Vibration-induced changes in movement-related EMG activity in humans

The effect of muscle tendon vibration during voluntary arm movement was studied in normal humans. Subjects made alternating step flexion and extension movements about the elbow. A small vibrator was mounted over either the biceps or the triceps muscle and vibration was applied during flexion or extension movements. The vibrator was turned off between movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the muscle antagonist to the movement being performed produced an undershoot of the required end-movement position. The undershoot was 20-30% of the total movement amplitude. In contrast, vibration of the muscle agonist to the movement resulted in no change in movement end position. The vibration-induced undershoot was associated with an increase in the EMG activity of the vibrated (antagonist) muscle and a resultant increase in the ratio of the antagonist to agonist EMG activity. The increase in antagonist EMG produced by the vibration occurred with a latency of approximately 60 ms from vibration onset. The observed results are consistent with vibration-induced activation of muscle spindle receptors in the lengthening muscle during movement. It is suggested that, during movement, the sensitivity of the spindle receptors in the shortening muscle is decreased and the information concerning limb position during movement comes primarily from the lengthening muscle.     

The effects of wrist muscle vibration on human voluntary elbow flexion-extension movements

Summary The effect of forearm muscle tendon vibration during alternating step flexion-extension movements about the elbow was studied in normal humans. In one experiment, a vibrator was mounted over either the forearm flexor or the extensor muscle. In a second experiment, a vibrator was mounted over either the forearm muscle or the biceps muscle. In both experiments, vibration was applied either to a single muscle or simultaneously to both muscles during elbow flexion-extension movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the forearm and the biceps muscle during extension movements produced an undershoot of the required end movement position. Moreover, application of high frequency vibration (100 Hz) to the forearm extensor and flexor muscle produced an overshoot of the required end-movement position. The observed results are consistent with vibration induced activation of muscle spindle receptors not only in the lengthening muscle during movement but also in the forearm muscles. It is suggested that the pattern of distribution of muscle spindle afferent from the forearm muscle onto -motoneurons of muscles acting at the elbow has played an important role of alternating step flexion-extension movements.     

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.     

Separate control of arm position and velocity demonstrated by vibration of muscle tendon in man

Summary The effect of muscle tendon vibration on the performance of some simple motor tasks and on kinesthesia was studied in normal humans. Subjects performed non-visually-guided slow arm movements to match either the position or the velocity of a visual target. In the experiments designed to study kinesthesia subjects indicated the perceived position or velocity of their passively moved arm. Vibration was applied over either the biceps or the triceps tendon. Position and velocity matching were found to be disturbed by vibration in essentially different ways, as were the perception of imposed position and the perception of imposed velocity. However, the vibration induced disturbance of position matching was congruent with the distortion of position perception. The effect of vibration on velocity matching was in accordance with the effect of vibration on the perception of velocity. It is concluded that the afferent information pathways that give rise to the perception of position and velocity respectively can be used separately in the control of slow movements under different conditions.     

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.     

Disturbed proprioception following a period of muscle vibration in humans

Forearm position matching tasks were performed by blindfolded subjects before and after applying vibration for 60 s to the biceps or triceps muscle of one arm. Following cessation of vibration, statistically significant alignment (proprioceptive) errors occurred when a movement lengthened the previously vibrated muscle. The error was such that the length of the post-vibrated muscle was greater than the length of the same muscle in the non-vibrated arm. This effect is the opposite to that which occurs during vibration.     

Experimental muscle pain changes motor control strategies in dynamic contractions

This study investigated the effect of muscle pain on muscle activation strategies during dynamic exercises. Ten healthy volunteers performed cyclic elbow flexion/extension movements at maximum speed for 2 min after injection of (1) hypertonic (painful) saline in the biceps brachii, (2) hypertonic saline in both biceps brachii and triceps brachii, and (3) isotonic (nonpainful) saline in the biceps brachii muscle. Surface electromyographic (EMG) signals were collected from the upper trapezius, biceps brachii, triceps brachii, and brachioradialis muscles (to estimate EMG amplitude) and with an electrode arrays from biceps brachii (to estimate muscle fiber conduction velocity [CV]). In all conditions, the acceleration of the movement decreased throughout the exercise, and kinematic parameters were not altered by pain. With respect to the control condition, pain induced a decrease of the biceps brachii (mean ± SE, –23±4%) and brachioradialis (–10±0.4%) integrated EMG (IEMG) in the beginning of the exercise, and an increase (45±3.5%) of the upper trapezius IEMG at all time points during the exercise. The biceps brachii IEMG decreased over time during the nonpainful exercises (–11±0.6%) while it remained constant in the painful condition. Biceps brachii CV decreased during painful conditions (–12.8±2.2%) while it remained constant during the nonpainful condition. In conclusion, muscle pain changes the motor control strategy to sustain the required dynamic task both in the relative contribution between synergistic muscles and in the motor unit activation within the painful muscle. Such a changed motor strategy may be highly relevant in models of occupational musculoskeletal pain conditions.     

The effect of muscle pain on elbow flexion and coactivation tasks

The effects of muscle pain on movement can easily be observed in daily life routines. However, the influence of muscle pain on motor control strategies has not been fully clarified. In this human experimental study it was hypothesized that muscle pain affects the motor control of elbow flexion movements, in different combinations of range of motion and target size, by decreased agonistic muscle activity and increased antagonistic muscle activity with consequent implications on kinematic parameters. The effects of experimentally induced muscle pain on movement strategy for: (1) small and large range of motion (ROM) elbow flexion movements towards a wide target, (2) large ROM flexion movements towards a narrow and wide target, and (3) subsequent coactivation of agonistic and antagonistic muscles to elbow flexion were assessed. Muscle pain induced by injections of hypertonic saline (1 ml, 5.8%) in either m. biceps brachii or m. triceps brachii caused similar effects on the movements. For low accurate movements the initial (100 ms) integrated electromyographic (EMG) activity of m. biceps brachii was decreased during muscle pain. In contrast, integrated EMG of the entire m. biceps brachii burst was decreased by muscle pain only for small ROM at a low accuracy, which also showed decreased EMG activity of m. triceps brachii and m. brachioradialis, together with increased activity of m. trapezius. Finally, high accurate movements and post-movement coactivation were generally not modulated by muscle pain. In summary, the present study shows that acute muscle pain can perturb the motor control strategy, which might be highly important in occupational settings where such a change may need compensatory actions from other muscles and thereby eventually contribute to the development of musculoskeletal pain problems.     

The attainment of target position during step-tracking movements despite a shift of initial position

Summary Subjects performed a step-tracking forearm movement at maximum velocity without visual guidance. A considerable shift of initial forearm position, which remained unnoticed by the subject, was induced by vibration of the biceps tendon. Notwithstanding the shift of initial position subjects attained the correct final position, irrespective of whether the vibration was switched off before the movement or continued during the movement. Recordings of biceps and triceps electromyograms show that muscle activities were programmed to produce a movement from the actual initial position to the target position. The findings indicate that correct information on forearm position is available to the central nervous system at a subconscious level even if position perception is disturbed through vibration.     

Influence of neural adjustments and muscle oxygenation on task failure during sustained isometric contractions with elbow flexor muscles

This study investigated the adjustments in muscle activation and oxygenation in biceps and triceps brachii during two tasks sustained to failure at 20 and 60% of the maximal voluntary contraction (MVC) force. The tasks required participants either to push against a rigid restraint (force task) or to support an inertial load (position task) with the elbow flexor muscles. The surface EMG was recorded for biceps brachii, brachioradialis, triceps brachii and trapezius superior muscles. Muscle oxygenation of biceps and triceps brachii was measured by near-infrared spectroscopy. The position task was briefer (404 ± 159 s) than the force task (533 ± 194 s) when performed at 20% MVC (P = 0.011), but endurance time did not differ at 60% MVC (54 ± 19 versus 64 ± 16 s, respectively; P = 0.13). Biceps brachii oxygenation decreased slightly (by ~7%) during tasks performed at 20% MVC, whereas it dropped (-40%) for tasks sustained at 60% MVC. However, the decrease in muscle oxygenation was not a significant predictor of time to failure at the two target forces, although its contribution to muscle fatigue cannot be completely ruled out at 60% MVC. In contrast, time to failure was predicted by the increase in EMG of biceps brachii for both tasks at 20% MVC, and EMG of brachioradialis and trapezius for both tasks at 60% MVC. These results suggest that neural adjustments rather than muscle oxygenation limited the time to failure for the force and position tasks at low and high target forces.     

Independent coactivation of shoulder and elbow muscles

 The aim of this study was to examine the possibility of independent muscle coactivation at the shoulder and elbow. Subjects performed rapid point-to-point movements in a horizontal plane from different initial limb configurations to a single target. EMG activity was measured from flexor and extensor muscles acting at the shoulder (pectoralis clavicular head and posterior deltoid) and elbow (biceps long head and triceps lateral head) and flexor and extensor muscles acting at both joints (biceps short head and triceps long head). Muscle coactivation was assessed by measuring tonic levels of electromyographic (EMG) activity after limb position stabilized following the end of the movements. It was observed that tonic EMG levels following movements to the same target varied as a function of the amplitude of shoulder and elbow motion. Moreover, for the movements tested here, the coactivation of shoulder and elbow muscles was found to be independent – tonic EMG activity of shoulder muscles increased in proportion to shoulder movement, but was unrelated to elbow motion, whereas elbow and double-joint muscle coactivation varied with the amplitude of elbow movement and were not correlated with shoulder motion. In addition, tonic EMG levels were higher for movements in which the shoulder and elbow rotated in the same direction than for those in which the joints rotated in opposite directions. In this respect, muscle coactivation may reflect a simple strategy to compensate for forces introduced by multijoint limb dynamics. Received: 7 July 1998 / Accepted: 28 July 1998     

Shoulder and elbow muscle activity in goal-directed arm movements

 This research examined the electromyographic (EMG) activity of shoulder and elbow muscles during reaching movements of the upper limb. Subjects performed goal-directed arm movements in the horizontal plane. Movements which varied in amplitude, speed, and direction were performed in different sections of the workspace. EMG activity was recorded from the pectoralis major, posterior deltoid, biceps brachii short head, brachioradialis, triceps brachii long head, and triceps brachii lateral head; motion recordings were obtained with an optoelectric system. The analysis focused on the magnitude and timing of opposing muscle groups at the shoulder and elbow joints. For hand movements within any given direction of the workspace direction, kinematic manipulations changed agonist and antagonist EMG magnitude and intermuscle timing in a manner consistent with previous single-joint findings. To produce reaching movements in different directions and areas of the workspace, shoulder and elbow agonist EMG magnitude increased for those hand motions which required higher angular velocities, while the timing between opposing muscle groups at each joint was invariant. Received: 11 January 1996 / Accepted: 24 February 1997     

Cardiovascular and respiratory responses to changes in central command during isometric exercise at constant muscle tension

1. Experiments were designed to show whether elements of the command descending from higher centres to exercising muscles provide an input for cardiovascular and respiratory control. Vibration, known to be a powerful stimulus to the primary afferents from muscle spindles, was applied to the biceps tendon of human subjects performing sustained isometric contractions with the biceps or the triceps muscle. When the biceps was contracting this activation of muscle spindle primary afferents in it provided an element of reflex excitation, so that less central command was required to achieve a given tension. When triceps was contracting, the activation of muscle spindle primary afferents in its antagonist, biceps, contributed an element of reflex inhibition, so that more central command than normally was required to achieve a given tension. The cardiovascular and respiratory responses to an isometric effort could thus be investigated at any tension when the central command was normal, decreased, or increased.2. Blood pressure, heart rate, and pulmonary ventilation all increase in an isometric effort. The increase in each is less when the central command is reduced. The increase in each is greater when the central command is increased.3. It is concluded that there is irradiation of cardiovascular and respiratory control centres by the descending central command during voluntary muscular contractions in man.     

Changes in neuronal activity of the monkey precentral cortex during preparation for movement

The aim of this study was to analyze the changes in neuronal activity of the motor cortex associated with preparation for movement. Monkeys were trained to perform a between-hands choice-reaction time (RT) task. They pressed upon two levers with both hands and, after a preparatory period of 1-s duration initiated by a warning tone, they must point at either a left- or right-located target, when illuminated as a response signal, with either the left or right hand. The level of preparation for performing either sided movement was changed by manipulating, across block of trials, the relative probabilities for the left and right hands to point at the corresponding target. When considering behavioral data averaged over daily sessions, it was found that RT significantly decreased as response probability increased, whereas movement time (MT) was not affected. However, large changes in this rule were observed when data collected within a single session were considered. Response probability manipulation was acting on RT alone in 27% of sessions, on MT alone in 19% of sessions, and on both RT and MT in 17% of sessions. Statistically significant trial-by-trial correlations between RT and MT were found; of positive sign for 34% of sessions and of negative sign for 12% of sessions. Electromyographic recordings from the biceps and triceps brachii showed that muscle activity remained stable during the preparatory period, both muscles of the performing arm began to be coactivated about 70-130 ms before the lever release, biceps activation being progressively larger than that of the triceps, and the rising slope of biceps activation was steeper for short than for long RTs. Single-cell recording techniques were used to study the neuronal activity of primary motor cortex during the performance of the task. One hundred and fifteen units in area 4, whose activity was recorded during a whole experimental session, were selected for further analysis. Disregarding 4% of units which did not exhibit any movement-related change in activity, it was found that twenty-one percent of units were related to contralateral movements only, 2% to ipsilateral movements only, and 73% to both sided movements. Among this last subset, 27% of units exhibited reciprocal movement-related changes in activity, the most often an excitation for contralateral movements and an inhibition for ipsilateral movements, and 50% of units exhibited a movement-related change in activity in the same direction for both movements, the most often an excitation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of brain activity during different types of proprioceptive inputs: a positron emission tomography study

It has been shown that the primary and secondary somatosensory cortex, as well as the supplementary motor area (SMA), are involved in central processing of proprioceptive signals during passive and active arm movements. However, it is not clear whether different cortical areas are involved in processing of different proprioceptive inputs (skin, joint, muscle receptors), what their relative contributions might be, where kinesthetic sensations are formed within the CNS, and how they interact when the full peripheral proprioceptive machinery acts. In this study we investigated the representation of the brain structures involved in the perception of passive limb movement and illusory movement generated by muscle tendon vibration. Changes in cortical activity as indicated by changes in regional cerebral blood flow (rCBF) were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) passive flexion-extension movement (PM) of the left forearm; (2) induced illusions of movements (VI) similar to the real PM, induced by alternating vibration of biceps and triceps tendons (70-80 Hz) at the elbow; (3) alternating vibration of biceps and triceps tendons (with 20-50 Hz) without induced kinesthetic illusions (VN); and (4) rest condition (RE). The results show different patterns of cortex activation. In general, the activation during passive movement was higher in comparison with both kinds of vibration, and activation during vibrations with induced illusions of movement was more prominent than during vibrations without induced illusions. When the PM condition was contrasted with the other conditions we found the following areas of activation -- the primary motor (MI) and somatosensory area (SI), the SMA and the supplementary somatosensory area (SSA). In conditions where passive movements and illusory movements were contrasted with rest, some temporal areas, namely primary and associative auditory cortex, were activated, as well as secondary somatosensory cortex (SII). Our data show that different proprioceptive inputs, which induce sensation of movement, are associated with differently located activation patterns in the SI/MI and SMA areas of the cortex. In general, the comparison of activation intensities under different functional conditions indicates the involvement of SII in stimulus perception generation and of the SI/MI and SMA areas in the processing of proprioceptive input. Activation of the primary and secondary auditory cortex might reflect the interaction between somatosensory and auditory systems in movement sense generation. SSA might also be involved in movement sense generation and/or maintenance.     

Physiological changes underlying bilateral isometric arm voluntary contractions in healthy humans

Many bilateral motor tasks engage simultaneous activation of distal and proximal arm muscles, but little is known about their physiological interactions. Here, we used transcranial magnetic stimulation to examine motor-evoked potentials (MEPs), interhemispheric inhibition at a conditioning-test interval of 10 (IHI(10)) and 40 ms (IHI(40)), and short-interval intracortical inhibition (SICI) in the left first dorsal interosseous (FDI) muscle during isometric index finger abduction. The right side remained at rest or performed isometric voluntary contraction with the FDI, biceps or triceps brachii, or the tibialis anterior. Left FDI MEPs were suppressed to a similar extent during contraction of the right FDI and biceps and triceps brachii but remained unchanged during contraction of the right tibialis anterior. IHI(10) and IHI(40) were decreased during contraction of the right biceps and triceps brachii compared with contraction of the right FDI. SICI was increased during activation of the right biceps and triceps brachii and decreased during activation of the right FDI. The present results indicate that an isometric voluntary contraction with either a distal or a proximal arm muscle, but not a foot dorsiflexor, decreases corticospinal output in a contralateral active finger muscle. Transcallosal inhibitory effects were strong during bilateral activation of distal hand muscles and weak during simultaneous activation of a distal and a proximal arm muscle, whereas GABAergic intracortical activity was modulated in the opposite manner. These findings suggest that in intact humans crossed interactions at the level of the motor cortex involved different physiological mechanisms when bilateral distal hand muscles are active and when a distal and a proximal arm muscle are simultaneously active.     

Electromechanical delay in skeletal muscle under normal movement conditions

Electromechanical delays (EMD), the time from onset of EMG activity to change in acceleration or deceleration of the forearm, were studied in concentric and eccentric contractions of biceps and triceps brachii muscles. Horizontal flexion and extension movements were performed at varying speeds by 10 subjects. EMD time in concentric contractions for biceps was 41 +/- 13 ms and for triceps was 26 +/- 11 ms and was not influenced by the velocity of the movement. In eccentric contractions at the slow velocity the biceps EMD time was 38 +/- 13 ms and shortened to 28 +/- 10 ms at the faster velocity. The eccentric triceps EMD, however, was not significantly altered by movement velocity and averaged 30 +/- 7 ms. The data provided support for the hypothesis that stretching of the series elastic component, to a point where muscle force can be detected, is the primary determinant of the EMD phenomenon. However, there are complex interactions of the effects on EMD of muscle fiber type composition, whether the contraction is concentric or eccentric, and the velocity of the movement as well as possible gamma system influence. These complications require that consideration of electromechanical delay be made when phasic relationships between muscle force or joint torque generation from different muscles are inferred from EMGs.     

Effects of tendon vibration on the spatiotemporal characteristics of human locomotion

The present study addressed the involvement of proprioceptive input of the muscle spindles in the spatiotemporal control of human locomotion. Blindfolded subjects walked along a walkway while tendon vibration, a powerful stimulus of Ia afferents, was applied to various muscles of the lower limb. The effects of tendon vibration were measured on joint kinematics and on intralimb and interlimb coordination. Tendon vibration of the tibialis anterior during locomotion led to a decreased plantar flexion at toe-off, whereas vibration of the triceps surae led to a decreased dorsiflexion during swing. Vibration of the quadriceps femoris at the knee led to a decreased knee flexion during swing. These local effects of vibration can be explained in the light of a lengthening illusion of the vibrated muscle in that phase of the gait cycle where the muscle is lengthened. Tendon vibration did not affect the qualitative features of intralimb coordination. With respect to interlimb coordination, only vibration of the biceps femoris showed a significant increase in phase lead of the vibrated limb. The present results suggest the involvement of Ia afferent input in the online control of joint rotations. Additionally it is hypothesized that the proprioceptive input of biceps femoris might be involved in the control of coordination between the limbs, whereas the coordination between the segments of one limb appears to be unaffected by disturbance of muscle spindle input of one muscle.     

Cutaneous receptors contribute to kinesthesia at the index finger, elbow, and knee

The neural mechanisms underlying the sense of joint position and movement remain controversial. While cutaneous receptors are known to contribute to kinesthesia for the fingers, the present experiments test the hypothesis that they contribute at other major joints. Illusory movements were evoked at the interphalangeal (IP) joints of the index finger, the elbow, and the knee by stimulation of populations of cutaneous and muscle spindle receptors, both separately and together. Subjects matched perceived movements with voluntary movements of homologous joints on the contralateral side. Cutaneous receptors were activated by stretch of the skin (using 2 intensities of stretch) and vibration activated muscle spindle receptors. Stimuli were designed to activate receptors that discharge during joint flexion. For the index finger, vibration was applied over the extensor tendons on the dorsum of the hand, to evoke illusory metacarpophalangeal (MCP) joint flexion, and skin stretch was delivered around the IP joints. The strong skin stretch evoked the illusion of flexion of the proximal IP joint in 6/8 subjects (12 +/- 5 degrees, mean +/- SE). For the group, strong skin stretch delivered during vibration increased the perceived flexion of the proximal IP joint by eight times with a concomitant decrease in perceived flexion of the MCP joint compared with vibration alone (P < 0.05). For the elbow, vibration was applied over the distal tendon of triceps brachii and skin stretch over the dorsal forearm. When delivered alone, strong skin stretch evoked illusory elbow flexion in 5/10 subjects (9 +/- 4 degrees). Simultaneous strong skin stretch and vibration increased the illusory elbow flexion for the group by 1.5 times compared with vibration (P < 0.05). For the knee, vibration was applied over the patellar tendon and skin stretch over the thigh. Skin stretch alone evoked illusory knee flexion in 3/10 subjects (8 +/- 4 degrees) and when delivered during vibration, perceived knee flexion increased for the group by 1.4 times compared with vibration (P < 0.05). Hence inputs from cutaneous receptors, muscle receptors, and combined inputs from both receptors likely subserve kinesthesia at joints throughout the body.     

Effect of load level and muscle pain intensity on the motor control of elbow-flexion movements

This study assessed interactions between mild/moderate muscle pain and inertial load on the control of human elbow-flexion movements. It is hypothesized that high inertial load combined with moderate muscle pain intensity affect the motor control more than for low inertial-load combined with low-intensity pain. Fifteen subjects performed horizontal pointing movements (70° range) under three load conditions: 0, 4, and 10 kg. Pain was induced by injection of 0.5 ml and 1.5 ml hypertonic saline into the biceps muscle. Subjects scored the muscle pain intensity on a visual analogue scale (VAS). Elbow joint position, VAS, and the electromyograms (EMG, m. biceps brachii, m. triceps brachii, m. brachioradialis, and m. trapezius) were recorded. Mild and moderate muscle pain attenuated acceleration profiles [6.1(0.9)%], effective movement amplitude [3.2 (0.7)%], peak velocity [5.8 (0.9)%] and prolonged the reaction time [21 (5)%]. No interaction between muscle pain intensity and inertial load was found for the kinematic parameters. EMG profiles from m. biceps brachii, m. triceps brachii, and m. brachioradialis were similarly attenuated [10.2 (0.80)%] by mild and moderate muscle pain in all inertial load conditions. For high inertial load, the initial agonist EMG burst activity was more attenuated [50 (5.3)%] by moderate muscle pain compared with mild muscle pain [34 (4.2)%]. These data suggest that for high effort-demanding tasks muscle pain differently affects the motor planning according to the pain-intensity level. Perturbations of motor planning lead to changes on movement strategies, which might be a potential cause of musculoskeletal problems.