Muscle-pair specific distribution and grip-type modulation of neural common input to extrinsic digit flexors

To gain insight into the synergistic control of hand muscles, we have recently quantified the strength of correlated neural activity across motor units from extrinsic digit flexors during a five-digit object-hold task. We found stronger synchrony and coherence across motor units from thumb and index finger flexor muscle compartment than between the thumb flexor and other finger flexor muscle compartments. The present study of two-digit object hold was designed to determine the extent to which such distribution of common input among thumb-finger flexor muscle compartments, revealed by holding an object with five digits, is preserved when varying the functional role of a given digit pair. We recorded normal force exerted by the digits and electrical activity of single motor units from muscle flexor pollicis longus (FPL) and two compartments of the m. flexor digitorum profundus (FDP2 and FDP3; index and middle finger, respectively). Consistent with our previous results from five-digit grasping, synchrony and coherence across motor units from FPL-FDP2 was significantly stronger than in FPL-FDP3 during object hold with two digits [common input strength: 0.49 +/- 0.02 and 0.35 +/- 0.02 (means +/- SE), respectively; peak coherence: 0.0054 and 0.0038, respectively]. This suggests that the distribution of common neural input is muscle-pair specific regardless of grip type. However, the strength of coherence, but not synchrony, was significantly stronger in two- versus five-digit object hold for both muscle combinations, suggesting the periodicity of common input is sensitive to grip type.     

Periodic modulation of motor-unit activity in extrinsic hand muscles during multidigit grasping

We recently examined the extent to which motor units of digit flexor muscles receive common input during multidigit grasping. This task elicited moderate to strong motor-unit synchrony (common input strength, CIS) across muscles (flexor digitorum profundus, FDP, and flexor pollicis longus, FPL) and across FDP muscle compartments, although the strength of this common input was not uniform across digit pairs. To further characterize the neural mechanisms underlying the control of multidigit grasping, we analyzed the relationship between firing of single motor units from these hand muscles in the frequency domain by computing coherence. We report three primary findings. First, in contrast to what has been reported in intrinsic hand muscles, motor units belonging to different muscles and muscle compartments of extrinsic digit flexors exhibited significant coherence in the 0- to 5- and 5- to 10-Hz frequency ranges and much weaker coherence in the higher 10-20 Hz range (maximum 0.0025 and 0.0008, respectively, pooled across all FDP compartment pairs). Second, the strength and incidence of coherence differed considerably across digit pairs. Third, contrary to what has been reported in the literature, across-muscle coherence can be stronger and more prevalent than within-muscle coherence, as FPL-FDP2 (thumb-index digit pair) exhibited the strongest and most prevalent coherence in our data (0.010 and 43% at 3 Hz, respectively). The heterogeneous organization of common input to these muscles and muscle compartments is discussed in relation to the functional role of individual digit pairs in the coordination of multiple digit forces in grasping.     

Common input to motor neurons innervating the same and different compartments of the human extensor digitorum muscle

Short-term synchronization of active motor units has been attributed in part to last-order divergent projections that provide common synaptic input across motor neurons. The extent of synchrony thus allows insight as to how the inputs to motor neurons are distributed. Our particular interest relates to the organization of extrinsic finger muscles that give rise distally to multiple tendons, which insert onto all the fingers. For example, extensor digitorum (ED) is a multi-compartment muscle that extends digits 2-5. Given the unique architecture of ED, it is unclear if synaptic inputs are broadly distributed across the entire pool of motor neurons innervating ED or segregated to supply subsets of motor neurons innervating different compartments. Therefore the purpose of this study was to evaluate the degree of motor-unit synchrony both within and across compartments of ED. One hundred and forty-five different motor-unit pairs were recorded in the human ED of nine subjects during weak voluntary contractions. Cross-correlation histograms were generated for all of the motor-unit pairs and the degree of synchronization between two units was assessed using the index of common input strength (CIS). The degree of synchrony for motor-unit pairs within the same compartment (CIS = 0.7 +/- 0.3; mean +/- SD) was significantly greater than for motor-unit pairs in different compartments (CIS = 0.4 +/- 0.22). Consequently, last-order synaptic projections are not distributed uniformly across the entire pool of motor neurons innervating ED but are segregated to supply subsets of motor neurons innervating different compartments.     

Common input to motor units of intrinsic and extrinsic hand muscles during two-digit object hold

Anatomical and physiological evidence suggests that common input to motor neurons of hand muscles is an important neural mechanism for hand control. To gain insight into the synaptic input underlying the coordination of hand muscles, significant effort has been devoted to describing the distribution of common input across motor units of extrinsic muscles. Much less is known, however, about the distribution of common input to motor units belonging to different intrinsic muscles and to intrinsic-extrinsic muscle pairs. To address this void in the literature, we quantified the incidence and strength of near-simultaneous discharges of motor units residing in either the same or different intrinsic hand muscles (m. first dorsal, FDI, and m. first palmar interosseus, FPI) during two-digit object hold. To extend the characterization of common input to pairs of extrinsic muscles (previous work) and pairs of intrinsic muscles (present work), we also recorded electromyographic (EMG) activity from an extrinsic thumb muscle (m. flexor pollicis longus, FPL). Motor-unit synchrony across FDI and FPI was weak (common input strength, CIS, mean ± SE: 0.17 ± 0.02). Similarly, motor units from extrinsic-intrinsic muscle pairs were characterized by weak synchrony (FPL-FDI: 0.25 ± 0.02; FPL-FPI: 0.29 ± 0.03) although stronger than FDI-FPI. Last, CIS from within FDI and FPI was more than three times stronger (0.70 ± 0.06 and 0.66 ± 0.06, respectively) than across these muscles. We discuss present and previous findings within the framework of muscle-pair specific distribution of common input to hand muscles based on their functional role in grasping.     

Motor-unit synchrony within and across compartments of the human flexor digitorum superficialis

An interesting feature of the muscular organization of the human hand is that the main flexors and extensors of the fingers are compartmentalized and give rise to multiple parallel tendons that insert onto all the fingers. Previous studies of motor-unit synchrony in extensor digitorum and flexor digitorum profundus indicated that synaptic input to motor neurons supplying these multitendoned muscles is not uniformly distributed across the entire pool of motor neurons but instead appears to be partially segregated to supply subsets of motor neurons that innervate different muscular compartments. Little is known, however, about the organization of the synaptic inputs to the motor neurons supplying another multitendoned finger muscle, the flexor digitorum superficialis (FDS). Therefore in this study, we estimated the extent of divergence of last-order inputs to FDS motor neurons by measuring the degree of short-term synchrony among motor units within and across compartments of FDS. The degree of synchrony for motor-unit pairs within the same digit compartment was nearly twofold that of pairs of motor units in adjacent compartments and more than fourfold that of pairs in nonadjacent compartments. Therefore like other multitendoned muscles of the hand, last-order synaptic inputs to motor neurons supplying the FDS appear to primarily supply subsets of motor neurons innervating specific finger compartments. Such an organization presumably enables differential activation of separate compartments to facilitate independent movements of the fingers.     

Role of across-muscle motor unit synchrony for the coordination of forces

Evidence from five-digit grasping studies indicates that grip forces exerted by pairs of digits tend to be synchronized. It has been suggested that motor unit synchronization might be a mechanism responsible for constraining the temporal relationships between grip forces. To evaluate this possibility and quantify the effect of motor unit synchrony on force relationships, we used a motor unit model to simulate force produced by two muscles using three physiological levels of motor unit synchrony across the two muscles. In one condition, motor units in the two muscles discharged independently of one another. In the other two conditions, the timing of randomly selected motor unit discharges in one muscle was adjusted to impose low or high levels of synchrony with motor units in the other muscle. Fast Fourier transform analysis was performed to compute the phase differences between forces from 0.5 to 17 Hz. We used circular statistics to assess whether the phase differences at each frequency were randomly or non-randomly distributed (Rayleigh test). The mean phase difference was then computed on the non-random distributions. We found that the number of significant phase-difference distributions increased markedly with increasing synchronization strength from 18% for no synchrony to 65% and 82% for modest and strong synchrony conditions, respectively. Importantly, most of the mean angles clustered at very small phase difference values (~0 to 10°), indicating a strong tendency for forces to be exerted in a synchronous fashion. These results suggest that motor unit synchronization could play a significant functional role in the coordination of grip forces.     

Thumb and finger forces produced by motor units in the long flexor of the human thumb

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers ( P < 0.001). On average, these motor units produced small but significant loading forces on the index finger (∼5.3% of their force on the thumb) with the same time-to-peak force as the thumb (∼50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2–10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.     

Enslaving effects in multi-finger force production

When a person produces isometric force with one, two, or three fingers, the other fingers of the hand also produce a certain force. Enslaving is the involuntary force production by fingers not explicitly involved in a force-production task. This study explored the enslaving effects (EE) in multi-finger tasks in which the contributions of the flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and intrinsic muscles (INT) were manipulated. A new experimental technique was developed that allows the redistribution of the muscle activity between the FDP, FDS, and INT muscles. In the experiment, ten subjects were instructed to perform maximal voluntary contractions with all possible one-, two-, three-, and four-finger combinations. The point of force application was changed in parallel for the index, middle, ring, and little fingers from the middle of the distal phalanx, to the distal interphalangeal joint, and then to the proximal interphalangeal joint. It was found that: (1) the EE of similar amplitude were present in various experimental conditions that involved different muscle groups for force production; (2) the EE were large on average--the slave fingers could produce forces reaching 67.5% of the maximal forces produced by themselves in a single-finger task; (3) the EE were larger for neighboring fingers; and (4) the EE were non-additive--in most cases, the EE from two or three fingers were smaller than the EE from at least one finger. EE among different muscles suggest a widespread neural interaction among the structures controlling flexor muscles in the hand as the main mechanism of finger enslaving.     

Short-term synchronization between motor units in different functional subdivisions of the human flexor digitorum profundus muscle

The ability to independently move the digits is limited by peripheral as well as central factors. A central limitation to independent finger movements might arise from the inability of the human nervous system to activate motor units (MUs) that exert force on one finger without also activating MUs that exert force on adjacent fingers. Short-term synchronization between MU pairs is thought to be the result of the two motoneurons receiving common input from last-order neuronal projections. The human flexor digitorum profundus (FDP) muscle contains four subdivisions, one for each of the fingers. We hypothesized that the distribution of MU synchrony within and between subdivisions of FDP might parallel the ability to selectively activate different functional subdivisions within FDP, and the ability to flex one digit independently of another. We found that the degree of MU synchrony indeed was not uniform among the different functional subdivisions of FDP; MUs acting on ulnar digits (d5, d4) were more synchronized than MUs acting on radial digits (d2, d3). Furthermore, synchrony was observed between MU pairs where each unit acted on a different digit and was highest when both units of a pair acted on the least-independent digits (d4, d5). This indicates that the CNS does not exert completely independent control over the different functional subdivisions of FDP. The strength of synchrony appears related to the inability to produce completely independent forces or movements with the digits. These observations reflect widespread divergence of last-order inputs within the FDP motoneuron pool, and we suggest that the organization of the CNS drive to this muscle contributes to the limited ability of humans to flex one digit in isolation from other digits.     

Voluntary activation of the different compartments of the flexor digitorum profundus

Flexor digitorum profundus (FDP), the sole flexor of the fingertips, is critical for tasks such as grasping. It is a compartmentalized multitendoned muscle with both neural and mechanical links between the fingers. We determined whether voluntary activation (VA), the level of neural drive to muscle, could be measured separately in its four compartments, whether VA differed between the fingers, and whether maximal voluntary contraction (MVC) force and VA changed when the non-test fingers were extended from full flexion to 90° flexion to partially "disengage" the test finger. Transcranial magnetic stimulation (TMS) of the motor cortex was used to measure VA, in a position in which only FDP generated force at the fingertip. Despite differences among the fingers in MVCs, VA for each finger was ～92% (n = 8), with no differences between fingers. When the test finger was partially disengaged by extending the other fingers to 90° flexion, performance was more variable both within and between subjects. MVCs decreased significantly by about 25-40% for the four fingers. However, VA was not significantly changed (n = 6) and was similar for the four fingers. In both positions, there were strong linear relationships between the voluntary forces and the superimposed twitch sizes, indicating that the method to measure VA was very reliable. Our results indicate that maximal VA is similar for all four compartments of FDP when force production by the other fingers is unconstrained. When altered mechanical connections between the compartments decrease voluntary force output there is little difference in neural drive.     

Systemic salt loading decreases body temperature and increases heat-escape/cold-seeking behaviour via the central AT1 and V1 receptors in rats

In humans, the flexor digitorum profundus (FDP), which is a multi-tendoned muscle, produces forces that flex the four distal interphalangeal joints of the fingers. We determined whether the force associated with activity in a single motor unit in the FDP was confined to a single finger or distributed to more than one finger during a natural grasp. The discharge of single low-threshold motor units ( n = 69) was recorded at sites across the muscle during weak voluntary grasping involving all fingers and spike-triggered averaging of the forces under each of the finger pads was used to assess the distribution pattern. Spike-triggered averaging revealed that time-locked changes in force occurred under the 'test' finger (that finger on which the unit principally acted) as well as under the 'non-test' fingers. However, for the index-, middle- and ring-finger units, the changes in force under non-test fingers were typically small (< 20 % of those under the test finger). For little-finger units, the mean changes in force under the adjacent ring finger were large (>50 % of those under the test finger). The distribution of forces by little-finger units differed significantly from that for each of the other three fingers. Apart from increases in force under non-test fingers, there was occasional unloading of adjacent fingers (22/267 combinations), usually affecting the index finger. The increases in force under the test finger correlated significantly with the background force for units acting on the middle, ring and little fingers. During a functional grasp, the activity of single units in the FDP allows for a relatively selective control of forces at the tips of the index, middle and ring fingers, but this is limited for little-finger units.     

High stiffness of human digital flexor tendons is suited for precise finger positional control

The objective of this study was to define the biomechanical properties of the human digital flexor tendons and to compare these biomechanical properties to other muscle-tendon units in the forearm. Mechanical measurements were performed on fresh-frozen tendons under physiological load and temperature conditions. Loads were determined by first measuring the physiological cross-sectional area of each digital belly of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) and estimating maximum tension (P(o)) of that specific muscle head. Loading each tendon to the appropriate P(o) resulted in no significant difference in tendon strain among any of the tendons within each muscle (P > 0.05; digits 2-5) or between muscle types (FDP vs. FDS). The one exception to this finding was that a significantly higher strain at Po was observed in the FDP tendon to the small finger (P < 0.05). Average absolute strains observed for the FDP and FDS tendons (1.20 +/- 0.38%, mean +/- SD; n = 39) were significantly lower than those observed previously in a study of the prime movers of the wrist. The measured strain of approximately 1.5% was less than half of that predicted to occur in muscles of this architectural design. Modeling sarcomere shortening magnitudes during FDP or FDS contraction yielded a value of only 0.10 microm, which would have a negligible effect on the force generating capacity of these muscles. Thus the high stiffness of the digital flexor tendons suits them well for fine positional control and would render their muscle spindles quite sensitive to length perturbations at the fingertips.     

Limited ability to extend the digits of the human hand independently with extensor digitorum

While the human hand has an extraordinary capacity to manipulate objects, movement of its digits is usually not completely independent. These limits have been documented for extrinsic flexor muscles, although hand skills also require selectivity for extension movements. Hence, we measured the degree of independent control of the major extrinsic extensor (extensor digitorum, ED). Subjects grasped a cylinder, with the thumb perpendicular to the fingers. Load cells were connected to the proximal phalanges of the fingers and the thumb's distal phalanx. Intramuscular recordings using needle electrodes were made from the individual digital compartments of ED. Subjects were instructed to extend each digit isometrically in a voluntary ramp contraction to 50% maximal force. In total, the behaviour of 283 single motor units was analysed. More than half of the units associated with one 'test' finger were recruited inadvertently when another digit contracted to 50% maximum, with most units being recruited by extension of the adjacent digits. Usually, test motor units were recruited at higher forces by extension of fingers further from the test finger. Unexpectedly, extension of the thumb recruited many motor units acting on the little finger. Across tasks, at recruitment of the test motor units, the force produced by the test finger often differed between the voluntary and inadvertent contractions. Overall, the independent control of the output of ED is limited; this may reflect 'spill-over' of motor commands to other digital extensor compartments. This level of control of the extrinsic extensor muscles is more independent than the control of the deep extrinsic flexor muscle but less independent than that of the superficial extrinsic flexor muscle.     

Relations between surface EMG of extrinsic flexors and individual finger forces support the notion of muscle compartments

The goal of this experiment was to investigate the relationship between individual fingertip forces and the surface EMG of multi-digit muscles. The surface EMG of the hand extrinsic flexors (flexor digitorum profundis and superficialis) was recorded in eight subjects during multi-digit force production tasks. In one session, subjects pressed with all four fingers (IMRL, I=index, M=middle, R=ring, and L=little finger) with the total force ranging from 10% and 90% of their maximum force (MVC). Results showed a close linear relationship between an integrated EMG index and force. In another session, subjects produced constant total force of either 10% or 30% of their IMRL MVC, with different finger combinations such that the degree of involvement of each finger was manipulated (15 finger combinations were tested). The EMG level of the flexors depended greatly on the finger combination (P<0.001). Multi-variable regression made it possible to describe the flexor EMG as a linear function of individual fingertip forces. These results suggest that: (1) hand extrinsic flexors muscles are arranged in functional compartments serving individual fingers, and (2) each compartment has a force/EMG relationship that is close to being linear. Electronic Publication     

Distribution of motor unit force in human extensor digitorum assessed by spike-triggered averaging and intraneural microstimulation

A peculiar aspect of the muscular organization of the human hand is that the main flexors and extensors of the fingers are muscles that each give rise to four parallel tendons that insert on all the fingers. It has been hypothesized that these multi-tendoned muscles are comprised of functional compartments, with each finger controlled by a discrete population of motor units. The purpose of this study was to determine the force distribution across the four fingers for motor units in human extensor digitorum (ED), a multi-tendoned muscle that extends the fingers. The force distribution was assessed by spike-triggered averaging and intraneural microstimulation for 233 and 18 ED units, respectively. A selectivity index from 0 (force equally distributed across the fingers) to 1.0 (force concentrated on a single finger) was used to quantify the distribution of motor unit force across the four digits. The mean selectivity index was high for ED motor units assessed with intraneural microstimulation (0.90 +/- 0.28) and was significantly greater than that obtained with spike-triggered averaging (0.38 +/- 0.14). Therefore it is likely that each finger is acted on by ED through a discrete population of motor units and that weak synchrony between motor units in different compartments of ED may have contributed to the appearance of spike-triggered average force on multiple fingers. Moreover, the high selectivity of motor units for individual fingers may provide the mechanical substrate needed for highly fractionated movements of the human hand.     

Force synergies for multifingered grasping

Compared with the control of precision grips involving the thumb and one or two fingers, the control of grasping using the entire hand involves a larger number of degrees of freedom that has to be controlled simultaneously, and it introduces indeterminacies in the distribution of grip forces suitable for holding an object. We studied the control of five-digit grasping by measuring contact forces when subjects lifted, held, and replaced a manipulandum. This study focused primarily on the patterns of coordination among the normal forces exerted by each of the digits, assessed by varying the center of mass of the manipulandum. The force patterns during the lift and hold phases were modulated as a function of the location of the center of mass. A frequency domain analysis revealed a consistent temporal synergy by which digits tended to exert normal forces in phase with each other across all experimental conditions. This tendency for in-phase covariations by the normal forces exerted by the digits extended over the entire functional frequency range (up to 10 Hz). When the effect of thumb force was removed, a second synergy was revealed in which forces in two fingers could be modulated 180 degrees out of phase (also prevailing throughout the range of frequencies studied). The first synergy suggests the presence of a "common drive" to all of the extrinsic finger muscles, whereas the second one suggests another input, ultimately resulting in a reciprocally organized pattern of activity of some of these muscles.     

The effect of muscle length on motor-unit recruitment during isometric plantar flexion in humans

The triceps surae muscle group, consisting of the mono-articular soleus (SOL) and bi-articular gastrocnemius (GAS) muscles, primarily generates plantar flexor torque. Since the GAS muscle crosses the knee joint, flexion of the knee reduces the length of this muscle, thus limiting its contribution to torque output. However, it is not clearly understood how the central nervous system activates muscles that are at inefficient or non-optimal force-producing lengths. Therefore, the present study was designed to determine the effect of muscle length on motor-unit recruitment in the medial GAS muscle. Single motor-unit activity was recorded from the medial GAS muscle while electromyographic (EMG) activity was recorded from the SOL muscle in nine male subjects. With the ankle angle held constant at 90 degrees, the knee angle was changed from 180 degrees to 90 degrees, corresponding to a long and short GAS muscle length, respectively. Levels of voluntary plantar flexor torque were produced at a rate of 2 Nm.s-1 until motor-unit activity was detected. A total of 229 motor units were recorded, of which 121 and 108 were obtained at the long and short muscle lengths, respectively. At the short length, onset of motor-unit activity occurred at significantly higher levels of plantar flexor torque and SOL EMG. Onset of motor-unit activity occurred at 2.97 +/- 7.78 Nm and 32.14 +/- 10.25 Nm, corresponding to 0.045 +/- 0.075 mV and 0.231 +/- 0.129 mV of SOL EMG in the long and short positions, respectively. No individual GAS motor unit could be recorded at both muscle lengths. Motor units in the shortened GAS muscle may be influenced by peripheral afferents capable of reducing the excitability of the motoneurone pool. This may also reflect a specific inhibition of motor units having shortened, non-optimal fascicle lengths, and they are thereby incapable of contributing to plantar flexor torque.     

Recovery of hand grip strength and hand steadiness after exhausting manual stretcher carriage

Rescue activities frequently require not only substantial and sustained hand-grip forces but also a subtle coordination of hand and finger muscles, e.g. when manipulating injection syringes after manual stretcher carriage. We investigated the recovery kinetics of manual coordination and muscle strength after exhausting stretcher carriage (4.5 km/h, load at each handle bar: 25 kg). Hand steadiness (frequency and duration of wall contacts when holding a metal pin into a small bore) and parameters of hand-grip strength were determined in 15 male volunteers before and immediately after the stretcher carriage. Measurements were repeated after 0.5, 1, 4 and 24 h of recovery. Mean carrying time was 215±87 s (SD), mean transport distance amounted to 264±104 m. During the carriage test, forces at the stretcher handles oscillated in the order of ±50 N within each gait cycle. Immediately after exhaustion, hand steadiness was significantly deteriorated (threefold increase in frequency and duration of wall contacts), maximum and mean hand-grip force over 15 s were reduced by almost 20%. While the recovery of hand steadiness was complete by minute 30 after stretcher carriage, a significant reduction in maximum and mean hand-grip force by 12% could still be observed after 24 h. The present findings demonstrate that hand steadiness recovers much faster than maximum hand-grip strength after exhaustive manual stretcher carriage (less than 30 min vs. more than 24 h). Probably, muscle damage induced in particular by the eccentric components during stretcher transport seems to affect only the generation of large forces. By contrast, the generation and coordination of the much lower forces required for hand-steadiness appears to be impaired only during the short transient of metabolic recovery.     

Muscular and postural synergies of the human hand

Because humans have limited ability to independently control the many joints of the hand, a wide variety of hand shapes can be characterized as a weighted combination of just two or three main patterns of covariation in joint rotations, or "postural synergies." The present study sought to align muscle synergies with these main postural synergies and to describe the form of membership of motor units in these postural/muscle synergies. Seventeen joint angles and the electromyographic (EMG) activities of several hand muscles (both intrinsic and extrinsic muscles) were recorded while human subjects held the hand statically in 52 specific shapes (i.e., shaping the hand around 26 commonly grasped objects or forming the 26 letter shapes of a manual alphabet). Principal-components analysis revealed several patterns of muscle synergy, some of which represented either coactivation of all hand muscles, or reciprocal patterns of activity (above and below average levels) in the intrinsic index finger and thumb muscles or (to a lesser extent) in the extrinsic four-tendoned extensor and flexor muscles. Single- and multiunit activity was generally a multimodal function of whole hand shape. This implies that motor-unit activation does not align with a single synergy; instead, motor units participate in multiple muscle synergies. Thus it appears that the organization of the global pattern of hand muscle activation is highly distributed. This organization mirrors the highly fractured somatotopy of cortical hand representations and may provide an ideal substrate for motor learning and recovery from injury.     

Incomplete functional subdivision of the human multitendoned finger muscle flexor digitorum profundus: an electromyographic study

The human flexor digitorum profundus (FDP) sends tendons to all 4 fingers. One might assume that this multitendoned muscle consists of 4 discrete neuromuscular compartments each acting on a different finger, but recent anatomical and physiological studies raise the possibility that the human FDP is incompletely subdivided. To investigate the functional organization of the human FDP, we recorded electromyographic (EMG) activity by bipolar fine-wire electrodes simultaneously from 2 or 4 separate intramuscular sites as normal human subjects performed isometric, individuated flexion, and extension of each left-hand digit. Some recordings showed EMG activity during flexion of only one of the 4 fingers, indicating that the human FDP has highly selective core regions that act on single fingers. The majority of recordings, however, showed a large amount of EMG activity during flexion of one finger and lower levels of EMG activity during flexion of an adjacent finger. This lesser EMG activity during flexion of adjacent fingers was unlikely to have resulted from recording motor units in neighboring neuromuscular compartments, and instead suggests incomplete functional subdivision of the human FDP. In addition to the greatest agonist EMG activity during flexion of a given finger, most recordings also showed EMG activity during extension of adjacent fingers, apparently serving to stabilize the given finger against unwanted extension. Paradoxically, the functional organization of the human FDP-with both incomplete functional subdivision and highly selective core regions-may contribute simultaneously to the inability of humans to produce completely independent finger movements, and to the greater ability of humans (compared with macaques) to individuate finger movements.