Differences between motor unit firing rate,twitch characteristics and fibre type composition in an agonistic muscle group in man

Summary A comparison was carried out between the motor unit (MU) firing rate and the characteristics of the twitch and the fibre type composition of anconeus and triceps brachii. Fibre type composition (type I, type II) was determined in whole cross-sections of cadaver specimens. The proportion of type I fibre was 60%–67% in anconeus and 32–40% in the lateral head of triceps brachii. Reflecting these histochemical differences, the contraction time of anconeus and triceps was 92±9 ms and 68±9 ms respectively. It follows that anconeus can be classified as a slow muscle, as opposed to the lateral head of triceps. The relationship between MU firing rate and isometric force, tested at 90° elbow flexion, differed between the two muscles for force values below 30% of maximal voluntary contraction. No significant increase in MU firing rate was found in anconeus at forces above 5% of maximal voluntary contraction. It is concluded that even within a single agonistic muscle group acting at a single joint there is an adaptation of MU firing rate to the contractile characteristics of each muscle.     

Motor unit discharge rates of the anconeus muscle during high-velocity elbow extensions

Motor unit recruitment and motor unit discharge rate (MUDR) have been widely studied in isometric conditions but minimally during velocity-dependent contractions. For isometric contractions, surface electromyography (EMG) activity of the elbow extensors plateaus at near maximal torques (Le Bozec et al. 1980; Le Bozec and Maton 1982). One study (Maton and Bouisset 1975) recorded single motor unit (MU) activity at maximal velocities; however, only the rate of the first interspike interval (ISI) was reported and likely was not representative of the average MUDR of the MU train. The purpose was to calculate average MUDRs of the anconeus during loaded velocity-dependent contractions from zero velocity (isometric) up to maximal velocity (V_max25) through a large range of motion. A Biodex dynamometer was used to record elbow extension torque, position, and velocity. Single MU potentials were collected from the anconeus with intramuscular EMG, and surface EMG was sampled from the lateral head of the triceps brachii during maximal voluntary isometric contractions (MVCs) and velocity-dependent contractions loaded at 25% MVC over 120° range of motion at five target velocities (0, 25, 50, 75, 100%V_max25). Elbow extension velocities ranged from 93 to 494°/s and average MUDR ranged from 11.8 Hz at 25%MVC to 39.0 Hz at 100%V_max25. Overall average MUDRs increased as a function of velocity, although the root mean square of triceps brachii surface EMG plateaued at 50%V_max25. Piecewise regression analysis revealed two distinct linear ranges each described by a unique equation, suggesting that MUDRs of the anconeus enter a secondary range of firing, characterized by a steeper slope as velocity approaches maximum.     

Age independent and position-dependent alterations in motor unit activity of the biceps brachii

In the biceps brachii, age-related differences in synaptic excitability and muscle architecture may affect motor unit (MU) activity differently depending on the position of the forearm. It was hypothesised that as a result of these age-related differences, greater changes in MU activity would accompany a change in forearm position in old when compared with young men. Six young (22 ± 3 years) and six old (84 ± 3 years) men maintained isometric elbow flexion at 10% of maximal voluntary contraction (MVC) during changes in forearm position. Forty-nine MUs in the short (SBB) and long (LBB) heads of the biceps brachii were followed. Motor unit recruitment and de-recruitment thresholds, motor unit discharge rates (MUDRs), and MU discharge variability were measured. Although an age-related decrease in MU recruitment thresholds, and increase in MU discharge variability was evident, changes in forearm position influenced MUDRs similarly in young and old men (P = 0.27). Motor unit recruitment thresholds of the SBB were highest in the pronated position (8.2 ± 2.9 %MVC), whereas in the LBB they were highest in the supinated position (8.6 ± 2.0 %MVC). Motor unit discharge rates of the LBB did not change with forearm position. In the SBB, MUDRs were highest when the forearm was supinated, and also greater when compared with the LBB in this position. No position-dependent changes were observed for MU discharge variability in the LBB, but the SBB exhibited greatest MU discharge variability in the pronated position. The results suggest that MU activity is modulated following a change in forearm position, but the response is similar in young and old adults.     

Modulation of corticospinal excitability dependent upon imagined force level

Motor imagery is defined as the mental execution of a movement without any muscle activity. In the present study, corticospinal excitability was assessed by motor evoked potentials (MEPs) when the subjects imagined isometric elbow flexion at various force levels. Electromyography was recorded from the right brachioradialis, the biceps brachii and the triceps brachii muscles. First, the maximum voluntary contraction (MVC) of elbow flexion was recorded in each subject. Subjects practiced performing 10, 30 and 60 % MVC using visual feedback. After the practice, MEPs were recorded during the imagery of elbow flexion with the forces of 10, 30 and 60 % MVC without any feedback. After the MEPs recording, we assigned subjects to reproduce the actual elbow flexion force at 10, 30 and 60 % MVC. The MEPs amplitudes in the brachioradialis and biceps brachii in the 60 % MVC condition were significantly greater than those in the 10 % MVC condition (p < 0.05). These findings suggest that the enhancement of corticospinal excitability during motor imagery is associated with an increase in imagined force level.     

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study

Summary The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%–30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%–30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.     

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.     

Modulation of slow and fast elbow extensor EMG tonic activity by stretch reflexes in man

Summary Reflex EMG responses to sudden passive flexion of the elbow were recorded from anconeus and triceps brachii in 5 human volunteers.While the subjects were required not to resist the flexion movement, they were required to maintain an extension torque of 3.5 or 7.0 Nm prior to its onset.Under these isotonic conditions, the latency and amplitude of the reflex activities from anconeus and triceps brachii did not differ significantly, in contrast to the findings of Le Bozec (1986) in actively relaxed subjects. The myotatic/postmyotatic EMG amplitude ratio did not provide a further quantitative way to distinguish between these muscles.The absence of a difference between the reflex activities of a slow (anconeus) and a fast (triceps brachii) muscle is interpreted as resulting from a strong drive of spindle activity on the whole extensor motoneuron pool, which outweights the differences in recruitment due to the differing relative amounts of type I and type II fibres in the two muscles. Differences like those described between finger and calf muscles by other authors are though to be due to the relative degree of corticalization of these muscles.All short and long latency responses of the muscles increased in magnitude and decreased in latency with increasing background EMG activity as well as with increasing initial length. The position and tonic activity dependency of these responses is explained in terms of alpha-gamma coactivation.     

Physiology of the fasciculation potentials in amyotrophic lateral sclerosis: which motor units fasciculate?

We set out to study whether in amyotrophic lateral sclerosis (ALS) fasciculation potentials (FPs) arise from the most excitable motor units (MUs). We studied 70 patients with ALS and 18 subjects with benign fasciculation syndrome (BFS). Of the 56 eligible ALS patients, 31 had signs of reinnervation in the right first dorsal interosseous muscle selected for study, and 25 did not. Two needle electrodes were placed in different MUs in each studied muscle. We defined the most excitable MU as that first activated by minimal voluntary contraction. In muscles without reinnervation, the recording site with most frequent FPs had a higher probability of showing the first recruited MU (p < 0.001). No significant difference was found in other patients or in BFS subjects. In very early affected muscles, fasciculating MUs are the most likely to be recruited volitionally. This probably represents hyperexcitability at lower motor neuronal level.     

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.     

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.     

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     

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.     

Amplitude and frequency measures of surface electromyography during dual task elbow torque production

Summary Studies of motor unit recruitment thresholds have demonstrated the existence of task-specific motor units within the muscles controlling the elbow. Two degree-of-freedom (df) task specificity was investigated at higher levels of elbow torque using the amplitude and frequency characteristics of surface electromyography (EMG). Flexion and supination torque data were collected together with EMG from electrode pairs on the brachioradialis (BRAD), biceps brachii short head, and medial and lateral aspects of biceps brachii long head, while subjects (n=14) performed the following four combinations of isometric tasks: (1) maximum voluntary contraction (MVC) flexion (F) and (2) MVC supination (S), each with a targeted torque of zero in the second d f; (3) MVC flexion with targeted MVC supination (FS); and (4) MVC supination with targeted MVC flexion (SF). Median power frequency (MEDF) and root mean square (RMS) amplitude under steady-state torque conditions were calculated and analyzed using ANCOVA models with planned contrasts (=0.05). A significant main effect for task was found in RMS, but not in MEDF. Contrasts showed a significant increase in RMS response in the dual MVC tasks (FS and SF) over the single MVC tasks of F and S. The lack of frequency changes with alterations in RMS data indicates that the underlying recruitment/rate coding scheme in use for dual-d f tasks may be different than in single-d f tasks, and provides possible support for the notion of motor unit task groups. Task-by-site interactions were found for both MEDF and RMS, and illustrated that the three biceps sites differed from BRAD in their responses to the F versus S tasks. These results provide further support that the synergy between biceps and BRAD is not fixed, and that the concept of flexor equivalence at the elbow does not hold under all torque task conditions     

The effects of massed versus distributed contractions on the variability of maximal isometric force

This study evaluated the effect of a massed versus distributed repetition schedule on the variability of force and surface electromyographic (sEMG) activity during maximal voluntary isometric elbow flexion contractions. The massed group (N = 13) performed 15 contractions on 1 day, while the distributed group (N = 13) performed 15 contractions across three consecutive days (five per day). Two retention tests (five contractions each) occurred 2 weeks and 3 months after the final trial of the initial test sessions. Force and sEMG of the biceps and triceps brachii muscles were recorded concurrently. Both groups had comparable increases in force and biceps brachii sEMG that continued over short- and longer-term retention tests (p < 0.05). Triceps brachii sEMG exhibited a more complicated pattern of successive decreases and increases (p < 0.05). The massed repetition schedule resulted in significantly (p < 0.05) less variability in maintaining a constant force [root mean square (RMS) error]. There was a significant decrease in the variability of the force–time and sEMG–time curves as assessed by the variance ratio (VR) (p < 0.05). Only biceps sEMG and VR correlated highly with force VR for the distributed group. Total (biceps + triceps) sEMG magnitude and variability correlated highly with both RMS error and force VR for the massed group. It was concluded that the massed contraction pattern allowed participants to learn how to regulate joint stiffness in addition to the variability of muscle activity. This allowed for greater decreases in RMS error than could be obtained by regulating the variability of muscle activity alone.     

Motor point map of upper body muscles

Purpose

The purpose of the present study was to systematically investigate the upper body motor point (MP) positions of selected muscles and to create an atlas of the identified MPs.

Methods

MPs were searched bilaterally in 15 male and 15 female subjects by scanning the skin with a special pen electrode at low stimulation frequency (3 Hz) and current amplitude (<10 mA). The following muscles were investigated: biceps brachii, triceps brachii, deltoideus, trapezius, latissimus dorsi, erector spinae (lumbar part), pectoralis minor and major, and rectus abdominis.

Results

A total of 1,563 MPs were identified. The MPs could be clustered into 31 distinct positions on each side of the body. However, the number of MPs per muscle varied between subjects: 2 MPs were found for the biceps brachii, 2–3 for the triceps brachii, 4–5 for the deltoideus, 2–3 for the pectoralis major, 1 MP for the pectoralis minor, 4–5 for the trapezius, 3–4 for the latissimus dorsi, 4–5 for the rectus abdominis, and 2–3 for the erector spinae in its lumbar part. Referring to the applied grid, upper limb and lower back muscles presented a low inter-individual variation, whereas MPs of the deltoideus, the pectoralis major, and the rectus abdominis were characterized by a poor homogeneity. All MPs were found to be highly symmetrical between both sides of the body (r = 0.96; p < 0.001).

Conclusion

The presented data and the corresponding map will help physiotherapists, and conditioning specialists improve their neuromuscular electrical stimulation therapy or training, respectively.     

Muscle architecture of biceps brachii, triceps brachii and supraspinatus in the horse

Three muscles from the proximal equine forelimb were dissected in order to investigate their potential to contribute to proximal limb mechanics. Muscle mass, fibre length, tendon mass and tendon length were measured from biceps brachii, triceps brachii, supraspinatus and lacertus fibrosus (biceps lateral head mass 171-343.4 g and fibre length 0.5-0.8 cm; biceps medial head mass 283-500 g and fibre length 2.2-4 cm; biceps tendon mass 121.8-260 g and tendon length 35-44 cm; triceps long head mass 3200-6663 g and fibre length 19-26.3 cm; triceps lateral head mass 513.8-1240 g and fibre length 17.5-24 cm; triceps medial head mass 85.2-270.6 g and fibre length 9-16.8 cm; supraspinatus mass 793-1546 g and fibre length 4.7-12.4 cm; lacertus fibrosus mass 4.6-12.4 g and length 10-16 cm). Physiological cross-sectional area (PCSA) and maximum isometric force were estimated for each muscle, and moment arm measurements were taken at the shoulder and elbow joints. Biceps has a greater isometric force-generating capacity than supraspinatus. It also appears to have a larger shoulder moment arm, so could therefore have the potential to make a greater contribution to the shoulder moment than supraspinatus. Supraspinatus is likely to function primarily as a shoulder stabilizer rather than a shoulder extensor. Biceps also functions as an elbow flexor and data here indicate that it has a greater PCSA and isometric force-generating capacity than its antagonist triceps brachii. Calculation of tendon forces showed that the biceps tendon can withstand much greater forces than lacertus fibrosus. This study will enable further investigation into the interaction between energy recycling in elastic tissues and the generation and absorption of mechanical work by adjacent muscle groups in the equine forelimb.     

Effect of neuromuscular electrical stimulation intensity over the tibial nerve trunk on triceps surae muscle fatigue

Purpose

This study was designed to investigate whether the intensity modulation of a neuromuscular electrical stimulation (NMES) protocol delivered over the nerve trunk of the plantar flexors would lead to differential peripheral and central contributions of muscle fatigue.

Methods

Three fatiguing isometric protocols of the plantar flexors matched for the same amount of isometric torque-time integral (TTI) were randomly performed including a volitional protocol at 20 % of the maximal voluntary contraction (MVC) and two NMES protocols (one at constant intensity, CST; the other at intensity level progressively adjusted to maintain 20 % of MVC, PROG).

Results

No time x protocol interaction was found for any of the variables. The MVC decreased similarly (≈12 %, p < 0.001) after all protocols, so did the potentiated twitch responses (p = 0.001). Although voluntary activation of the plantar flexors did not change, maximal H-reflex to M-wave ratio of the soleus (SOL) and the gastrocnemius medialis (GM) muscles showed an overall increase (SOL: p = 0.037, GM: p = 0.041), while it remained stable for the gastrocnemius lateralis muscle (p = 0.221). A main time effect was observed only for the SOL maximal V-wave to the superimposed M-wave ratio (p = 0.024) and to the superimposed H-reflex (p = 0.008). While similar central and peripheral adaptations were observed after the three fatiguing protocols, the individual contribution of the three different triceps surae muscles was different.

Conclusion

Whether the current intensity was increased or not, the adaptations after a NMES protocol yield to similar muscle fatigue adaptations as voluntary contractions likely through similar pathways matching a similar TTI.     

Mechanomyographic responses during voluntary ramp contractions of the human first dorsal interosseous muscle

The aim of this study was to examine the mechanomyogram (MMG) and force relationship of the first dorsal interosseous (FDI) muscle as well as the biceps brachii (BB) muscle during voluntary isometric ramp contractions, and to elucidate the MMG responses resulting from the intrinsic motor unit (MU) activation strategy of FDI muscle with reference to the MMG of BB muscle. The subjects were asked to exert ramp contractions of FDI and BB muscle from 5% to 70% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. In FDI muscle, the root-mean-squared amplitude (RMS) of the MMG decreased slowly with force up to 21%, and then a progressive increase was followed by a relatively rapid decrease beyond 41% MVC. The RMS/%MVC relationship in BB muscle consisted of an initial slow increase followed by a rapid increase from 23% MVC and a progressive decrease beyond 61% MVC. With respect to the mean power frequency (MPF), FDI muscle demonstrated no obvious inflection point in the MPF/%MVC relationship compared with that in BB muscle. Namely, the MPF of FDI muscle increased linearly through the force levels exerted. In contrast to FDI muscle, the MPF/%MVC relationship in BB muscle was decomposed into four specific regions: (1) a relative rapidly increase (<34% MVC), (2) a slow increment (34–53% MVC), (3) a temporary reduction (53–62% MVC), and (4) a further rapid increase (>62% MVC). The different MMG responses between FDI and BB muscles are considered to reflect the fact that the MU activation strategy varies among different muscles in relation to their morphology and histochemical type. Namely, the rate coding of the MUs plays a more prominent role in force production in relatively small FDI muscle than does MU recruitment compared with their respective roles in the relatively large BB muscle.     

The synergy of elbow extensor muscles during dynamic work in man

Summary This study concerns the synergy of the elbow extensor muscles during voluntary elbow extension performed at various velocities against different inertia and in a horizontal plane.The quantity of excitation (Q) of the three heads of the triceps brachii muscle increases linearly with the external work (W). On the contrary, the anconeus Q increases rapidly for low values of W while for higher values it increases very slowly but linearly with the W. These results imply a proportionality between Q and W of the different muscles except in the case of anconeus for low values of W. Furthermore, it is shown that the position of the shoulder does not influence the relative contribution of the different muscles to W but leads to an adjustment of the activity of the long head.The different extensor muscles do not come into play simultaneously. The chronology of the muscular activities depends on velocity and inertia.The importance of anconeus activity for slow movements is pointed out, and it is suggested that there exists a specific control of the motoneurons of the different elbow extensor muscles. The limits of the concept of muscle equivalent are also discussed.     

EMG activation patterns during force production in precision grip. III. Synchronisation of single motor units

Motor unit (MU) synchronisation during isometric force production in the precision grip was analysed in five subjects performing a visually guided steptracking motor task with three different force levels. With this aim multi-unit electromyographic (EMG) activity of 14 intrinsic and extrinsic finger muscles from 15 experimental sessions was decomposed into the potentials of single MUs. The behaviour of 62 intrinsic and 30 extrinsic MUs in the motor task was quantified. Most MUs displayed a positive correlation between firing rate and grip force. Compared to MUs in extrinsic muscles, intrinsic MUs had steeper regression lines with negative intercepts indicating higher force sensitivity and higher recruitment thresholds. A cross-correlation analysis was performed for 69 intra- and 166 intermuscular MU pairs while steady grip force was exerted at the three force levels. Synchronisation, for at least one force level, was found in 78% of the intra- and 45% of the intermuscular pairs. The occurrence of synchronisation was not stable over the force range tested. Factors influencing the fluctuations in occurrence and strength of synchronisation were investigated. Force increase was not paralleled by increased synchronisation; in contrast, in most MU pairs, especially intermuscular pairs, synchronisation occurred preferentially at the lower force levels. The recruitment threshold appeared to play a determining role in synchronisation: the more similar the thresholds of two MUs, the greater the probability of them being synchronised at this force level. Synchronised MUs fired on average at a lower frequency than non-synchronised ones. Finally, synchronisation at the multi-unit EMG level does not indicate that all underlying MUs are synchronised, nor does the absence of temporal coupling at the multi-unit level indicate that none of the MUs is synchronised.