Human forearm position sense after fatigue of elbow flexor muscles

After a period of eccentric exercise of elbow flexor muscles of one arm in young, adult human subjects, muscles became fatigued and damaged. Damage indicators were a fall in force, change in resting elbow angle and delayed onset of soreness. After the exercise, subjects were asked to match the forearm angle of one arm, whose position was set by the experimenter, with their other arm. Subjects matched the position of the unsupported reference arm, when this was unexercised, with a significantly more flexed position in their exercised indicator arm. Errors were in the opposite direction when the reference arm was exercised. The size of the errors correlated with the drop in force. Less consistent errors were observed when the reference arm was supported. A similar pattern of errors was seen after concentric exercise, which does not produce muscle damage. The data suggested that subjects were using as a position cue the perceived effort required to maintain a given forearm angle against the force of gravity. The fall in force from fatigue after exercise meant more effort was required to maintain a given position. That led to matching errors between the exercised and unexercised arms. It was concluded that while a role for muscle spindles in kinaesthesia cannot be excluded, detailed information about static limb position can be derived from the effort required to support the limb against the force of gravity.     

Position sense at the human forearm in the horizontal plane during loading and vibration of elbow muscles

When blindfolded subjects match the position of their forearms in the vertical plane they rely on signals coming from the periphery as well as from the central motor command. The command signal provides a positional cue from the accompanying effort sensation required to hold the arm against gravity. Here we have asked, does a centrally generated effort signal contribute to position sense in the horizontal plane, where gravity cannot play a role? Blindfolded subjects were required to match forearm position for the unloaded arm and when flexors or extensors were bearing 10%, 25% or 40% of maximum loads. Before each match the reference arm was conditioned by contracting elbow muscles while the arm was held flexed or extended. For the unloaded arm conditioning led to a consistent pattern of errors which was attributed to signals from flexor and extensor muscle spindles. When elbow muscles were loaded the errors from conditioning converged, presumably because the spindles had become coactivated through the fusimotor system during the load-bearing contraction. However, this convergence was seen only when subjects supported a static load. When they moved the load differences in errors from conditioning persisted. Muscle vibration during load bearing or moving a load did not alter the distribution of errors. It is concluded that for position sense of an unloaded arm in the horizontal plane the brain relies on signals from muscle spindles. When the arm is loaded, an additional signal of central origin contributes, but only if the load is moved.     

Effect of muscle fatigue on the sense of limb position and movement

We have recently shown that in an unsupported forearm-matching task blindfolded human subjects are able to achieve an accuracy of 2–3°. If one arm was exercised to produce significant fatigue and the matching task was repeated, it led subjects to make position-matching errors. Here that result is confirmed using fatigue from a simple weight-lifting exercise. A 30% drop in maximum voluntary force after the exercise was accompanied by a significant matching error of 1.7° in the direction of extension when the reference arm had been fatigued, and 1.9° in the direction of flexion when the indicator arm had been fatigued. We also tested the effect of fatigue on a simple movement tracking task where the reference forearm was moved into extension at a range of speeds from 10 to 50°s⁻¹. Fatigue was found not to significantly reduce the movement-tracking accuracy. In a second experiment, movement tracking was measured while one arm was vibrated. When it was the reference arm, the subject perceived the movement to be significantly faster (3.7°s⁻¹) than it actually was. When it was the indicator, it was perceived to be slower (4.6°s⁻¹). The data supports the view that muscle spindles are responsible for the sense of movement, and that this sense is not prone to the disturbance from fatigue. By contrast, the sense of position can be disturbed by muscle fatigue. It is postulated, that the sense of effort experienced by holding the arm against the force of gravity is able to provide information about the position in space of the limb and that the increased effort from fatigue produces positional errors.     

The fall in force after exercise disturbs position sense at the human forearm

We reported previously that concentric or eccentric exercise can lead to errors in human limb position sense. Our data led us to conclude that the errors, post-exercise, were not due to an altered responsiveness of the proprioceptive afferents, and we proposed that they resulted from central changes in the processing of the afferent input. However, it remained uncertain what was responsible for triggering those changes, the volume of afferent traffic during the exercise or the developing fatigue. The afferent traffic hypothesis was tested by subjects carrying out a series of 250 lightly loaded concentric contractions of elbow flexors that produced little fatigue (6 %). This did not lead to significant position errors. In a second experiment, a series of fatiguing isometric contractions, which kept movements of the muscle to a minimum, led to a 24 % fall in force and significant position errors (3°, direction of extension). In the third experiment, at 24 h after eccentric exercise, when the short-term effects of fatigue and accumulated metabolites were gone, but force was still 28 % below control values, this was accompanied by significant position errors in the direction of extension, 3.2° in the relaxed arm and 3.3° in the self-supported arm. It is concluded that it is the fall in force accompanying exercise which is responsible for disturbing limb position sense. It is suggested that the exercise effects are generated in the brain, perhaps as a result of an alteration of the body map, triggered by the fall in force.     

Changes in kinematic variables at various muscle lengths of human elbow flexors following eccentric exercise

Exercise-induced muscle damage causes a disproportionally larger drop in maximal force when measured at short versus optimal or long muscle lengths, resulting in a shift of the length (angle)-force relationship towards longer lengths. However, little attention has been given to the potential effect of this shift on the rate of force development (RFD) and isotonic function at different muscle lengths. This study examined RFD at various elbow angles and kinematic variables at two different ranges of elbow flexion, so as to include mainly the ascending (S condition) or the descending limb (L condition) of the angle-force curve, following eccentric exercise. Seven male volunteers performed an eccentric exercise protocol with the elbow flexors, which caused significant changes in indicators of muscle damage (P?相似文献   

Tongue-placed tactile biofeedback suppresses the deleterious effects of muscle fatigue on joint position sense at the ankle

Whereas the acuity of the position sense at the ankle can be disturbed by muscle fatigue, it recently also has been shown to be improved, under normal ankle neuromuscular state, through the use of an artificial tongue-placed tactile biofeedback. The underlying principle of this biofeedback consisted of supplying individuals with supplementary information about the position of their matching ankle position relative to their reference ankle position through electrotactile stimulation of the tongue. Within this context, the purpose of the present experiment was to investigate whether this biofeedback could mitigate the deleterious effect of muscle fatigue on joint position sense at the ankle. To address this objective, sixteen young healthy university students were asked to perform an active ankle-matching task in two conditions of No-fatigue and Fatigue of the ankle muscles and two conditions of No-biofeedback and Biofeedback. Measures of the overall accuracy and the variability of the positioning were determined using the absolute error and the variable error, respectively. Results showed that the availability of the biofeedback allowed the subjects to suppress the deleterious effects of muscle fatigue on joint position sense at the ankle. In the context of sensory re-weighting process, these findings suggested that the central nervous system was able to integrate and increase the relative contribution of the artificial tongue-placed tactile biofeedback to compensate for a proprioceptive degradation at the ankle.     

A comparison of critical force and electromyographic fatigue threshold for isometric muscle actions of the forearm flexors

The purpose of this study was twofold: (1) to determine if the mathematical model used for estimating the EMG_FT during cycle ergometry was applicable to isometric muscle actions; and (2) to compare the mean torque level from the CF test to that of the EMG_FT test. The CF was defined as the slope coefficient of the linear relationship between total “isometric work” (W _lim in N m s) and time to exhaustion (T _lim). The EMG_FT was defined as the y-intercept of the isometric torque versus EMG fatigue curve slope coefficient relationship. There was a significant (p < 0.05) mean difference between CF (6.6 ± 3.2 N m) and EMG_FT (10.9 ± 4.7 N m). The results of the present study suggested that, during isometric muscle actions of the forearm flexors, fatigue thresholds estimated from the W _lim versus T _lim relationship (CF) are different from those estimated from electromyographic fatigue curves (EMG_FT).     

The role of muscle proprioceptors in human limb position sense: a hypothesis

In this mini-review I have proposed that there are two kinds of position sense, one a sense of the position of one part of the body relative to another, the other a sense of the location in space of our body and its limbs. A common method used to measure position sense is to ask subjects to match with one arm the position adopted by the other. Here all of the evidence points to muscle spindles as the major proprioceptors, with cutaneous receptors acting as proprioceptors providing a supporting role. Other senses such as vision do not play a major role. The sense of localisation in space measured by pointing to the arm, rather than matching its position, I propose, is not served by proprioceptors but by exteroceptors, vision, touch and hearing. Here the afferent input is relayed to sensory areas of the brain, to address the postural schema, a cortical map of the body and limbs, specifying its size and shape. It is here that spatial location is computed. This novel interpretation of position sense as two separate entities has the advantage of proposing new, future experiments and if it is supported by the findings, it will represent an important step forward in our understanding of the central processing of spatial information.     

The effect of muscle vibration on human position sense during movements controlled by lengthening muscle contraction

Summary Muscle vibration studies suggest that during voluntary movement limb position is coded by muscle spindle information derived from the lengthening, antagonist muscle. However, these investigations have been limited to movements controlled by shortening contractions. This study further examined this property of kinesthesia during movements controlled by lengthening contraction. Subjects performed a horizontal flexion of the right forearm to a mechanical stop randomly positioned at 30, 50 and 70° from the starting position. The movement was performed against a flexor load (1 kg) requiring contraction of the triceps muscle. Vision was occluded and movements were performed under three conditions: no vibration, vibration of the right biceps and vibration of the right triceps. The perceived position of the right forearm was assessed by instructing subjects to simultaneously match the right limb position with the left limb. Vibration of the shortening biceps muscle had no effect on limb matching accuracy. However, triceps vibration resulted in significant overestimation of the vibrated limb position (10–13°). The variability in movement distance was uninfluenced by muscle vibration. During movements controlled by lengthening contraction, there is a concurrent gamma dynamic fusimotor input that would enhance primary afferent discharge. Despite this additional regulating input to the muscle spindle, it appears that muscle spindle information from the lengthening muscle is important for the accurate perception of limb movement and/or position.     

Contribution of joint and muscle afferents to position sense at the human proximal interphalangeal joint.

Experiments were carried out to examine the perceived position of the human index finger about the proximal interphalangeal joint. In protocol I, the finger was moved from an intermediate position at velocities ranging from 200 deg/min to 2 deg/min, then held still at one of three positions. The subject's task was visually to align a finger silhouette that was coaxial with the joint to the kinaesthetically perceived position of the unseen finger. Judgements of position were found to be quite accurate, and unaffected by previous velocity. Protocol II showed that although the direction of joint displacements of 0.01 and 0.1 deg could not be detected at any velocity, 1 deg could be detected at 200 deg/min and 10 deg at 20 and 2 deg/min. In protocol III the finger was moved at 2 deg/min and maintained at either 105 or 175 deg. It was found that the position of the unanaesthetized finger was quite accurately known, but with digital nerve block, subjects clearly perceived the finger to be at the mid-position (approximately 130-150 deg). This suggests that the absence of joint and cutaneous afference is interpreted by the CNS as indicating mid-position. The slight bias of the sensed position towards the objective position shown by the results also indicates that muscle afferents can provide a crude signal related to joint position. This finding was further supported by the observation that splinting the distal interphalangeal joint into flexion resulted in flexion bias in the perceived angle of the proximal interphalangeal joint. Anaesthesia of the middle finger, thumb and distal portion of the index finger (leaving proximal joint unaffected), had little effect on position matching performance, suggesting that the large error in position sense during anaesthesia of the whole finger is due to loss of afference specifically related to the proximal interphalangeal joint, and not due to loss of non-specific facilitatory influences from cutaneous and joint afferents. The results argue for an important proprioceptive role for joint afferents at finger joints.     

Strategies for muscle activation during isometric torque generation at the human elbow

1. We studied the patterns of electromyographic (EMG) activity in elbow muscles of 14 normal human subjects. The activity of five muscles that act in flexion-extension and forearm supination-pronation was simultaneously recorded during isometric voluntary torque generation, in which torques generated in a plane orthogonal to the long axis of the forearm were voluntarily coupled with torques generated about the long axis of the forearm (i.e., supination-pronation). 2. When forearm supination torques were superimposed on a background of elbow flexion torque, biceps brachii activity increased substantially, as expected; however, brachioradialis and brachialis EMG levels decreased modestly, a less predictable outcome. The pronator teres was also active during pure flexion and flexion coupled with mild supination (even though no pronation torque was required). This was presumably to offset inappropriate torque contributions of other muscles, such as the biceps brachii. 3. When forearm supination torque was superimposed on elbow extension torque, again the biceps brachii was strongly active. The pronator teres also became mildly active during extension with added pronation torque. These changes occurred despite the fact that both the pronator and biceps muscles induce elbow flexion. 4. In these same elbow extension tasks, triceps brachii activity was also modulated with both pronation or supination loads. It was most active during either supination or pronation loads, again despite the fact that it has no mechanical role in producing forearm supination-pronation torque. 5. Recordings of EMG activity during changes in forearm supination-pronation angle demonstrated that activation of the biceps brachii followed classic length-tension predictions, in that less EMG activity was required to achieve a given supination torque when the forearm was pronated (where biceps brachii is relatively longer). On the other hand, EMG activity of the pronator teres did not decrease when the pronator was lengthened. Triceps EMG was also more active when the forearm was supinated, despite its having no direct functional role in this movement. 6. Plots relating EMG activity in biceps brachii, brachialis, and brachioradialis at three different forearm positions revealed that there was a consistent positive near-linear relationship between brachialis and brachioradialis and that biceps brachii is often most active when brachioradialis and brachialis are least active. 7. We argue that, for the human elbow joint at least, fixed muscle synergies are rather uncommon and that relationships between muscle activities are situation dependent.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense

The present study investigated the effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense in the upper extremity. Twenty-four healthy subjects (12 males and 12 females) performed a 45-min standardized mouse-operated computer task on two occasions. The task consisted of painting rectangles that were presented on the screen. On one occasion, time pressure and precision demands were imposed (more demanding task, MDT), whereas, on the other occasion, no such restraints were added (less demanding task, LDT). The order of the two task versions was randomized. Tissue oxygen saturation in the trapezius and extensor carpi radialis muscles was recorded throughout, and the position-matching ability of the wrist was measured before and after the tasks. In addition, measurements of autonomic nervous system reactivity and subjective ratings of tenseness and physical fatigue were obtained. Performance was measured in terms of the number of rectangles that were painted during the task. During MDT, oxygen saturation in extensor carpi radialis decreased (P<0.05) compared to LDT. These data were paralleled by increased electrodermal activity (P<0.05), skin blood flow (P<0.05), ratings of tenseness and fatigue (P<0.01), and increased performance (P<0.01) during MDT. Females exhibited lower oxygen saturation than males, during rest as well as during the computer tasks (P<0.01). Wrist repositioning error increased following LDT as compared to MDT (P<0.05). In conclusion, computer mouse work under time pressure and precision demands caused a decrease in forearm muscle oxygenation, but did not affect wrist position sense accuracy. We attribute our changes in oxygenation more to increased oxygen consumption as a result of enhanced performance, than to vasoconstriction.Presented in part at the 49th Nordiska Arbetsmiljömötet in Savonlinna Finland, August 25–27, 2003     

The effect of quadriceps muscle fatigue on position matching at the knee

This is a report of the effects of exercise on position matching at the knee. Young adult subjects were required to step down a set of stairs (792 steps), representing eccentric-biased exercise of the quadriceps muscle, or step up them, concentric-biased exercise. Immediately after eccentric exercise subjects showed a mean force drop of 28% (± 6%, s.e.m. ) of the control value in their exercised quadriceps muscle, which was accompanied by 4.8 deg (± 0.8 deg) of error between reference and matching legs in a position matching task at the knee. Similarly concentric exercise was followed by a force drop of 15% (± 3%) and matching errors of 3.7 deg (± 0.4 deg). These effects were significant. The direction of the errors suggested that subjects perceived their exercised muscles to be longer that they actually were. This finding was not consistent with the hypothesis that the increase in effort required to support the leg after fatigue from exercise was responsible for the errors. It is hypothesized that position sense in an unsupported leg arises, in part, from operation of an internal forward model. When the motor command is increased to compensate for the effects of fatigue, the comparison between predicted and actual feedback from quadriceps leads to the impression that the muscle is longer than it actually is. The exercise effects on proprioception may have implications for sports injuries and for evaluation of the factors leading to falls in the elderly.     

Effects of body orientation, load and vibration on sensing position and movement at the human elbow joint

Experiments were carried out to study the ability of human subjects to match the position of their forearms relative to the horizontal. The normal, arms-in-front position with the hands aligned and little forward flexion at the shoulder was called the reference position. When the arms were rotated to the side, one arm was raised, or both arms were raised, matching ability deteriorated compared with the reference position, when expressed as an increase in the standard deviation of matching errors. It was concluded that particular significance was assigned by the brain to the arms-in-front position, with the hands in their normal working space. Increases in errors were also observed when the reference arm was made weightless or its weight was increased by means of an adjustable load. This suggested that lifting the arm against gravity provided additional positional information. In a second experiment, dependence of the illusion of muscle lengthening evoked by vibration was tested after two different forms of muscle conditioning, a co-contraction of elbow muscles with the arm held flexed or with it held extended. The speed of the illusory extension of flexor muscles during their vibration increased three-fold after flexion conditioning compared with extension conditioning. Since after flexion conditioning, muscle spindles in flexor muscles are expected to be more sensitive to vibration than after extension conditioning, this observation provides additional support for the view that muscle spindles make an important contribution to kinaesthesia at the elbow joint.     

The effect of agonist/antagonist muscle vibration on human position sense

Summary During voluntary movement, muscle spindles of both the agonist and antagonist muscles potentially can supply information about position of the limb. Muscle vibration is known to increase muscle spindle discharge and cause systematic distortions of limb position sense in humans. The following two experiments attempted to examine these contributions by separately vibrating over the triceps and biceps muscles during forearm positioning. In the first experiment, subjects performed a horizontal flexion or extension of the right arm to a mechanical stop randomly positioned at 20, 40 or 60°. Vision was occluded and vibration was applied to the right arm. The perceived position of the right limb was assessed by instructing subjects to simultaneously match the right arm position with the left limb. Vibration of the shortening, agonist muscle had no effect on limb matching accuracy. However, antagonist muscle vibration resulted in a significant overestimation of the vibrated limb position by 6–13°. The procedures for the second experiment were similar to the first, except that movements of the right limb were self-terminated and only flexion movements were performed. A screen was mounted over the arms and subjects were instructed to move the right arm until it was positioned beneath a marker on the screen. Vibration of the shortening agonist muscle had no effect on either the positioning accuracy of the right limb or matching accuracy of the left limb. However, antagonist muscle vibration resulted in significantly shorter movements (6–10°) by the right limb and an overestimation of right limb position by the left, matching limb. These findings support the hypothesis that muscle spindle afferent information from the lengthening antagonist muscle contributes to limb position sense during voluntary movement.     

The combined effect of muscle contraction history and motor commands on human position sense

Along with afferent information, centrally generated motor command signals may play a role in joint position sense. Isometric muscle contractions can produce a perception of joint displacement in the same direction as the joint would move if unrestrained. Contradictory findings of perceived joint displacement in the opposite direction have been reported. As this only occurs if muscle spindle discharge in the contracting muscle is initially low, it may reflect increased muscle spindle firing from fusimotor activation, rather than central motor command signals. Methodological differences including the muscle contraction task and use of muscle conditioning could underlie the opposing findings. Hence, we tested perceived joint position during two contraction tasks (‘hold force’ and ‘hold position’) at the same joint (wrist) and controlled muscle spindle discharge with thixotropic muscle conditioning. We expected that prior conditioning of the contracting muscle would eliminate any effect of increased fusimotor activation, but not of central motor commands. Muscle conditioning altered perceived wrist position as expected. Further, during muscle contractions, subjects reported wrist positions displaced ~12° in the direction of contraction, despite no change in wrist position. This was similar for ‘hold force’ and ‘hold position’ tasks and occurred despite prior conditioning of the agonist muscle. However, conditioning of the antagonist muscle did reduce the effect of voluntary contraction on position sense. The errors in position sense cannot be explained by fusimotor activation. We propose that central signals combine with afferent signals to determine limb position and that multiple sources of information are weighted according to their reliability.