Distribution of the forces produced by motor unit activity in the human flexor digitorum profundus

Evolution, biomechanics, and neurobiology converge to explain selective finger motor control

Humans use their fingers to perform a variety of tasks, from simple grasping to manipulating objects, to typing and playing musical instruments, a variety wider than any other species. The more sophisticated the task, the more it involves individuated finger movements, those in which one or more selected fingers perform an intended action while the motion of other digits is constrained. Here we review the neurobiology of such individuated finger movements. We consider their evolutionary origins, the extent to which finger movements are in fact individuated, and the evolved features of neuromuscular control that both enable and limit individuation. We go on to discuss other features of motor control that combine with individuation to create dexterity, the impairment of individuation by disease, and the broad extent of capabilities that individuation confers on humans. We comment on the challenges facing the development of a truly dexterous bionic hand. We conclude by identifying topics for future investigation that will advance our understanding of how neural networks interact across multiple regions of the central nervous system to create individuated movements for the skills humans use to express their cognitive activity.

Unintentional drifts in performance during one-hand and two-hand finger force production

We explored the phenomena of force drifts and unintentional finger force production (enslaving), and their dependence on visual feedback. Predictions have been drawn based on the theory of control with spatial referent coordinates for condition with feedback on instructed (master) finger force, enslaved finger force, and total force for one-hand and two-hand tasks. Subjects produced force under the different feedback conditions without their knowledge. No feedback condition was also used for the one-hand tasks. Overall, feedback of master finger force led to an increase in the enslaved force, feedback on the slave finger force led to a drop in the master force, feedback on the total force led to balanced drifts in the master force down and enslaved force up, and under the no-feedback condition, master and total force drifted down with large variability in the enslaved force drifts. The patterns were the same in both hands in the two-hand tasks when feedback was provided on the forces of one hand only (without subject's knowledge). The index of enslaving always drifted toward higher values. We interpret the findings as reflecting three main factors: drifts in the referent coordinates toward actual finger coordinates, spread of cortical excitation over representations of the fingers, and robust sharing of referent coordinates between the two hands in bimanual tasks. The large consistent drifts in enslaving toward higher values have to be considered in studies of multi-finger synergies.

How the hand has shaped sign languages

In natural languages, biological constraints push toward cross-linguistic homogeneity while linguistic, cultural, and historical processes promote language diversification. Here, we investigated the effects of these opposing forces on the fingers and thumb configurations (handshapes) used in natural sign languages. We analyzed over 38,000 handshapes from 33 languages. In all languages, the handshape exhibited the same form of adaptation to biological constraints found in tasks for which the hand has naturally evolved (e.g., grasping). These results were not replicated in fingerspelling—another task where the handshape is used—thus revealing a signing-specific adaptation. We also showed that the handshape varies cross-linguistically under the effects of linguistic, cultural, and historical processes. Their effects could thus emerge even without departing from the demands of biological constraints. Handshape’s cross-linguistic variability consists in changes in the frequencies with which the most faithful handshapes to biological constraints appear in individual sign languages.

Support for mechanical advantage hypothesis of grasping cannot be explained only by task mechanics

Successful object interaction during daily living involves maintaining the grasped object in static equilibrium by properly arranging the fingertip contact forces. According to the mechanical advantage hypothesis of grasping, during torque production tasks, fingers with longer moment arms would produce greater normal force than those with shorter moment arms. Previous studies have probed this hypothesis by investigating the force contributions of individual fingers through systematic variations (or perturbations) of the properties of the grasped handle. In the current study, we examined the validity of this hypothesis in a paradigm wherein the thumb tangential force was constrained to a minimal constant magnitude. This was achieved by placing the thumb on a freely movable slider platform. The total mass of the handle was systematically varied by adding external loads directly below the center of mass of the handle. Our findings suggest that the mechanical advantage hypothesis manifests only during the heaviest loading condition when a threshold difficulty is reached. We infer that the support for the mechanical advantage hypothesis depends not only on the physical parameters but also on the individual ability to manage the task.

The neural mechanisms of manual dexterity

The hand endows us with unparalleled precision and versatility in our interactions with objects, from mundane activities such as grasping to extraordinary ones such as virtuoso pianism. The complex anatomy of the human hand combined with expansive and specialized neuronal control circuits allows a wide range of precise manual behaviours. To support these behaviours, an exquisite sensory apparatus, spanning the modalities of touch and proprioception, conveys detailed and timely information about our interactions with objects and about the objects themselves. The study of manual dexterity provides a unique lens into the sensorimotor mechanisms that endow the nervous system with the ability to flexibly generate complex behaviour.

The Nature of Finger Enslaving: New Results and Their Implications

We present a review on the phenomenon of unintentional finger action seen when other fingers of the hand act intentionally. This phenomenon (enslaving) has been viewed as a consequence of both peripheral (e.g., connective tissue links and multifinger muscles) and neural (e.g., projections of corticospinal pathways) factors. Recent studies have shown relatively large and fast drifts in enslaving toward higher magnitudes, which are not perceived by subjects. These and other results emphasize the defining role of neural factors in enslaving. We analyze enslaving within the framework of the theory of motor control with spatial referent coordinates. This analysis suggests that unintentional finger force changes result from drifts of referent coordinates, possibly reflecting the spread of cortical excitation.

Adaptation of the Corticomuscular and Biomechanical Systems of Pianists

Independent control of movements between the fingers plays a role in hand dexterity characterizing skilled individuals. However, it remains unknown whether and in what manner neuromuscular and biomechanical constraints on the movement independence of the fingers depend on motor expertise. Here, we compared motor dexterity, corticospinal excitability of multiple muscles, muscular activation, and anatomical features of the fingers between the pianists and nonpianists. When the ring finger was passively moved by a robot, passive motions produced at the adjacent fingers were smaller for the pianists than the nonpianists, indicating reduced biomechanical constraint of fingers in the pianists. In contrast, when the ring finger moved actively, we found no group difference in passive motions produced at the adjacent fingers; however, reduced inhibition of corticospinal excitability of the adjacent fingers in the pianists compared with the nonpianists. This suggests strengthened neuromuscular coupling between the fingers of the pianists, enhancing the production of coordinated finger movements. These group differences were not evident during the index and little finger movements. Together, pianists show expertise-dependent biomechanical and neurophysiological adaptations, specifically at the finger with innately low movement independence. Such contrasting adaptations of pianists may subserve dexterous control of both the individuated and coordinated finger movements.

Attentional focus modulates automatic finger-tapping movements

The majority of human behaviors are composed of automatic movements (e.g., walking or finger-tapping) which are learned during nurturing and can be performed simultaneously without interfering with other tasks. One critical and yet to be examined assumption is that the attention system has the innate capacity to modulate automatic movements. The present study tests this assumption. Setting no deliberate goals for movement, we required sixteen participants to perform personalized and well-practiced finger-tapping movements in three experiments while focusing their attention on either different component fingers or away from movements. Using cutting-edge pose estimation techniques to quantify tapping trajectory, we showed that attention to movement can disrupt movement automaticity, as indicated by decreased inter-finger and inter-trial temporal coherence; facilitate the attended and inhibit the unattended movements in terms of tapping amplitude; and re-organize the action sequence into distinctive patterns according to the focus of attention. These findings demonstrate compelling evidence that attention can modulate automatic movements and provide an empirical foundation for theories based on such modulation in controlling human behavior.

Examining Additional Aspects of Muscle Function with a Digital Handgrip Dynamometer and Accelerometer in Older Adults: A Pilot Study

Background: Maximal handgrip strength (HGS) could be an incomplete and unidimensional measure of muscle function. This pilot study sought to examine the relationships between maximal HGS, radial and ulnar digit grip strength, submaximal HGS force control, HGS fatigability, neuromuscular HGS steadiness, and HGS asymmetry in older adults. Methods: A digital handgrip dynamometer and accelerometer was used to collect several HGS measurements from 13 adults aged 70.9 ± 4.0 years: maximal strength, radial and ulnar digit grip strength, submaximal force control, fatigability, neuromuscular steadiness, and asymmetry. Pearson correlations determined the relationships between individual HGS measurements. A principal component analysis was used to derive a collection of new uncorrelated variables from the HGS measures we examined. Results: The individual HGS measurements were differentially correlated. Maximal strength (maximal HGS, radial digit strength, ulnar digits strength), contractile steadiness (maximal HGS steadiness, ulnar digit grip strength steadiness), and functional strength (submaximal HGS force control, HGS fatigability, HGS asymmetry, HGS fatigability steadiness) emerged as dimensions from the HGS measurements that we evaluated. Conclusion: Our findings suggest that these additional measures of muscle function may differ from maximal HGS alone. Continued research is warranted for improving how we assess muscle function with more modern technologies, including handgrip dynamometry and accelerometry.

Comparable behaviour of ring and little fingers due to an artificial reduction in thumb contribution to hold objects

Background

The human hand plays a crucial role in accomplishing activities of daily living. The contribution of each finger in the human hand is remarkably unique in establishing object stabilization. According to the mechanical advantage hypothesis, the little finger tends to exert a greater normal force than the ring finger during a supination moment production task to stabilize the object. Similarly, during pronation, the index finger produces more normal force when compared with the middle finger. Hence, the central nervous system employs the peripheral fingers for torque generation to establish the equilibrium as they have a mechanical advantage of longer moment arms for normal force. In our study, we tested whether the mechanical advantage hypothesis is supported in a task in which the contribution of thumb was artificially reduced. We also computed the safety margin of the individual fingers and thumb.

Methodology

Fifteen participants used five-finger prismatic precision grip to hold a custom-built handle with a vertical railing on the thumb side. A slider platform was placed on the railing such that the thumb sensor could move either up or down. There were two experimental conditions. In the “Fixed” condition, the slider was mechanically fixed, and hence the thumb sensor could not move. In the “Free” condition, the slider platform on which the thumb sensor was placed could freely move. In both conditions, the instruction was to grasp and hold the handle (and the platform) in static equilibrium. We recorded tangential and normal forces of all the fingers.

Results

The distribution of fingertip forces and moments changed depending on whether the thumb platform was movable (or not). In the free condition, the drop in the tangential force of thumb was counteracted by an increase in the normal force of the ring and little finger. Critically, the normal forces of the ring and little finger were statistically equivalent. The safety margin of the index and middle finger did not show a significant drop in the free condition when compared to fixed condition.

Conclusion

We conclude that our results does not support the mechanical advantage hypothesis at least for the specific mechanical task considered in our study. In the free condition, the normal force of little finger was comparable to the normal force of the ring finger. Also, the safety margin of the thumb and ring finger increased to prevent slipping of the thumb platform and to maintain the handle in static equilibrium during the free condition. However, the rise in the safety margin of the ring finger was not compensated by a drop in the safety margin of the index and middle finger.

The effect of wrist posture on extrinsic finger muscle activity during single joint movements

Wrist posture impacts the muscle lengths and moment arms of the extrinsic finger muscles that cross the wrist. As a result, the electromyographic (EMG) activity associated with digit movement at different wrist postures must also change. We sought to quantify the posture-dependence of extrinsic finger muscle activity using bipolar fine-wire electrodes inserted into the extrinsic finger muscles of able-bodied subjects during unrestricted wrist and finger movements across the entire range of motion. EMG activity of all the recorded finger muscles were significantly different (p < 0.05, ANOVA) when performing the same digit movement in five different wrist postures. Depending on the wrist posture, EMG activity changed by up to 70% in individual finger muscles for the same movement, with the highest levels of activity observed in finger extensors when the wrist was extended. Similarly, finger flexors were most active when the wrist was flexed. For the finger flexors, EMG variations with wrist posture were most prominent for index finger muscles, while the EMG activity of all finger extensor muscles were modulated in a similar way across all digits. In addition to comprehensively quantifying the effect of wrist posture on extrinsic finger EMG activity in able-bodied subjects, these results may contribute to designing control algorithms for myoelectric prosthetic hands in the future.

Mapping of finger fascicles within the flexor digitorum superficialis and profundus muscles

INTRODUCTION

In this study we aimed to systematically investigate and map localization of the individual finger fascicles within the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP) muscles.

METHODS

Using B-mode ultrasonographic assessments, the right forearms of ten healthy participants (five males and five females) were examined during active and passive finger movements.

RESULTS

A topographical map indicating clear core areas of fascicle activation within the FDS and FDP muscles was created. Borders that could, to some degree, define individual differences were also indicated.

DISCUSSION

Our findings offer a detailed image of fascicle distributions within the FDS and FDP muscles. Various challenges, such as managing individual muscular profiles or the existence of the palmaris longus muscle, are discussed and should always be taken into consideration. The current map can serve as a general orientation for future measurements or injection therapies.

Effect of immobilization of the distal interphalangeal joint of fingers on grip strength

Arthrodesis of the distal interphalangeal joint of the finger is an effective salvage treatment for end-stage arthropathy of the joint. This study aimed to evaluate the effect of simulated fusion of individual distal interphalangeal joints on the overall grip strength of the hand. Custom moulded thermoplastic splints were used to simulate fusion by immobilizing the index, middle, ring and little fingers' distal interphalangeal joints in turn in both hands of 56 healthy participants. Testing was performed with no immobilization and after immobilization of each of the individual digits. Grip strengths reduced significantly following immobilization of the distal interphalangeal joint. The degree of reduction became progressively more pronounced from the index to the little fingers (12%, 18%, 24% and 25%, respectively) and was similar for the dominant and non-dominant hands. This information may have clinical application when counselling patients regarding fusion of the distal interphalangeal joint of the fingers.

The Effect of Isolated Finger Stiffness on Adjacent Digit Function

BACKGROUND

Isolated stiffness in a single finger can affect the function of adjacent digits and decrease overall hand function due to the quadriga phenomenon. This study objectively quantifies the dysfunctional impact of each individual stiff finger upon the remaining digits.

METHODS

Twenty-five individuals (10 men and 15 women) with a mean age of 31 years (range, 18-58 years) without any upper limb pathology, neuropathy, or systemic illness were recruited. Volar-based finger splints were used to hold individual digits of the dominant hand (24 right and 1 left) sequentially in full extension at the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints. Motion of the remaining 3 nonsplinted digits was assessed using a finger goniometer and linear scale to measure the total active range of motion (TAM) and fingertip-to-distal palmar crease (DPC) distance. TAM before and after splinting for each digit was compared using 1-way analysis of variance (ANOVA).

RESULTS

Splinting of any individual finger resulted in a significant reduction in the TAM of all adjacent fingers, regardless of which finger was splinted ( P < .001). Digits immediately adjacent to the splinted finger were more heavily impacted compared with nonadjacent digits. Splinting of the ring finger produced the greatest detriment, with a 26% to 47% reduction in the TAM and a DPC distance greater than 40 mm in a third of participants. The index finger caused the least disturbance to remaining digital motion.

CONCLUSIONS

Isolated finger stiffness causes a variable degree of dysfunction on adjacent normal digits. This emphasizes the need for a focused and proactive approach to restore full active motion following isolated finger injuries to prevent persistent functional sequelae of the hand.

Ulnar digits contribution to grip strength in patients with thumb carpometacarpal osteoarthritis is less than in normal controls

BACKGROUND

Grip testing is commonly used as an objective measure of strength in the hand and upper extremity and is frequently used clinically as a proxy measure of function. Increasing knowledge of hand biomechanics, muscle strength, and prehension patterns can provide us with a better understanding of the functional capabilities of the hand. The objectives of this study were to determine the contribution of ulnar digits to overall grip strength in individuals with thumb carpometacarpal (CMC) osteoarthritis (OA).

METHODS

Thirty-seven subjects participated in the study. This group consisted of 19 patients with CMC OA (aged 60-88 years) and 18 healthy subjects (60-88 years). Three hand configurations were used by the subjects during grip testing: use of the entire hand (index, middle, ring, and little fingers) (IMRL); use of the index, middle, and ring fingers (IMR); and use of only the index and middle fingers (IM).

RESULTS

Grip strength findings for the two groups found that compared to their healthy counterparts, CMC OA patients had, on average, a strength deficiency of 45.6, 35.5, and 28.8 % in IMRL, IMR, and IM, respectively. The small finger contribution to grip is 14.3 % and the ring and small finger contribute 34 % in subjects with CMC OA.

DISCUSSION

Grip strength decreases as the number of digits contributing decreased in both groups. The ulnar digits contribution to grip strength is greater than one third of total grip strength in subjects with CMC OA. Individuals with CMC OA demonstrate significantly decreased grip strength when compared to their healthy counterparts.

Bilateral force transients in the upper limbs evoked by single-pulse microstimulation in the pontomedullary reticular formation

Neurons in the pontomedullary reticular formation (PMRF) give rise to the reticulospinal tract. The motor output of the PMRF was investigated using stimulus-triggered averaging of electromyography (EMG) and force recordings in two monkeys (M. fascicularis). EMG was recorded from 12 pairs of upper limb muscles, and forces were detected using two isometric force-sensitive handles. Of 150 stimulation sites, 105 (70.0%) produced significant force responses, and 139 (92.5%) produced significant EMG responses. Based on the average flexor EMG onset latency of 8.3 ms and average force onset latency of 15.9 ms poststimulation, an electromechanical delay of ∼7.6 ms was calculated. The magnitude of force responses (∼10 mN) was correlated with the average change in EMG activity (P < 0.001). A multivariate linear regression analysis was used to estimate the contribution of each muscle to force generation, with flexors and extensors exhibiting antagonistic effects. A predominant force output pattern of ipsilateral flexion and contralateral extension was observed in response to PMRF stimulation, with 65.3% of significant ipsilateral force responses directed medially and posteriorly (P < 0.001) and 78.6% of contralateral responses directed laterally and anteriorly (P < 0.001). This novel approach permits direct measurement of force outputs evoked by central nervous system microstimulation. Despite the small magnitude of poststimulus EMG effects, low-intensity single-pulse microstimulation of the PMRF evoked detectable forces. The forces, showing the combined effect of all muscle activity in the arms, are consistent with reciprocal pattern of force outputs from the PMRF detectable with stimulus-triggered averaging of EMG.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Synergy-based hand pose sensing: Reconstruction enhancement

Many applications in human–machine interfaces, information visualization, rehabilitation and entertainment require hand pose reconstruction systems that are both accurate and economic. Unfortunately, economically and ergonomically viable sensing gloves provide limited precision due to the imperfect and incomplete correspondence of sensing models with the anatomical degrees of freedom of the human hand, and because of measurement noise. This paper examines the problem of optimally estimating the posture of a human hand using non-ideal sensing gloves. The main idea is to maximize their performance by exploiting knowledge of how humans most frequently use their hands. To increase the accuracy of pose reconstruction without modifying the glove hardware — hence basically at no extra cost — we propose the collection, organization, and exploitation of information on the probabilistic distribution of human hand poses in common tasks. We discuss how a database of such a priori information can be built, represented in a hierarchy of correlation patterns or postural synergies, and fused with glove data in a consistent way, so as to provide good hand pose reconstruction in spite of insufficient and inaccurate sensing data. Simulations and experiments on a low-cost glove are reported which demonstrate the effectiveness of the proposed techniques.

Effects of the index finger position and force production on the flexor digitorum superficialis moment arms at the metacarpophalangeal joints - a magnetic resonance imaging study

BACKGROUND

The purpose of this study was to use magnetic resonance imaging to measure the moment arm of the flexor digitorum superficialis tendon about the metacarpophalangeal joint of the index, middle, ring, and little fingers when the position and force production level of the index finger was altered. A secondary goal was to create regression models using anthropometric data to predict moment arms of the flexor digitorum superficialis about the metacarpophalangeal joint of each finger.

METHODS

The hands of subjects were scanned using a 3.0 T magnetic resonance imaging scanner. The metacarpophalangeal joint of the index finger was placed in: flexion, neutral, and extension. For each joint configuration subjects produced no active force (passive condition) and exerted a flexion force to resist a load at the fingertip (active condition).

RESULTS

The following was found: (1) The moment arm of the flexor digitorum superficialis at the metacarpophalangeal joint of the index finger (a) increased with the joint flexion and stayed unchanged with finger extension; and (b) decreased with the increase of force at the neutral and extended finger postures and did not change at the flexed posture. (2) The moment arms of the flexor digitorum superficialis tendon of the middle, ring, and little fingers (a) did not change when the index metacarpophalangeal joint position changed (P>0.20); and (b) The moment arms of the middle and little fingers increased when the index finger actively produced force at the flexed metacarpophalangeal joint posture. (4) The moment arms showed a high correlation with anthropometric measurements.

INTERPRETATION

Moment arms of the flexor digitorum superficialis change due to both changes in joint angle and muscle activation; they scale with various anthropometric measures.

Task-dependent inhomogeneous muscle activities within the bi-articular human rectus femoris muscle

The motor nerve of the bi-articular rectus femoris muscle is generally split from the femoral nerve trunk into two sub-branches just before it reaches the distal and proximal regions of the muscle. In this study, we examined whether the regional difference in muscle activities exists within the human rectus femoris muscle during maximal voluntary isometric contractions of knee extension and hip flexion. Surface electromyographic signals were recorded from the distal, middle, and proximal regions. In addition, twitch responses were evoked by stimulating the femoral nerve with supramaximal intensity. The root mean square value of electromyographic amplitude during each voluntary task was normalized to the maximal compound muscle action potential amplitude (M-wave) for each region. The electromyographic amplitudes were significantly smaller during hip flexion than during knee extension task for all regions. There was no significant difference in the normalized electromyographic amplitude during knee extension among regions within the rectus femoris muscle, whereas those were significantly smaller in the distal than in the middle and proximal regions during hip flexion task. These results indicate that the bi-articular rectus femoris muscle is differentially controlled along the longitudinal direction and that in particular the distal region of the muscle cannot be fully activated during hip flexion.

Constraints for control of the human hand

More than 30 muscles drive the hand to perform a multitude of essential dextrous tasks. Here we consider new views on the evolution of hand structure and on peripheral and central constraints for independent control of the digits of the hand. The human hand is widely assumed to have evolved from hands like those of African apes, yet recent studies have shown that our hands and those of the earliest hominids are very similar and unlike those of living apes. Understanding the limits of hand function may come from investigation of our last common ancestor with the great apes, rather than the great apes themselves. In the periphery, movement across the full range of joint space can be limited by mechanical linkages among the extrinsic muscles. Further, peripheral limits occur when the hand adopts some positions in which the contraction of muscles fails to move the joints on which they usually act; there is muscle 'disengagement' and functional paralysis for some actions. Surprisingly, the central nervous system drives the hand seamlessly through this landscape of mechanical limits. Central constraints on control of the individual digits include the spillover of neural drive to neighbouring muscles and their 'compartments', and the inability to make maximal muscle forces when multiple digits contract strongly which produces a force deficit. The pattern of these latter constraints correlates with amounts of daily use of each digit and favours enslaved extension to lift fingers from an object but selective flexion of fingers to contact it.

Changes in spatial distribution of flexor digitorum superficialis muscle activity is correlated to finger's action

Multitendoned extrinsic muscles of the human hand can be divided into several neuromuscular compartments (NMCs), each of which contributes to the ability of human finger to produce independent finger movements or force. The aim of this study was to investigate the changes in the spatial activation of flexor digitorum superficialis (FDS) during the fingertip force production with non-invasive multichannel surface electromyography (sEMG) technique. 7 healthy Subjects were instructed to match the target force level for 5s using individual index finger (I), individual middle finger (M) and the combination of the index and middle finger (IM) respectively. Simultaneously, a 2 × 6 electrode array was employed to record multichannel sEMG from FDS as finger force was produced. The entropy and center of gravity of the sEMG root mean square (RMS) map were computed to assess the spatial inhomogeneity in muscle activation and the change in spatial distribution of EMG amplitude related to the force generation of specific task finger. The results showed that the area and intensity of high amplitude region increased with force production, and the entropy increased with force level under the same task finger. The findings indicate that the change of spatial distribution of multitendoned extrinsic hand muscle activation is correlated to specific biomechanical functions.

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.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

In humans, hand performance has evolved from a crude multidigit grasp to skilled individuated finger movements. However, control of the fingers is not completely independent. Although musculotendinous factors can limit independent movements, constraints in supraspinal control are more important. Most previous studies examined either flexion or extension of the digits. We studied differences in voluntary force production by the five digits, in both flexion and extension tasks. Eleven healthy subjects were instructed either to maximally flex or extend their digits, in all single- and multidigit combinations. They received visual feedback of total force produced by “instructed” digits and had to ignore “noninstructed” digits. Despite attempts to maximally flex or extend instructed digits, subjects rarely generated their “maximal” force, resulting in a “force deficit,” and produced forces with noninstructed digits (“enslavement”). Subjects performed differently in flexion and extension tasks. Enslavement was greater in extension than in flexion tasks ( P = 0.019), whereas the force deficit in multidigit tasks was smaller in extension ( P = 0.035). The difference between flexion and extension in the relationships between the enslavement and force deficit suggests a difference in balance of spillover of neural drive to agonists acting on neighboring digits and focal neural drive to antagonist muscles. An increase in drive to antagonists would lead to more individualized movements. The pattern of force production matches the daily use of the digits. These results reveal a neural control system that preferentially lifts fingers together by extension but allows an individual digit to flex so that the finger pads can explore and grasp.

Contribution of the ulnar digits to grip strength

PURPOSE

To determine the contribution of ulnar digits to overall grip strength.

SUBJECTS

Fifty individuals (25 men and 25 women; 100 hands) with a mean age of 35.6 years (range 19 to 62 years) were tested. Exclusion criteria included previous history of hand injuries, entrapment neuropathies and systemic diseases.

METHODS

Ethics approval was granted before testing. A calibrated Jamar dynamometer (Lafayette Instrument Company, USA) was used to test subjects in three configurations: entire hand - index, middle, ring and little fingers; index, middle and ring fingers; and index and middle fingers. Little and ring fingers were excluded using generic hand-based finger splints. The order of testing was kept constant, and subjects were tested three times on each hand for each configuration. The average of the three trials at each configuration was recorded. Subjects received 1 min of rest between each testing configuration. The data were analyzed using a 3×2 repeated measures ANOVA with hand dominance and configuration as the within-subject factors, followed by two independent sample t tests to compare flexor digitorum superficialis (FDS) independence and FDS nonindependence on right and left hand grip strength measurements in the index, middle, ring and little condition.

RESULTS

Univariate results demonstrated that grip strength was significantly predicted by the interaction between hand dominance and configuration, while the parsing of the interaction term demonstrated greater grip strength across all levels of configuration for the dominant and nondominant hand. There were no significant differences between FDS independence and FDS nonindependence for either hand on grip strength.

DISCUSSION

The results indicate a significant decrease in grip strength as ulnar fingers were excluded. Furthermore, exclusion of the little finger has differing effects on the grip strength of the dominant and nondominant hands - the dominant hand had a greater loss of strength with the little finger excluded than the nondominant hand.

CONCLUSIONS

The ulnar two digits play a significant role in overall grip strength of the entire hand. In the present study, exclusion of the ulnar two digits resulted in a 34% to 67% decrease in grip strength, with a mean decrease of 55%. Exclusion of the little finger from a functional grip pattern decreased the overall grip strength by 33%. Exclusion of the ring finger from a functional grip pattern decreased the overall grip strength by 21%. It is clear that limitation of one or both of the ulnar digits adversely affects the strength of the hand. In addition, there was no significant difference between grip strength of FDS-independent and FDS-nonindependent subjects for either hand.

Selective activation of human finger muscles after stroke or amputation

Individuated finger movements of the human hand require selective activation of particular sets of muscles. Such selective activation is controlled primarily by the motor cortex via the corticospinal tract. Is this selectivity therefore lost when lesions damage the corticospinal tract? Or when the motor cortex reorganizes after amputation? We studied finger movements in normal human subjects and in patients who had recovered substantially from pure motor hemiparesis caused by lacunar strokes, which damage the corticospinal tract without affecting other pathways. Even after substantial recovery from these strokes, individuation of finger movements remained reduced-both for flexion/extension and for adduction/ abduction motion of the fingers. Stroke subjects regained the ability to move the instructed digit through a normal range, but unintentional motion of other digits was increased. This increase did not result from a change in the passive biomechanical coupling of the fingers. Rather, voluntary contractions of muscles that move the intended digit were accompanied by inappropriate contractions in muscles acting on additional digits. These observations suggest that the normal corticospinal system produces individuated finger movements not only by selectively activating certain muscles, but also by suppressing activation of other muscles during voluntary effort to move a given digit. In a separate experiment, reversible amputation of the hand was produced in normal subjects by ischemic nerve block at the wrist. Motor output to the intrinsic muscles and sensory input both become blocked under these conditions, effectively amputating the hand from the nervous system. But the long extrinsic muscles that flex and extend the digits remain normally innervated, and thus flexion forces still can be generated at the fingertips. During reversible amputation of the hand produced by ischemic nerve block, the ability of subjects to activate subdivisions of extrinsic muscles and to exert flexion force at individual fingertips continued to show essentially normal selectivity. Voluntary activation of the remaining muscles thus continues to be selective after amputation, in spite of both the loss of sensory input from the amputated hand, and reorganization within the primary motor cortex. During cortical reorganization after amputation, then, voluntary patterns of motor output intended for finger muscles may not be lost. We therefore examined activity in the stump muscles of above-elbow amputees, who have no remaining hand muscles. Different movements of the phantom hand were accompanied by different patterns of EMG in remaining proximal muscles, distinct from the EMG patterns associated with movement of the phantom elbow. We infer that voluntary motor output patterns that normally control finger movements after amputation may become diverted to remaining proximal muscles.

Multifinger prehension: an overview

The authors review the available experimental evidence on what people do when they grasp an object with several digits and then manipulate it. The article includes three parts, each addressing a specific aspect of multifinger prehension. In the first part, the authors discuss manipulation forces (i.e., the resultant force and moment of force exerted on the object) and the digits' contribution to such forces' production. The second part deals with internal forces defined as forces that cancel each other and do not disturb object equilibrium. The authors discuss the role of the internal forces in maintaining the object stability, with respect to such issues as slip prevention, tilt prevention, and resistance to perturbations. The third part is devoted to the motor control of prehension. It covers such topics as prehension synergies, chain effects, the principle of superposition, interfinger connection matrices and reconstruction of neural commands, mechanical advantage of the fingers, and the simultaneous digit adjustment to several mutually reinforcing or conflicting demands.

Interaction of finger enslaving and error compensation in multiple finger force production

Previous studies have documented two patterns of finger interaction during multi-finger pressing tasks, enslaving and error compensation, which do not agree with each other. Enslaving is characterized by positive correlation between instructed (master) and non-instructed (slave) finger(s) while error compensation can be described as a pattern of negative correlation between master and slave fingers. We hypothesize that pattern of finger interaction, enslaving or compensation depends on the initial force level and the magnitude of the targeted force change. Subjects were instructed to press with four fingers (I index, M middle, R ring, and L little) from a specified initial force to target forces following a ramp target line. Force-force relations between master and each of three slave fingers were analyzed during the ramp phase of trials by calculating correlation coefficients within each master-slave pair and then two-factor ANOVA was performed to determine effect of initial force and force increase on the correlation coefficients. It was found that, as initial force increased, the value of the correlation coefficient decreased and in some cases became negative, i.e. the enslaving transformed into error compensation. Force increase magnitude had a smaller effect on the correlation coefficients. The observations support the hypothesis that the pattern of inter-finger interaction--enslaving or compensation--depends on the initial force level and, to a smaller degree, on the targeted magnitude of the force increase. They suggest that the controller views tasks with higher steady-state forces and smaller force changes as implying a requirement to avoid large changes in the total force.

Finger force perception during ipsilateral and contralateral force matching tasks

The aims of the present study were to compare matching performance between ipsilateral and contralateral finger force matching tasks and to examine the effect of handedness on finger force perception. Eleven subjects were instructed to produce reference forces by an instructed finger (index-I or little-L finger) and to reproduce the same amount force by the same or a different finger within the hand (i.e., ipsilateral matching task), or by a finger of the other hand (i.e., contralateral matching task). The results of the ipsilateral and contralateral tasks in the present study commonly showed that (1) the reference and matching forces were matched closely when the two forces were produced by the same or homologous finger(s) such as I/I task; (2) the weaker little finger underestimated the magnitude of reference force of the index finger (I/L task), even with the higher level of effort (relative force), but the two forces were matched when considering total finger forces; (3) the stronger index finger closely matched the reference force of the little finger with the lower level of relative force (i.e., L/I task); (4) when considering the constant errors, I/L tasks showed an underestimation and L/I tasks showed an overestimation compared to I/I tasks. There was no handedness effect during ipsilateral tasks. During the contralateral task, the dominant hand overestimated the force of the non-dominant hand, while the non-dominant hand attempted to match the absolute force of the dominant hand. The overall results support the notion that the absolute, rather than relative, finger force is perceived and reproduced during ipsilateral and contralateral finger force matching tasks, indicating the uniqueness of finger force perception.

Hand digit control in children: age-related changes in hand digit force interactions during maximum flexion and extension force production tasks

We studied the finger interactions during maximum voluntary force (MVF) production in flexion and extension in children and adults. The goal of this study was to investigate the age-related changes and flexion-extension differences of MVF and finger interaction indices, such as finger inter-dependency (force enslaving (FE): unintended finger forces produced by non-instructed fingers during force production of an instructed finger), force sharing (FS; percent contributions of individual finger forces to the total force at four-finger MVF), and force deficit (FD; force difference between single-finger MVF and the force of the same finger at four-finger MVF). Twenty-five right-handed children of 6-10 years of age and 25 adults of 20-24 years of age participated as subjects in this study (five subjects at each age). During the experiments, the subjects had their forearms secured in armrests. The subjects inserted the distal phalanges of the right hand into C-shaped aluminum thimbles affixed to small force sensors with 200 of flexion about the metacarpophalangeal (MCP) joint. The subjects were instructed to produce their maximum isometric force with a single finger or all four fingers in flexion or extension. In order to examine the effects of muscle-force relationship on MVF and other digit interaction indices, six subjects were randomly selected from the group of 25 adult subjects and asked to perform the same experimental protocol described above. However, the MCP joint was at 800 of flexion. The results from the 20' of MCP joint flexion showed that (1) MVF increased and finger inter-dependency decreased with children's age, (2) the increasing and decreasing absolute slopes (N/year) from regression analysis were steeper in flexion than extension while the relative slopes (%/year) with respect to adults' maximum finger forces were higher in extension than flexion, (3) the larger MVF, FE, and FD were found in flexion than in extension, (4) the finger FS was very similar in children and adults, (5) the FS pattern of individual fingers was different for flexion and extension, and (6) the differences between flexion and extension found at 20 degrees MCP joint conditions were also valid at 80 degrees MCP joint conditions. We conclude that (a) the finger strength and independency increase from 6 to 10 years of age, and the increasing trends are more evident in flexion than in extension as indexed by the absolute slopes, (b) the finger strength and finger independency is greater in flexion than in extension, and (c) the sharing pattern in children appears to develop before 6 years of age or it is an inherent property of the hand neuromusculoskletal system. One noteworthy observation, which requires further investigation, was that FE was slightly smaller in the 80 degrees condition than in the 20 degrees condition for flexion, but larger for extension for all subjects. This may be interpreted as a greater FE when flexor or extensor muscles are stretched.

Neuromuscular determinants of force coordination during multidigit grasping

The biomechanical structure of the hand and its underlying neurophysiology contribute to the coordination of the kinematics and kinetics necessary for multidigit grasping. We recently examined the neural organization of inputs to different extrinsic finger flexors during multi-digit object hold and found moderate to strong motor unit short-term synchrony. This suggests a common neural input to the motoneurons innervating these different hand muscles/muscle compartments, which may in turn influence the coordination of grip forces. To further characterize this common input to the hand muscles during multidigit grasping, we used the frequency-based measure of coherence. Motor unit coherence provides information with regards to the oscillatory frequency of a common input, as well as the coupling of the discharges of a motor unit pair at both short and long latencies. Preliminary results indicate that a large proportion of trials are characterized by significant coherence in the 1-12 Hz frequency range, which is more pronounced in the within- than between-muscle/muscle compartment analysis. This indicates a differential organization of common oscillatory inputs to pairs of motoneurons innervating the same vs. different muscles/ muscle compartments. The functional role of the 1-12 Hz oscillatory modulation of motor unit behavior is currently being investigated.

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 ( approximately 5.3% of their force on the thumb) with the same time-to-peak force as the thumb ( approximately 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.

Perception of finger forces within the hand after index finger fatiguing exercise

The effect of fatigue on finger force perception within a hand during ipsilateral finger force matching was examined. Thirteen subjects were instructed to match a reference force of an instructed finger using the same or different finger within the hand before and after index finger fatigue. Absolute reference force targets for the index or little finger were identical during pre- and post-fatigue sessions. Fatigue was induced by a 60-s sustained maximal voluntary contraction (MVC) of the index finger. Index finger MVC decreased approximately 29%, while there was a non-significant (about 5%) decrease in the little finger MVC. The results showed that: (1) the absolute reference and matching forces of the instructed fingers were not significantly changed after fatigue, while the total forces (sum of instructed and uninstructed finger forces) were increased after fatigue. (2) The relative forces (with respect to corresponding pre- and post-fatigue MVCs) of the index finger increased significantly in both reference and matching tasks, while the relative forces of the little finger remained unchanged after fatigue. (3) Matching errors remained unchanged after fatigue when the fatigued index finger produced the reference force, while the errors increased significantly when the fatigued index finger produced the matching force. (4) Enslaving (difference between total and instructed finger forces) increased significantly after fatigue, especially during force production by the fatigued index finger and when the little finger produced matching forces at higher force levels. (5) Enslaving significantly increased matching errors particularly after fatigue. Taken together, our results suggest that absolute finger forces within the hand are perceived within the CNS during ipsilateral finger force matching. Perception of absolute forces of the fatigued index finger is not altered after fatigue. The ability of the fatigued index finger to reproduce little finger forces is impaired to a certain degree, however. The impairment is likely to be attributable to altered afferent/efferent relationships of the fatigued index finger.

Prehension synergies: principle of superposition and hierarchical organization in circular object prehension

This study tests the following hypotheses in multi-digit circular object prehension: the principle of superposition (i.e., a complex action can be decomposed into independently controlled sub-actions) and the hierarchical organization (i.e., individual fingers at the lower level are coordinated to generate a desired task-specific outcome of the virtual finger at the higher level). Subjects performed 25 trials while statically holding a circular handle instrumented with five six-component force/moment sensors under seven external torque conditions. We performed a principal component (PC) analysis on forces and moments of the thumb and virtual finger (VF: an imagined finger producing the same mechanical effects of all finger forces and moments combined) to test the applicability of the principle of superposition in a circular object prehension. The synergy indices, measuring synergic actions of the individual finger (IF) moments for the stabilization of the VF moment, were calculated to test the hierarchical organization. Mixed-effect ANOVAs were used to test the dependent variable differences for different external torque conditions and different fingers at the VF and IF levels. The PC analysis showed that the elemental variables were decoupled into two groups: one group related to grasping stability control (normal force control) and the other group associated with rotational equilibrium control (tangential force control), which supports the principle of superposition. The synergy indices were always positive, suggesting error compensations between IF moments for the VF moment stabilization, which confirms the hierarchical organization of multi-digit prehension.

The effect of enslaving on perception of finger forces

The primary purpose was to examine the effect of enslaving on finger force perception during isometric finger force production using an ipsilateral force-matching paradigm. Fourteen subjects were instructed to produce varying levels of reference forces [10, 20, 30, and 40% maximal voluntary contraction (MVC)] force using one finger (index, I or little, L) and to reproduce these forces using the same finger (homo-finger tasks, I/I and L/L) or a different finger (hetero-finger tasks, I/L and L/I). Forces of all fingers were recorded. During homo-finger tasks, no differences were found in force magnitude or relative level of force (expressed as a proportion of MVC). The index finger matching force magnitudes were greater than the little finger reference force magnitudes, with significantly lower levels of relative force during L/I tasks; while the little finger matching forces underestimated the index finger reference forces with significantly higher levels of relative force during I/L tasks. The difference in the matching and reference forces by the instructed finger(s), i.e., matching error, was larger in hetero-finger tasks than in homo-finger tasks, particularly at high reference force levels (30, 40% MVC). When forces of all fingers were considered, enslaving (uninstructed finger forces) significantly minimized matching errors of the total force during both I/L and L/I hetero-finger tasks, especially at high reference force levels. Our results show that there is a tendency to match the absolute magnitude of the total force during ipsilateral finger force-matching tasks. This tendency is likely related to enslaving effects. Our results provide evidence that all (instructed and uninstructed) finger forces are sensed, thus resulting in perception of the absolute magnitude of total finger force.

The functional form of the lognormal distribution as a sum of decaying exponential and sinusoidal terms applied to the isometric pinch force of human fingers

The transient data of the pinch force produced between the human forefinger and thumb have been shown to fit the functional form of the well-known lognormal density function. Isometeric force generation is achieved by the stochastic recruitment of individual motor units, which sum together. Evidence from animal and human experiments demonstrates that the force generation can be modelled by underdamped terms. It is shown that a lognormal time series (distribution) can be fitted to a sum of exponential decaying sinusoidal terms.

Selective recruitment of single motor units in human flexor digitorum superficialis muscle during flexion of individual fingers

Flexor digitorum superficialis (FDS) is an extrinsic multi-tendoned muscle which flexes the proximal interphalangeal joints of the four fingers. It comprises four digital components, each with a tendon that inserts onto its corresponding finger. To determine the degree to which these digital components can be selectively recruited by volition, we recorded the activity of a single motor unit in one component via an intramuscular electrode while the subject isometrically flexed each of the remaining fingers, one at a time. The finger on which the unit principally acted was defined as the 'test finger' and that which flexed isometrically was the 'active' finger. Activity in 79 units was recorded. Isometric finger flexion forces of 50% maximum voluntary contraction (MVC) activated less than 50% of single units in components of FDS acting on fingers that were not voluntarily flexed. With two exceptions, the median recruitment threshold for all active-test finger combinations involving the index, middle, ring and little finger test units was between 49 and 60% MVC (60% MVC being the value assigned to those not recruited). The exceptions were flexion of the little finger while recording from ring finger units (median: 40% MVC), and vice versa (median: 2% MVC). For all active-test finger combinations, only 35/181 units were activated when the active finger flexed at less than 20% MVC, and the fingers were adjacent for 28 of these. Functionally, to recruit FDS units during grasping and lifting, relatively heavy objects were required, although systematic variation occurred with the width of the object. In conclusion, FDS components can be selectively activated by volition and this may be especially important for grasping at high forces with one or more fingers.

Intraneural microstimulation of motor axons in the study of human single motor units

Single motor unit activity has been studied in depth since the first intramuscular electrodes were developed more than 70 years ago. Many techniques have been combined or used in isolation since then. Intraneural motor axon microstimulation allows the detailed study of single motor units in awake human subjects in a manner most analogous to that used in reduced animal preparations. A microelectrode, inserted percutaneously into a peripheral nerve, stimulates the axon of a single alpha-motoneuron at a site remote from the contracting muscle, allowing detailed analyses of the contractile properties of a single motor unit in an otherwise quiescent muscle, that is, without interference of simultaneously active motor units or the presence of an electrode within the muscle. The methods and results obtained using this technique are described and compared to those of other studies of single motor units in human subjects. Differences have been found between human and animal motor units and between motor units of various muscles. Studying human and animal motor units using an analogous technique provides insight into the interpretation of human data when results differ from animal data, and when human motor units cannot be examined in the same way, or at a similar level of detail, as animal motor units.

Effects of motor imagery on finger force responses to transcranial magnetic stimulation

The purpose of this study was to investigate whether characteristics of finger interaction seen in voluntary finger force production tasks could also be observed during motor imagery. Transcranial magnetic stimulation (TMS) was applied over the contralateral M1 hand area. Three conditions were tested in eight young healthy volunteers: At rest, during motor imagery of maximal force production by the index finger (ImIn), and during motor imagery of maximal force production by all four fingers simultaneously (ImAll). We obtained measures of motor threshold (MT), motor-evoked potentials (MEP) from the contralateral flexor digitorium superficialis, and TMS-induced forces from individual fingers. Increased MEP and decreased MT during motor imagery tasks suggested enhanced excitability of structures involved in the generation of TMS-induced responses. TMS-induced forces were larger during motor imagery tasks than at rest. This effect was present, albeit significantly smaller, in the middle, ring, and little fingers during ImIn as compared to ImAll. This finding has been interpreted as a correlate of the phenomenon of unintended finger force production (enslaving). The motor imagery effect on finger forces evoked by TMS was significantly larger during ImIn (4% MVC) than during ImAll (2.8% MVC) tasks, corresponding to the phenomenon of force deficit. These results provide direct evidence of the neural origin of the main phenomena of finger interaction. Furthermore, the similarities between characteristics of finger interaction during motor imagery and during voluntary movement suggest the involvement of similar neural structures (including M1).

Common input to motor units of digit flexors during multi-digit grasping

The control of whole hand grasping relies on complex coordination of multiple forces. While many studies have characterized the coordination of finger forces and torques, the control of hand muscle activity underlying multi-digit grasping has not been studied to the same extent. Motor-unit synchrony across finger muscles or muscle compartments might be one of the factors underlying the limited individuation of finger forces. Such "unwanted" coupling among finger forces, however, might be desirable when a high level of force coupling is required to prevent object slip during grasping. The goal of this study was to quantify the strength of synchrony between single motor units from extrinsic hand muscles as subjects held a device with a five-digit grasp. During the hold phase, we recorded the normal force exerted by each digit and the electrical activity of single motor units from each of the four divisions of the muscle flexor digitorum profundus (FDP) and one thumb flexor muscle, m. flexor pollicis longus (FPL). The strength of motor-unit synchrony was quantified by the common input strength index (CIS). We found moderate to strong motor-unit synchrony between FPL and the index FDP compartment [CIS: 0.49 +/- 0.03 (SE)] and across most FDP compartments (0.34 +/- 0.02). Weak synchrony, however, was found between FPL and the middle, ring, and little finger FDP compartments (0.25 +/- 0.01). This difference might reflect the larger force contribution of the thumb-index finger pair relative to other thumb-finger combinations in five-digit grasping.

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 (approximately 0 to 10 degrees), 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.

Human finger independence: limitations due to passive mechanical coupling versus active neuromuscular control

We studied the extent to which mechanical coupling and neuromuscular control limit finger independence by studying passive and active individuated finger movements in healthy adults. For passive movements, subjects relaxed while each finger was rotated into flexion and extension by a custom-built device. For active movements, subjects moved each finger into flexion and extension while attempting to keep the other, noninstructed fingers still. Active movements were performed through approximately the same joint excursions and at approximately the same speeds as the passive movements. We quantified how mechanical coupling limited finger independence from the passive movements, and quantified how neuromuscular control limited finger independence using an analysis that subtracted the indices obtained in the passive condition from those obtained in the active condition. Finger independence was generally similar during passive and active movements, but showed a trend toward less independence in the middle, ring, and little fingers during active, large-arc movements. Mechanical coupling limited the independence of the index, middle, and ring fingers to the greatest degree, followed by the little finger, and placed only negligible limitations on the independence of the thumb. In contrast, neuromuscular control primarily limited the independence of the ring, and little fingers during large-arc movements, and had minimal effects on the other fingers, especially during small-arc movements. For the movement conditions tested here, mechanical coupling between the fingers appears to be a major factor limiting the complete independence of finger movement.

Hand function: peripheral and central constraints on performance

The hand is one of the most fascinating and sophisticated biological motor systems. The complex biomechanical and neural architecture of the hand poses challenging questions for understanding the control strategies that underlie the coordination of finger movements and forces required for a wide variety of behavioral tasks, ranging from multidigit grasping to the individuated movements of single digits. Hence, a number of experimental approaches, from studies of finger movement kinematics to the recording of electromyographic and cortical activities, have been used to extend our knowledge of neural control of the hand. Experimental evidence indicates that the simultaneous motion and force of the fingers are characterized by coordination patterns that reduce the number of independent degrees of freedom to be controlled. Peripheral and central constraints in the neuromuscular apparatus have been identified that may in part underlie these coordination patterns, simplifying the control of multi-digit grasping while placing certain limitations on individuation of finger movements. We review this evidence, with a particular emphasis on how these constraints extend through the neuromuscular system from the behavioral aspects of finger movements and forces to the control of the hand from the motor cortex.

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.

Reduced muscle selectivity during individuated finger movements in humans after damage to the motor cortex or corticospinal tract

We investigated how damage to the motor cortex or corticospinal tract affects the selective activation of finger muscles in humans. We hypothesized that damage relatively restricted to the motor cortex or corticospinal tract would result in unselective muscle activations during an individuated finger movement task. People with pure motor hemiparesis attributed to ischemic cerebrovascular accident were tested. Pure motor hemiparetic and control subjects were studied making flexion/extension and then abduction/adduction finger movements. During the abduction/adduction movements, we recorded muscle activity from 3 intrinsic finger muscles: the abductor pollicis brevis, the first dorsal interosseus, and the abductor digit quinti. Each of these muscles acts as an agonist for only one of the abduction/adduction movements and might therefore be expected to be active in a highly selective manner. Motor cortex or corticospinal tract damage in people with pure motor hemiparesis reduced the selectivity of finger muscle activation during individuated abduction/adduction finger movements, resulting in reduced independence of these movements. Abduction/adduction movements showed a nonsignificant trend toward being less independent than flexion/extension movements in the affected hands of hemiparetic subjects. These changes in the selectivity of muscle activation and the consequent decrease in individuation of movement were correlated with decreased hand function. Our findings imply that, in humans, spared cerebral motor areas and descending pathways that remain might activate finger muscles, but cannot fully compensate for the highly selective control provided by the primary motor cortex and the crossed corticospinal system.