in which they produced controlled levels of isometric force with their indicator arm and rated their perceived effort using the Borg CR10 scale. These tasks were performed before and after the biceps brachii of the indicator arm was submitted to a fatiguing NMES program that generated maximal force losses of 10-15%. Contrary to voluntary muscle fatigue, the sense of effort decreased post-NMES in both tasks despite increased neural outputs to the elbow flexors of the fatigued indicator arm. This shows that the relationship between motor command magnitude and effort perception was completely modified by NMES. It is proposed that NMES alters the sensory structures responsible for effort signal integration.

Influence of preceding muscle activity on movement-related cortical potential during superimposed ballistic contraction

The purpose of current study was to clarify the influence of preceding muscle activity on the force production and movement-related cortical potential (MRCP) during superimposed ballistic contractions. The participants performed the ballistic force production at 40 % of maximum voluntary contraction (MVC) using the isometric abduction force of the metacarpophalangeal joint of the index finger. They were asked to match the peak of force curve with a horizontal target line displayed on the computer monitor. We compared the MRCP amplitude during force exertion detected from Fz, C4, C3, Cz and Pz electrodes during ballistic force production with (active condition) and without (resting condition) preceding muscle activity. The results showed that the MRCP amplitudes of Fz, C4, C3 and Cz electrodes were significantly smaller for the active condition than the resting condition. This was the case even though the peak force values during both conditions were identical. This result suggests that the facilitation of spinal motoneuron excitability by preceding muscle activity could reduce the required central motor command to produce the identical force level. In addition, we examined the MRCP amplitude during ballistic force production of the active condition without a visually displayed target. In this condition, the participants had to perform the force production based on aiming point of target force level (40 %MVC). As a result, the mean of peak force without a visual target was 54 %MVC, which overshot the aiming force level. However, the MRCP amplitudes of five electrodes during the 54 %MVC force production in the active condition were equivalent to the case of the 40 %MVC force production in the resting condition. These results suggest that the MRCP amplitude is consistent with participants' sense of effort involved in the force production, rather than the actual produced force level.

Finger Force Matching and Verbal Reports: Testing Predictions of the Iso-Perceptual Manifold Concept

We used force matching and verbal reports of finger force to explore a prediction of the iso-perceptual manifold concept, which assumes that stable percepts are associated with a manifold in the afferent-efferent space. Young subjects produced various force magnitudes with the index finger, middle finger, or both fingers together. Further, they reported the force level using a verbal scale and by matching the force with fingers of the contralateral hand. Force matching, but not verbal reports, showed larger variable errors for individual fingers in the two-finger task compared to the single-finger tasks. We discuss possible differences in afferent and efferent contributions to force perception at low and high forces based on the idea of motor control with referent coordinates for the effectors. The differences between the force matching and verbal reports are possibly related to neural circuitry differences between perceiving without action and perceiving-to-act.

Laws of nature that define biological action and perception

We describe a physical approach to biological functions, with the emphasis on the motor and sensory functions. The approach assumes the existence of biology-specific laws of nature uniting salient physical variables and parameters. In contrast to movements in inanimate nature, actions are produced by changes in parameters of the corresponding laws of nature. For movements, parameters are associated with spatial referent coordinates (RCs) for the effectors. Stability of motor actions is ensured by the abundant mapping of RCs across hierarchical control levels. The sensory function is viewed as based on an interaction of efferent and afferent signals leading to an iso-perceptual manifold where percepts of salient sensory variables are stable. This approach offers novel interpretations for a variety of known neurophysiological and behavioral phenomena and makes a number of novel testable predictions. In particular, we discuss novel interpretations for the well-known phenomena of agonist-antagonist co-activation and vibration-induced illusions of both position and force. We also interpret results of several new experiments with unintentional force changes and with analysis of accuracy of perception of variables produced by elements of multi-element systems. Recently, this approach has been expanded to interpret motor disorders including spasticity and consequences of subcortical disorders (such as Parkinson's disease). We suggest that the approach can be developed for cognitive functions.

Force perceptual bias caused by muscle activity in unimanual steering

This study sought to investigate whether force perceptual bias was affected by differences in posture while steering an automobile using a psychophysical experiment to examine the relationship with muscle activity. The human perceptual characteristics of weight and force are known to be nonlinear, and a perceptual bias can occur, that is, bias that causes a perception of something that is larger or smaller than the actual scale. This is considered to be caused by physical and/or psychological conditions. Sense of effort is believed to be one influential factor. It is known to correlate with muscle activity intensity, and bias may be caused by muscle activity changes. In the current study, we hypothesized that force perceptual bias would depend on posture due to the intensity of muscle activity changes caused by changing postures during steering operation. By investigating this hypothesis, we can clarify the relationship between sense of effort and muscle activity. To investigate this issue, we conducted a psychophysical experiment to confirm postural dependence, and estimated muscle activity using a three-dimensional musculoskeletal model simulation with postural and arm force data during the experiment. In addition, prediction of bias was conducted based on a simulation in the psychophysical experiment using these data. The results revealed that bias existed, as measured by differences in postures. Additionally, a significant moderate correlation was found between the predicted bias and the actual bias, indicating the existence of a relationship between muscle activity and bias.

Accuracy of Individuals Post-hemiparetic Stroke in Matching Torques Between Arms Depends on the Arm Referenced

Background: Prior work indicates that 50–75% of individuals post-hemiparetic stroke have upper-extremity weakness and, in turn, inaccurately judge the relative torques that their arms generate during a bimanual task. Recent findings also reveal that these individuals judge the relative torques their arms generate differently depending on whether they reference their paretic vs. non-paretic arm.

Objective: Our goal was to determine whether individuals with hemiparetic stroke inaccurately matched torques between arms, regardless of the arm that they referenced.

Methods: Fifteen participants with hemiparetic stroke and 10 right-hand dominant controls matched torques between arms. Participants performed this task with their right arm referencing their left arm, and vice versa. Participants generated (1) 5 Nm and (2) 25% of their reference elbow's maximum voluntary torque (MVT) in flexion and extension using their reference arm while receiving audiovisual feedback. Then, participants matched the reference torque using their opposite arm without receiving feedback on their matching performance.

Results: Participants with stroke had greater magnitudes of error in matching torques than controls when referencing their paretic arm (p < 0.050), yet not when referencing their non-paretic arm (p > 0.050). The mean magnitude of error when participants with stroke referenced their paretic and non-paretic arm and controls referenced their dominant and non-dominant arm to generate 5 Nm in flexion was 9.4, 2.6, 4.2, and 2.5 Nm, respectively, and in extension was 5.3, 2.8, 2.5, and 2.3 Nm, respectively. However, when the torques generated at each arm were normalized by the corresponding MVT, no differences were found in matching errors regardless of the arm participants referenced (p > 0.050).

Conclusions: Results demonstrate the importance of the arm referenced, i.e., paretic vs. non-paretic, on how accurately individuals post-hemiparetic stroke judge their torques during a bimanual task. Results also indicate that individuals with hemiparetic stroke judge torques primarily based on their perceived effort. Finally, findings support the notion that training individuals post-hemiparetic stroke to accurately perceive their self-generated torques, with a focus of their non-paretic arm in relation to their paretic arm, may lead to an improved ability to perform bimanual activities of daily living.

Force perception at the shoulder after a unilateral suprascapular nerve block

There are two key sources of information that can be used to match forces-the centrally generated sense of effort and afferent signals from mechanical receptors located in peripheral tissues. There is currently no consensus on which source of information is more important for matching forces. The corollary discharge hypothesis argues that subjects match forces using the centrally generated sense of effort. The purpose of this study was to investigate force matching at the shoulder before and after a suprascapular nerve block. The nerve block creates a sensory and muscle force mismatch between sides when matching loads. The torque matching accuracy did not change after the nerve block was administered. Directionally, the torque error was in the direction proposed by the corollary discharge hypothesis. However, the mismatch between deltoid EMG was substantially greater compared to the changes in the torque matching error after the block. The results support that sensory information is used during force matching tasks. However, since the nerve block also created a sensory disruption between sides, it is not clear how sensory information is reweighted following the nerve block and a role for sense of effort is still implicated.

The Effects of Knee Joint Angle and Contractor Intensity on Perceived Exertion

This study examined knee joint angle and knee muscle contraction intensity effects on perceived exertion during isometric contractions. Fourteen healthy young adults participated in five experimental exercise sessions in which knee angles varied randomly (10°, 30°, 50°, 70°, and 90°), each separated by one week. During each session, subjects performed five isometric maximal voluntary contractions (MVCs) of knee extension, followed by nine, randomly ordered submaximal contractions (10%-90% MVC, 10% increments). The participants repeated the identical procedure for the knee flexor muscles. Immediately following each submaximal contraction, participants rated their perceived exertion using a modified Borg category-ratio scale. We found that the participants' overall ratings of perceived exertion were significantly ( p < .05) greater at the 90° than at the 70° and 10° positions during the knee extensor contractions. There were also several significant angle by contraction intensity interactions in that perceived exertion was significantly greater across 60% to 70% MVC at 30° than at 50° ( p < .01), while the opposite pattern was observed across 70% to 80% MVC ( p < .01). During knee flexor contractions, perceived exertion was significantly greater ( p < .05) at 90°, when compared with all other knee angles. There were also significant ( p < .05) angle by contraction intensity interactions between the 50° and 70° knee positions across contraction intensities of 30-40%, 40-50%, 50-60%, and 60-70% MVC. We conclude that the higher perceived exertion rating at 90° during knee extension and flexion contractions suggests different peripheral and central contributors between both muscle groups, due to differences in muscle length.