Localization and contractile properties of intrinsic longitudinal motor units of the rat tongue

Tongue dysfunction is a hallmark of many human clinical disorders, yet we lack even a rudimentary understanding of tongue neural control. Here, the location and contractile properties of intrinsic longitudinal motor units (MUs) of the rat tongue body are described to provide a foundation for developing and testing theories of tongue motor control. One hundred and sixty-five MUs were studied by microelectrode penetration and stimulation of individual motor axons coursing in the terminal portion of the lateral (retrusor) branch of the hypoglossal nerve in the rat. Uniaxial MU force was recorded by a transducer attached to the protruded tongue tip, and MU location was estimated by electromyographic (EMG) electrodes implanted into the anterior, middle, and posterior portions of the tongue body. All MUs produced retrusive force. MU twitch force ranged from 2-129 mg (mean = 35 mg) and tetanic force ranged from 9-394 mg (mean = 95 mg). MUs reached maximal twitch force in 8-33 ms (mean = 15 ms) and were resistant to fatigue; following 2 min of stimulation, MUs (n = 11) produced 78-131% of initial force. EMG data were collected for 105 MUs. For 65 of these MUs, the EMG response was confined to a single electrode location: for 26 MUs to the anterior, 21 MUs to the middle, and 18 MUs to the posterior portion of the tongue. Of the remaining MUs, EMG responses were observed in two (38/40) or all three (2/40) tongue regions. These data provide the first contractile measures of identified intrinsic tongue body MUs and the first evidence that intrinsic longitudinal MUs are restricted to a portion of tongue length. Localization of MU territory suggests a role for intrinsic MU in the regional control of the mammalian tongue observed during feeding and speech.     

Motor unit recruitment order during voluntary and electrically induced contractions in the tibialis anterior

 The recruitment order of motor units (MU) was compared during voluntary and electrically induced contractions. With the use of spike-triggered averaging, a total of 302 MUs with recruitment thresholds ranging from 1% to 88% of maximal voluntary contraction were recorded in the human tibialis anterior muscle in five subjects. The mean (±SD) MU force was 98.3±93.3 mN (mean torque 16.8±15.9 mNm) and the mean contraction time (CT) 46.2±12.7 ms. The correlation coefficients (r) between MU twitch force and CT versus the recruitment threshold in voluntary contractions were +0.68 and –0.38 (P<0.001), respectively. In voluntary contractions, MUs were recruited in order of increasing size except for only 6% of the cases; whereas, during transcutaneous electrical stimulation (ES) at the muscle motor point, MU pairs showed a reversal of recruitment order in 28% and 35% of the observations, respectively, when the pulse durations were 1.0 ms or 0.1 ms. This recruitment reversal during ES was not related to the magnitude of the difference in voluntary recruitment thresholds between MUs. It is concluded that if the reversal of MU recruitment observed during ES is biophysically controlled by differences in their nerve axon input impedance, in percutaneous stimulation at the motor point, other factors such as the size and the morphological organisation of the axonal branches can also influence the order of activation. Received: 24 May 1996 / Accepted: 30 September 1996     

Collateral nerve sprouting and twitch forces of single motor units in conditions with partial denervation in man

Single motor units (MUs) were studied in the first dorsal interosseus muscle of 7 patients with slight partial denervation and in 10 controls. MU action potentials were recorded using the macro-EMG technique and their size was taken to assess collateral nerve sprouting. Simultaneously, the muscle force was monitored to determine the voluntary recruitment thresholds of the MUs and spike-triggered-averaging was applied to measure their twitch forces. At comparable force recruitment thresholds, both macro-EMG potentials and twitch forces were increased in the patients. We conclude, that collateral nerve sprouting increases MU force and can compensate for MU loss.     

Motor unit properties from three synergistic muscles during ramp isometric elbow extensions

Many tasks require synergistic activation of muscles that possess different architectural, mechanical, and neural control properties. However, investigations of the motor unit (MU) mechanisms which modulate force are mostly restricted to individual muscles and low forces. To explore the pattern of MU recruitment and discharge behavior among three elbow extensors (lateral and long heads of the triceps brachii, and anconeus) during ramp isometric contractions, recruitment thresholds of 77 MUs in five young men were determined and corresponding MU discharge rates were tracked in 1-s epochs over forces ranging from 0 to 75 % of maximal voluntary isometric force (MVC). Across all forces, MUs in the lateral head discharged at higher rates than those in the anconeus (p < 0.001, Δ = 0.23). When all MUs were considered, recruitment thresholds in the long head of the triceps brachii were higher than the lateral head (p < 0.05, Δ = 0.70) with a trend (p = 0.08, Δ = 0.48) for higher recruitment thresholds in the long head compared with the anconeus. Together, these data indicate a potential mechanical disadvantage of the long head of the triceps brachii at 0° shoulder flexion. However, among low-threshold MUs (<10 % MVC), recruitment thresholds were lower in the anconeus than in both heads of the triceps brachii consistent with the expected twitch contractile and fiber type differences among these muscles. These findings illustrate the importance of considering synergistic relations among muscles used for a coordinated task, and the sensitivity of synergies to muscle architectural, mechanical, and possibly specific synaptic input factors.     

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production. In order to determine whether electromyogram (EMG) and mechanomyogram (MMG) are suitable for the noninvasive estimation of the motor unit (MU) activation strategy, the EMG/force and MMG/force relationships were examined simultaneously during isometric ramp contractions in biceps brachii muscle. The highest mean power frequency (MPF) of the EMG, which reflects the full MU recruitment, was determined at 51% MVC. Two obvious inflection points were identified on the MMG-amplitude/force relationship that showed an initial slow increase followed by a rapid increase and a progressive decrease at higher force levels. Our results suggest that the MMG amplitude allows the estimation of the beginning of recruitment of MUs that innervate the first-twitch fibers in addition to identification of the full MU recruitment. The rate coding strategy was qualitatively reflected by the MMG-MPF/force relationship. We conclude that the MU activation strategy is estimated in more detail by the MMG than by the EMG.     

Assessment of across-muscle coherence using multi-unit vs. single-unit recordings

Coherence between electromyographic (EMG) signals has been used to identify correlated neural inputs to motor units (MUs) innervating different muscles. Simulations using a motor-unit model (Fuglevand et al. 1992) were performed to determine the ability of coherence between two multi-unit EMGs (mEMG) to detect correlated MU activity and the range of correlation strengths in which mEMG coherence can be usefully employed. Coherence between motor-unit and mEMG activities in two muscles was determined as we varied the strength of a 30-Hz periodic common input, the number of correlated MU pairs and variability of MU discharge relative to the common input. Pooled and mEMG coherence amplitudes positively and negatively accelerated, respectively, toward the strongest and most widespread correlating inputs. Furthermore, the relation between pooled and mEMG coherence was also nonlinear and was essentially the same whether correlation strength varied by changing common input strength or its distribution. However, the most important finding is that while the mEMG coherence saturates at the strongest common input strengths, this occurs at common input strengths greater than found in most physiological studies. Thus, we conclude that mEMG coherence would be a useful measure in many experimental conditions and our simulation results suggest further guidelines for using and interpreting coherence between mEMG signals.     

Relation between the motor units recruitment threshold and their potentials propagation velocity at isometric activity.

The relationship between the propagation velocity of the excitation along the muscle fibers of the motor units (MUs) and their threshold of recruitment at different level of isometric voluntary contraction was investigated. The threshold of recruitment was measured by the value of the muscle force, expressed in percents from the maximal voluntary contraction (MVC) at which the first impulse of the MU appeared. A wire subcutaneous branched electrode was used to select the potentials from a single MU. The selected in this way MU impulses were used as a trigger to average two electromyographic (EMG) signals picked up by means of two monopolar surface electrodes with small leading-off areas mounted on a common plate at a distance of 10 mm from one another. The propagation velocity of the extraterritorial potentials of the MUs increased non-linearly with the increase of the recruitment threshold. The relationship was fitted as V = square root of a+b.theta, where v is the propagation velocity, theta is the threshold of recruitment and a and b are constants. The consideration of the velocity of propagation as a "size principle parameter" was discussed and the limitations of the latter are pointed out.     

Non-invasive assessment of single motor unit mechanomyographic response and twitch force by spike-triggered averaging

A method for non-invasive assessment of single motor unit (MU) properties from electromyographic (EMG), mechanomyographic (MMG) and force signals is proposed. The method is based on the detection and classification of single MU action potentials from interference multichannel surface EMG signals and on the spike-triggered average of the MMG (detected by an accelerometer) and force signals. The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles were investigated at contraction levels of 2% and 5% of the maximum voluntary contraction (MVC) force. A third contraction was performed by selective activation of a single MU with surface MU action potential visual feedback provided to the subject. At 5% MVC, the mean (±standard error) single MU MMG peak-to-peak value was 11.0±1.8 mm s⁻² (N=17) and 32.3±6.5 mm s⁻² (N=20) for the FDI and AMD muscles, respectively. The peak of the twitch force was, at the same contraction livel, 7.41±1.34 mN and 14.42±2.92 mN, for the FDI and ADM muscles, respectively. The peak-to-peak value of the MMG was significantly different for the same MU at different contraction levels, indicating a non-linear summation of the single MU contributions. For the FDI muscle, the MMG peak-to-peak value of individual MUs was 21.5±7.8 mm s⁻², when such MUs were activated with visual feedback provided to the subject, whereas, for the same MUs, it was 11.8±3.8 mm s⁻², when the subject maintained a constant force level of 2% MVC. The method proposed allows the non-invasive assessment of single MU membrane and contractile properties during voluntary contractions.     

Discharge behavior of motor units in knee extensors during the initial stage of constant-force isometric contraction at low force level

Abstract. To elucidate the strategy of the activity of motor units (MUs) to maintain a constant-force isometric contraction, I examined the behavior of MUs in knee extensor muscles [(vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF)] during a sustained contraction at 5% of maximal voluntary contraction for 5 min. In all cases, the spike interval exhibited an elongating trend, and two discharge patterns were observed, continuous discharge and decruitment. In continuous-discharge MUs, the trend slope was steep immediately after the onset of constant force (steep phase), and then became gentle (gentle phase). Decruitments were observed frequently during each phase, and additional MU recruitment was observed throughout the contraction. The mean value of recruitment threshold force did not differ among the extensors. The mean spike interval at the onset of constant-force isometric contractions was shorter in RF than in VL. However, there were no differences in the duration and extent of the elongating trend, decruitment time and recruitment time among the extensors. The electromyogram of the antagonist biceps femoris muscle revealed no compensatory change for extensor activity. These results indicated that at a low force level, the strategy employed by the central nervous system to maintain constant force appears to involve cooperation among elongating trends in the spike interval, decruitment following elongation, and additional MU recruitment in synergistic muscles. Electronic Publication     

Effects of short-term training on physiologic properties of human motor units.

The human neuromuscular system possesses a remarkable ability to adapt to any change in the demands placed upon it. Adaptation to training or disuse is reflected in the activation patterns and physiologic properties of the motor unit (MU) pool in a given muscle group. This review summarizes the studies that have examined such adaptation at the level of the single MU. Disuse, as revealed by electrophysiologic studies, results in impaired MU recruitment and smaller twitch tensions in low and high threshold MUs. Alternatively, shortterm training improves MU recruitment and generally results in larger MU twitch tensions. A method has been developed to physiologically characterize and longitudinally follow single thenar MUs. Studies utilizing these methods have demonstrated that MUs differing in their baseline physiologic characteristics respond very differently to identical short-term training programs. These observations at the single MU level provide insight into training adaptations in whole muscles and muscle groups.     

Human motor unit activity during concentric and eccentric movements

Single motor units (MUs) activity was investigated in human m. biceps brachii during movements against an elastic load. A total of sixty-five MUs were studied by means of subcutaneously placed fine-wire branched electrodes. Subjects were asked to perform active shortening and lengthening of the muscle with approximately constant velocities at two different speeds--slow and fast. Both recruitment (RT) and decruitment (DT) thresholds of MU were found to be lower in movement with higher velocity. The recruitment order of MUs was approximately one and the same during concentric movements with a different but constant velocity. The firing onset of MUs is organized so that the peak of the first twitch contraction occurs at approximately the same force level irrespective of how fast the movement is. In contrast, during the eccentric movements the peak of the last twitch contraction of MU occurs at different torque levels depending on the velocity. The decruitment of the MUs during eccentric movement was in a reverse order to their recruitment during concentric movements. Generally, at one and the same velocity the RT of a given MU was lower than DT. Nevertheless, the peaks of the first and the last twitch contractions during concentric and eccentric movements with one and the same velocity occurred at approximately one and the same torque level.     

Motor unit differentiation and integrated surface EMG in voluntary isometric contraction

The role of the differentiation of motor units (MU's) in the relation between integrated surface electromyogram and force was studied here for the biceps brachii, in the course of static contraction in man. The global EMG of the biceps brachii was recorded by two silver electrodes fixed to the skin. The MU's activity was simultaneously lead off by three bipolar wire electrodes inserted in three points of the muscle. MU firing frequency and recruitment were estimated by counting the action potentials on the three elementary records. The comparison between the increase of the integrated surface EMG and MU recruitment as a function of force shows that MU recruitment in itself cannot account for the increase in the integrated global EMG, particularly for high values of force. The difference between MU recruitment and integrated global EMG, which can neither come from a possible MU's synchronisation as was shown in the discussion, is being suggested to be connected to the progressive firing of “phasic? MU's. This type of recruitment may also play an important role in the gradation of isometric force in normal man.     

Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruitment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and subsequent recovery. Eight female subjects sustained a 30° shoulder abduction, requiring 11–12% of maximal voluntary contraction (MVC), for 30 min. At 10 and 30 min into the recovery period, the shoulder abduction was repeated for 1 min. The rating of perceived exertion for the shoulder region increased to “close to exhaustion” during the prolonged contraction, and the surface electromyography (EMG) recorded from the deltoid and trapezius muscles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle electrodes. Of the identified MUs, 95% were continuously active during the 8-s recordings, indicating a low degree of MU rotation. The mean (range) MU firing rate was 11.2 (5.7–14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abduction, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0–6 min) to the end (24–29 min) of the contraction period, indicating recruitment of larger MUs in response to fatigue. In contrast, after 10 min of recovery the average MU amplitude was smaller than seen initially in the prolonged contraction, but not different after 30 min, while the MU firing rate was higher during both tests. In conclusion, MU recruitment plays a significant role during fatigue, whereas rate coding has a major priority during recovery. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs during prolonged contractions. Accepted: 6 June 2000     

Physiological properties of the motor units of the wrist extensor muscles in man

Summary The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. MU recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase=0.25 N·s^-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 MUs recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r=-0.75) between the frequency rise times (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the frequency rise times increase when the twitch contraction times decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the MUs is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.     

The role of synchrony and oscillations in the motor output

 There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20–30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20–30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date. Received: 17 July 1998 / Accepted: 10 December 1998     

Evaluation of motor unit firing rates by standard concentric needle electromyography.

Motor unit firing rates at slight voluntary contraction were studied by standard concentric needle electromyography. Employing digital signal analysis techniques firing rates of motor units (MU) could be evaluated as long as four or less different MUs were activated in the vicinity of the concentric needle electrode. The extension of the recording area is defined by the recording properties of the electrode and the upper limit of rise-time for all MUPs being evaluated. Distant MUs, generating volume conducted potentials with rise-times greater than 0.8 ms, were excluded. In biceps muscles of 15 healthy controls the firing rate of the MU activated first was evaluated at that moment, when a second MU was recruited and was found to be 12.1 +/- 2.1 Hz (mean +/- S.D., n = 40). The firing rate of the fastest MU out of 2, 3, or 4 simultaneously active MUs was 10.7 +/- 2.5 Hz, 10.9 +/- 2.5 Hz and 10.6 +/- 2.4 Hz respectively. Hence at low innervation level there is no increase of firing rate with rising number of activated MUs. The upper normal limit of MU firing rate (3 sigma interval) is calculated as 17 Hz, irrespective whether 1, 2, 3 or 4 MUs are active within the recording area. Fifteen patients with partially denervated biceps muscles were investigated. Maximal firing rates were increased in 10 patients, all showing moderate or severe paresis (grade 1-3). In 10 patients suffering from myopathies firing rates always were normal. The presented data may serve as an additional criterion in evaluating MU firing rates during standard clinical EMG.     

Influence of motor unit synchronization on amplitude characteristics of surface and intramuscularly recorded EMG signals

The increase in muscle strength without noticeable hypertrophic adaptations is very important in some sports. Motor unit (MU) synchronisation and higher rate of MU activation are proposed as possible mechanisms for such a strength and electromyogram (EMG) increase in the early phase of a training regimen. Root mean square and/or integrated EMG are amplitude measures commonly used to estimate the adaptive changes in efferent neural drive. EMG amplitude characteristics could change also because of alteration in intracellular action potential (IAP) spatial profile. We simulated MUs synchronization under different length of the IAP profile. Different synchronization was simulated by variation of the percent of discharges in a referent MU, to which a variable percent of remaining MUs was synchronized. Population synchrony index estimated the degree of MU synchronization in EMG signals. We demonstrate that the increase in amplitude characteristics due to MU synchronization is stronger in surface than in intramuscularly detected EMG signals. However, the effect of IAP profile lengthening on surface detected EMG signals could be much stronger than that of MU synchronization. Thus, changes in amplitude characteristics of surface detected EMG signals with progressive strength training could hardly be used as an indicator of changes in neural drive without testing possible changes in IAPs.     

Comparison of F-waves of motor unit action potentials activated during voluntary contraction

The system for classifying F-waves was developed to study the properties of F-wave and to compare single motor unit (MU) F-waves with motor unit action potentials (MUAPs) activated during voluntary contraction. The F-waves evoked by submaximal stimulation as well as the EMG signals during voluntary contraction at 6 levels of 10-100% of maximum voluntary contraction (MVC) were measured in the tibialis anterior muscles of 3 healthy volunteers. Nine channel F-wave waveforms in a selected electrode array were classified using a template-matching method. After the detection procedure of MUAPs in voluntary EMG signals, the MUAPs were also classified by the same method. Most of the F-waves (88.4%) were composed of a single MUAP. The numbers of MU classified from single MU F-waves in 3 subjects were 12, 12 and 15, and the numbers of MU classified from the voluntary EMG signals at 6 contraction levels were 20, 27 and 24, respectively. A total of 26 single MU F-waves were identified with the MUs extracted from the data during voluntary contractions. The results suggest that the F-waves are composed of a population of the MUs, which are recruited at a wide range of contraction levels. The classification procedures of F-waves and voluntary EMG signals have made it possible to recognize the same MU in both signals and to analysis the firing thresholds of F-waves.     

Discharge properties of primate forearm motor units during isometric muscle activity

Activity of single motor units (MUs) was recorded in forelimb muscles of rhesus macaques while they generated isometric ramp-and-hold torques about the wrist. Multiunit electromyographic (EMG) activity was recorded from 10-12 identified flexor and extensor muscles of the wrist and digits with implanted EMG wire electrodes. Single MUs from these muscles were recorded with a remotely controlled tripolar microelectrode array. The parent muscle of each MU was determined by compiling MU-triggered averages of multiunit EMGs. The MU firing patterns during the isometric task were determined from response histograms aligned with change in torque. At moderate torque levels, MUs (n = 86) exhibited four types of discharge patterns during the ramp-and-hold trajectory: phasic-tonic (23%), tonic (33%), decrementing (39%), and phasic (5%). Phasic-tonic MUs exhibited a phasic burst of activity during the torque ramp which exceeded the firing rate during the static hold period. Both phasic-tonic and tonic MUs exhibited a constant mean firing rate during the hold period; the discharge of decrementing MUs gradually decreased during the static hold. Phasic MUs fired only during the change in force. The relation between MU firing rate and torque was investigated as the monkeys generated responses of different levels of static torque during the hold period. Mean firing rate during the hold was found to be proportional to static torque up to moderate torque levels, where it reached a maximum. In the linear range, the mean rate-torque slope was 3.4 +/- 1.9 imp/s per 10(5) dyn . cm (n = 9).     

Activation and intermuscular coherence of distal arm muscles during proximal muscle contraction

In the human upper extremity (UE), unintended effects of proximal muscle activation on muscles controlling the hand could be an important aspect of motor control due to the necessary coordination of distal and proximal segments during functional activities. This study aimed to elucidate the effects of concurrent activation of elbow muscles on the coordination between hand muscles performing a grip task. Eleven healthy subjects performed precision grip tasks while a constant extension or flexion moment was applied to their elbow joints, inducing a sustained submaximal contraction of elbow muscles to counter the applied torque. Activation of four hand muscles was measured during each task condition using surface electromyography (EMG). When concurrent activation of elbow muscles was induced, significant changes in the activation levels of the hand muscles were observed, with greater effects on the extrinsic finger extensor (23.2 % increase under 30 % elbow extensor activation; p = 0.003) than extrinsic finger flexor (14.2 % increase under 30 % elbow flexor activation; p = 0.130). Elbow muscle activation also induced involuntary changes in the intrinsic thumb flexor activation (44.6 % increase under 30 % elbow extensor activation; p = 0.005). EMG–EMG coherence analyses revealed that elbow muscle activation significantly reduced intermuscular coherence between distal muscle pairs, with its greatest effects on coherence in the β-band (13–25 Hz) (average of 17 % decrease under 30 % elbow flexor activation). The results of this study provide evidence for involuntary, muscle-specific interactions between distal and proximal UE muscles, which may contribute to UE motor performance in health and disease.