Index Finger Force Research Articles

Estimation of the effects of hand growth on muscle activation patterns: A musculoskeletal modeling study.

Throughout childhood growth and development, both the nervous and the musculoskeletal systems undergo rapid change. The goal of this study was to examine the impact of growth-related changes in skeletal size and muscle strength on the neural control of finger force generation. By modifying an existing OpenSim hand model in accordance with pediatric anthropometric data, we created 10 distinct models representing males and females at each year of development from 6 to 10years old. We then used the static optimization tool to estimate the requisite muscle activations to create a maximal palmar force with the index finger in two different postures (metacarpophalangeal, proximal interphalangeal, distal interphalangeal) - Posture 1: (0°, 30°, 0°) and Posture 2: (0°, 60°, 30°). For Posture 1, multiple regression analysis revealed a significant effect of both age and sex on activation for all muscles (p<0.035) with exception of the flexor digitorum profundus. For Posture 2, only the extensor digitorum communis activation had a significant relationship with age (p=0.010), while no other muscles showed a significant relationship with age, sex, or the age-sex interaction activation (p>0.054). Exchanging the activation patterns between the youngest and oldest models altered both the predicted index finger force and direction. Therefore, our simulations suggest that the changes in hand size and morphology associated with growth may necessitate changes in muscle activation patterns to be able to continue to perform a given hand function. Children may need to substantially adjust or even relearn motor control strategies throughout childhood.

Voluntary activation of maximal single and all finger power grip contractions.

When all 4 fingers are engaged together during a grip strength contraction, the force produced by an individual finger is less than the force produced when it acts in isolation. The purpose of this study was to evaluate if the reduced force output of a digit during an all-finger grip contraction is due to a decline in voluntary activation. Fifteen young adults (n=7 females) completed voluntary contractions of the index finger in isolation and all fingers together in a dynamometer capable of separately recording forces from each finger during voluntary and electrically evoked contractions. The median and ulnar nerves were electrically stimulated simultaneously at the elbow to record individual finger flexion forces from doublet (100 Hz) pulses. Doublet stimulations were applied during and immediately following contractions at 50, 65, 85, and 100% maximal voluntary contraction (MVC) forces. Two-way ANOVAs were used to compare the effects of sex and finger (single vs all) on flexion forces and voluntary activation. The index finger produced ∼25% more force when engaged in isolation compared to the all-finger contraction; however, there were no differences in voluntary activation between the single and all finger MVCs (p=0.344). The index finger force deficit was larger in females compared to males (34 vs 18%, p=0.030), but this was not explained by sex-related differences in voluntary activation. These data indicate that the additional force produced during single-finger contractions is not due to an alteration in voluntary activation, as all-finger contractions display near maximal activation of each digit.

Effects of wrist and finger posture on finger independence

Intended actions of one finger produce involuntary movement or force in other fingers. Mechanical and neural factors limit finger independence. The interplay between anatomical factors, wrist and finger postures, and finger independence remain unclear. The purpose of this study was to determine the effects of wrist and metacarpophalangeal (MCP) posture on involuntary finger forces and extensor digitorum (ED) activity. Twenty participants performed submaximal isometric finger extensions in three wrist positions (30° extension, neutral, and 30° flexion) and two MCP postures (straight and 90° flexion). Involuntary index finger force increased with MCP flexion, suggesting the importance of intertendinous connections in finger independence. Consistent with previous research, ED activity was generally higher in wrist extension than neutral and flexed postures. Understanding the role of passive properties within the hand may help us improve finger rehabilitation strategies.

Helping Surgeons’ Hands: A Biomechanical Evaluation of Ergonomic Instruments

"Ergonomic" is a common descriptor for a desk or computer workspace but is a term rarely used to describe a surgical instrument. Instead, surgeons spend many hours in inconvenient positions, often using instruments that are not ergonomic. Improving the ergonomics of surgical instruments may decrease the required force for simple tasks and allow for more efficient surgery. To evaluate the impact of ergonomic surgical instruments, the authors developed ergonomic screwdriver handles. The shape and size of these handles were engineered using previous dental studies and 3-dimensional modeling to create an ideal handle for specific glove sizes. Participants were recruited to test 3 different ergonomic handle sizes against a standard screwdriver while assessing digital peak force, digital contact area, and participant preference. Ten participants (3 women) with glove sizes ranging from 6 to 8 were evaluated. Ergonomic screwdriver handles sized for glove sizes 6 and 7 required significantly less thumb peak force than the standard screwdriver for all participants (702 N for glove size 6 and 567 N for glove size 7 ergonomic screwdrivers, vs 1780 N for "one size fits all" standard screwdriver). Participants consistently preferred screwdrivers that required lower thumb and index finger forces. All ergonomic handles required lower thumb and index finger force. Eighty percent of participants preferred a screwdriver modeled within 1 glove size of their own. Improved ergonomic handles require less force and are preferred by surgeons. The significant decrease in thumb peak force for glove sizes 6 and 7 suggests that there is room for ergonomic improvement in instruments, especially for surgeons with smaller hands. Manufacturing ergonomic screwdriver handles and using the evolving convenience of 3-dimensional printing may help to develop a more comfortable work environment for surgeons.

External six-bar mechanism rehabilitation device for index finger: Development and shape synthesis

This work proposes a novel external Stephenson-III six-bar mechanism-based rehabilitation device. This device has been designed to rehabilitate the patient’s finger for the action of grabbing, also known as flexion and extension. A predefined trajectory is used to synthesize the mechanism using TeachingLearning-Optimization algorithm (TLBO) and Particle swarm optimization algorithm (PSO). The trajectory data was obtained after image processing by grabbing a particular object 30 times to record the flexion and extension motion of the index finger. An optimization problem was formulated and results of TLBO and PSO were compared. The mechanism obtained from TLBO algorithm was deemed better in terms of precision and feasible configuration. Using clinical biomechanical data for flexion/extension of index finger, position and static force analysis are performed. The CAD model of the mechanism was then tested for feasibility in a CAD/Software. Excess mass was removed using topology optimization and a 20% mean reduction for every link was achieved. An index finger rehabilitation device employing an external six-bar mechanism was obtained, that would help a patient with motor control loss to rehabilitate and bring normalcy to life. The design of exoskeleton was able to match the trajectory of the index. Shape synthesis ensured a 20% reduction in overall mass of the linkages.

Triggermuscle: Exploring Weight Perception for Virtual Reality Through Adaptive Trigger Resistance in a Haptic VR Controller

It is challenging to provide users with a haptic weight sensation of virtual objects in VR since current consumer VR controllers and software-based approaches such as pseudo-haptics cannot render appropriate haptic stimuli. To overcome these limitations, we developed a haptic VR controller named Triggermuscle that adjusts its trigger resistance according to the weight of a virtual object. Therefore, users need to adapt their index finger force to grab objects of different virtual weights. Dynamic and continuous adjustment is enabled by a spring mechanism inside the casing of an HTC Vive controller. In two user studies, we explored the effect on weight perception and found large differences between participants for sensing change in trigger resistance and thus for discriminating virtual weights. The variations were easily distinguished and associated with weight by some participants while others did not notice them at all. We discuss possible limitations, confounding factors, how to overcome them in future research and the pros and cons of this novel technology.

Recovery and Prediction of Dynamic Precision Grip Force Control After Stroke

Background and Purpose- Dexterous object manipulation, requiring generation and control of finger forces, is often impaired after stroke. This study aimed to describe recovery of precision grip force control after stroke and to determine clinical and imaging predictors of 6-month performance. Methods- Eighty first-ever stroke patients with varying degrees of upper limb weakness were evaluated at 3 weeks, 3 months, and 6 months after stroke. Twenty-three healthy individuals of comparable age were also studied. The Strength-Dexterity test was used to quantify index finger and thumb forces during compression of springs of varying length in a precision grip. The coordination between finger forces (CorrForce), along with Dexterity-score and Repeatability-score, was calculated. Anatomical magnetic resonance imaging was used to calculate weighted corticospinal tract lesion load (wCST-LL). Results- CorrForce, Dexterity-score, and Repeatability-score in the affected hand were dramatically lower at each time point compared with the less-affected hand and the control group, even in patients with mild motor impairment according to Fugl-Meyer assessment. Improved performance over time occurred in CorrForce and Dexterity-score but not in Repeatability-score. The Fugl-Meyer assessment hand subscale, sensory function, and wCST-LL best predicted CorrForce and Dexterity-score status at 6 months (R2=0.56 and 0.87, respectively). wCST-LL explained substantial variance in CorrForce (R2=0.34) and Dexterity-score (R2=0.50) at 6 months; two-point discrimination and Fugl-Meyer score accounted for considerable additional variance. Absence of recovery in CorrForce was predicted by wCST-LL >4 cc and in Dexterity-score by wCST-LL >6 cc. Conclusions- Findings highlight persisting deficits in the ability to grasp and control finger forces after stroke. wCST-LL was the strongest predictor of performance at 6 months, but early two-point discrimination and Fugl-Meyer score had substantial additional predictive value. Clinical Trial Registration- URL: https://www.clinicaltrials.gov. Unique identifier: NCT02878304.

Concussion history is negatively associated with visual-motor force complexity: evidence for persistent effects on visual-motor integration

ABSTRACTObjectives: Long-term monitoring of concussion recovery requires time- and cost-effective methods. Physiologic complexity may be useful in evaluating visual-motor integration following concussion. The purpose of this study was to quantify the extent to which prior number of concussions influenced visual-motor tracking force complexity.Methods: Thirty-five individuals with a self-reported concussion history (age: 20.92 ± 1.98) and 15 without (age: 20.92 ± 2.21) performed an isometric visual-motor tracking task, using index finger force to trace a straight line across a computer screen. Finger force root mean square error (RMSE), multi-scale complexity, and average power from 0 to 12 Hertz (Hz) were calculated. Individual multiple regressions were fit to these outcomes.Results: Force complexity decreased linearly with an increasing number of concussions (R2 = 0.101). Males had more complex force overall (R2 = 0.219) and greater 4–8 Hz average power (R2 = 0.193). The 8–12 Hz average power decreased significantly for individuals with prior loss of consciousness (LOC) and increasing numbers of concussions (R2 = 0.143).Conclusion: Individuals exhibited linear decreases in visual-motor tracking force complexity with increasing numbers of concussions, influenced by both gender and a history of LOC. These findings indicate cumulative changes in the ways in which previously concussed individuals process and integrate visual information to guide behaviour.

Coherence and interlimb force control: Effects of visual gain

Neural coupling across hemispheres and homologous muscles often appears during bimanual motor control. Force coupling in a specific frequency domain may indicate specific bimanual force coordination patterns. This study investigated coherence on pairs of bimanual isometric index finger force while manipulating visual gain and task asymmetry conditions. We used two visual gain conditions (low and high gain = 8 and 512 pixels/N), and created task asymmetry by manipulating coefficient ratios imposed on the left and right index finger forces (0.4:1.6; 1:1; 1.6:0.4, respectively). Unequal coefficient ratios required different contributions from each hand to the bimanual force task resulting in force asymmetry. Fourteen healthy young adults performed bimanual isometric force control at 20% of their maximal level of the summed force of both fingers. We quantified peak coherence and relative phase angle between hands at 0–4, 4–8, and 8–12 Hz, and estimated a signal-to-noise ratio of bimanual forces. The findings revealed higher peak coherence and relative phase angle at 0–4 Hz than at 4–8 and 8–12 Hz for both visual gain conditions. Further, peak coherence and relative phase angle values at 0–4 Hz were larger at the high gain than at the low gain. At the high gain, higher peak coherence at 0–4 Hz collapsed across task asymmetry conditions significantly predicted greater signal-to-noise ratio. These findings indicate that a greater level of visual information facilitates bimanual force coupling at a specific frequency range related to sensorimotor processing.

An open-source model and solution method to predict co-contraction in the finger

A novel open-source biomechanical model of the index finger with an electromyography (EMG)-constrained static optimization solution method are developed with the goal of improving co-contraction estimates and providing means to assess tendon tension distribution through the finger. The Intrinsic model has four degrees of freedom and seven muscles (with a 14 component extensor mechanism). A novel plugin developed for the OpenSim modelling software applied the EMG-constrained static optimization solution method. Ten participants performed static pressing in three finger postures and five dynamic free motion tasks. Index finger 3D kinematics, force (5, 15, 30 N), and EMG (4 extrinsic muscles and first dorsal interosseous) were used in the analysis. The Intrinsic model predicted co-contraction increased by 29% during static pressing over the existing model. Further, tendon tension distribution patterns and forces, known to be essential to produce finger action, were determined by the model across all postures. The Intrinsic model and custom solution method improved co-contraction estimates to facilitate force propagation through the finger. These tools improve our interpretation of loads in the finger to develop better rehabilitation and workplace injury risk reduction strategies.

Effects of Carpal Tunnel Syndrome on Force Coordination and Muscle Coherence during Precision Pinch.

Carpal tunnel syndrome (CTS), caused by entrapment of the median nerve in the carpal tunnel, impairs hand function including dexterous manipulation. The purpose of this study was to investigate the effects of CTS on force coordination and muscle coherence during low-intensity sustained precision pinch while the wrist assumed different postures. Twenty subjects (10 CTS patients and 10 asymptomatic controls) participated in this study. An instrumented pinch device was used to measure the thumb and index finger forces while simultaneously collecting surface electromyographic activities of the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles. Subjects performed a sustained precision pinch at 10% maximum pinch force for 15 sec with the wrist stabilized at 30° extension, neutral, or 30° flexion using customized splints. The force discrepancy and the force coordination angle between the thumb and index finger forces were calculated, as well as the β-band (15-30 Hz) coherence between APB and FDI. The index finger applied greater force than the thumb (p < 0.05); this force discrepancy was increased with wrist flexion (p < 0.05), but was not affected by CTS (p > 0.05). The directional force coordination was not significantly affected by wrist posture or CTS (p > 0.05). In general, digit force coordination during precision pinch seems to be sensitive to wrist flexion, but is not affected by CTS. The β-band muscular coherence was increased by wrist flexion for CTS patients (p < 0.05), which could be a compensatory mechanism for the flexion-induced exacerbation of CTS symptoms. This study demonstrates that wrist flexion negatively influences muscle and force coordination in CTS patients supporting the avoidance of flexion posture for symptom exacerbation and functional performance.

Carpal tunnel syndrome impairs index finger responses to unpredictable perturbations

The fine-tuning of digit forces to object properties can be disrupted by carpal tunnel syndrome (CTS). CTS’ effects on hand function have mainly been investigated using predictable manipulation tasks; however, unpredictable perturbations are commonly encountered during manual tasks, presenting situations which may be more challenging to CTS patients given their hand impairments. The purpose of this study was to investigate muscle and force responses of the index finger to unpredictable perturbations in patients with CTS. Nine CTS patients and nine asymptomatic controls were instructed to stop the movement of a sliding plate by increasing index finger force following an unexpected perturbation. The electrical activity of the first dorsal interosseous muscle and forces exerted by the index finger were recorded. CTS patients demonstrated 20.9% greater muscle response latency and 12.0% greater force response latency compared to controls (p<0.05). The duration of plate sliding was significantly different between groups (p<0.05); the CTS group’s duration was 142.2±5.8ms compared to the control group’s duration of 133.1±8.4ms. Although CTS patients had increased muscle and force response durations comparatively, these differences were not statistically significant. Findings from this study suggest CTS-induced sensorimotor deficits interfere with accurate detection, processing and response to unpredictable perturbations. These deficits could be accounted for at multiple levels of the peripheral and central nervous systems. Delayed and decreased responses may indicate inefficient object manipulation by CTS patients and may help to explain why CTS patients tend to drop objects.

Cortical activity predicts good variation in human motor output.

Human movement patterns have been shown to be particularly variable if many combinations of activity in different muscles all achieve the same task goal (i.e., are goal-equivalent). The nervous system appears to automatically vary its output among goal-equivalent combinations of muscle activity to minimize muscle fatigue or distribute tissue loading, but the neural mechanism of this "good" variation is unknown. Here we use a bimanual finger task, electroencephalography (EEG), and machine learning to determine if cortical signals can predict goal-equivalent variation in finger force output. 18 healthy participants applied left and right index finger forces to repeatedly perform a task that involved matching a total (sum of right and left) finger force. As in previous studies, we observed significantly more variability in goal-equivalent muscle activity across task repetitions compared to variability in muscle activity that would not achieve the goal: participants achieved the task in some repetitions with more right finger force and less left finger force (right > left) and in other repetitions with less right finger force and more left finger force (left > right). We found that EEG signals from the 500milliseconds (ms) prior to each task repetition could make a significant prediction of which repetitions would have right > left and which would have left > right. We also found that cortical maps of sites contributing to the prediction contain both motor and pre-motor representation in the appropriate hemisphere. Thus, goal-equivalent variation in motor output may be implemented at a cortical level.

Effects of Tactile Sensitivity on Structural Variability of Digit Forces during Stable Precision Grip.

This study investigated the effects of fingertip tactile sensitivity on the structural variability of thumb and index finger forces during stable precision grip. Thirty right-handed healthy subjects participated in the experiment. Transient perturbation of tactile afferents was achieved by wrapping up the distal pads of the thumb or index finger with transparent polyethylene films. The time-dependent structure of each digit force and the variability of interdigit force correlation were examined by detrended fluctuation analysis (DFA) and detrended cross-correlation analysis (DCCA), respectively. Results showed that the tactile sensitivity affected αDFA of the vertical shear force Fx (F 3,239 = 6.814, p < 0.001) and αDCCA of Fx (χ 2 = 16.440, p < 0.001). No significant difference was observed in αDFA or αDCCA of the normal forces produced by the thumb or index finger. These results suggested that with blurred tactile sensory inputs the central nervous system might decrease the vertical shear force flexibility and increase the interdigit shear force coupling in order to guarantee a stable grip control of an object against gravity. This study shed light on the feedback and feed-forward strategies involved in digit force control and the role of SA-II afferent fibers in regulation of vertical shear force variability for precision grip.

Coordination of digit force variability during dominant and non-dominant sustained precision pinch.

This study examined the effects of handedness on the inter-digit coordination of force variability with and without concurrent visual feedback during sustained precision pinch. Twenty-four right-handed subjects were instructed to pinch an instrumented apparatus with their dominant and non-dominant hands, separately. During the pinch, the subjects were required to maintain a stable force output at 5N for 1min. Visual feedback was given for the first 30s and removed for the second 30s. Coefficient of variation and detrended fluctuation analysis were employed to examine the amount and structural variability of the thumb and index finger forces. Similarly, correlation coefficient and detrended cross-correlation analysis were applied to quantify the inter-digit correlation of force amount and structural variability. Results showed that, compared to the non-dominant hand, the dominant hand had higher inter-digit difference in the amount of digit force variability. Without visual feedback, the dominant hand exhibited lower digit force structural variability but higher inter-digit force structural correlation than the non-dominant hand. These results implied that the dominant hand would be more independent, less flexible and with lower dynamic degrees of freedom than the non-dominant hand in coordination of the thumb and index finger forces during sustained precision pinch. The effects of handedness on inter-digit force coordination were dependent on sensory condition, which shed light on higher-level sensorimotor mechanisms that may be responsible for the asymmetries in coordination of digit force variability.

Directional Coordination of Thumb and Finger Forces during Precision Pinch

The human opposable thumb enables the hand to perform dexterous manipulation of objects, which requires well-coordinated digit force vectors. This study investigated the directional coordination of force vectors generated by the thumb and index finger during precision pinch. Fourteen right-handed, healthy subjects were instructed to exert pinch force on an externally stabilized apparatus with the pulps of the thumb and index finger. Subjects applied forces to follow a force-ramp profile that linearly increased from 0 to 12 N and then decreased to 0 N, at a rate of ±3 N/s. Directional relationships between the thumb and index finger force vectors were quantified using the coordination angle (CA) between the force vectors. Individual force vectors were further analyzed according to their projection angles (PAs) with respect to the pinch surface planes and the shear angles (SAs) within those planes. Results demonstrated that fingertip force directions were dependent on pinch force magnitude, especially at forces below 2 N. Hysteresis was observed in the force-CA relationship for increasing and decreasing forces and fitted with exponential models. The fitted asymptotic values were 156.0±6.6° and 150.8±9.3° for increasing and decreasing force ramps, respectively. The PA of the thumb force vector deviated further from the direction perpendicular to the pinching surface planes than that of the index finger. The SA showed that the index finger force vector deviated in the ulnar-proximal direction, whereas the thumb switched its force between the ulnar-proximal and radial-proximal directions. The findings shed light on the effects of anatomical composition, biomechanical function, and neuromuscular control in coordinating digit forces during precision pinch, and provided insight into the magnitude-dependent force directional control which potentially affects a range of dexterous manipulations.

Motor Recovery after Spinal Cord Injury Enhanced by Strengthening Corticospinal Synaptic Transmission

The corticospinal tract is an important target for motor recovery after spinal cord injury (SCI) in animals and humans. Voluntary motor output depends on the efficacy of synapses between corticospinal axons and spinal motoneurons, which can be modulated by the precise timing of neuronal spikes. Using noninvasive techniques, we developed tailored protocols for precise timing of the arrival of descending and peripheral volleys at corticospinal-motoneuronal synapses of an intrinsic finger muscle in humans with chronic incomplete SCI. We found that arrival of presynaptic volleys prior to motoneuron discharge enhanced corticospinal transmission and hand voluntary motor output. The reverse order of volley arrival and sham stimulation did not affect or decreased voluntary motor output and electrophysiological outcomes. These findings are the first demonstration that spike timing-dependent plasticity of residual corticospinal-motoneuronal synapses provides a mechanism to improve motor function after SCI. Modulation of residual corticospinal-motoneuronal synapses may present a novel therapeutic target for enhancing voluntary motor output in motor disorders affecting the corticospinal tract.

Long‐range correlations in motor unit discharge times at low forces are modulated by visual gain and age

The purpose of this investigation was to examine the association between visual information and the size and temporal structure of the variability in index finger force and motor unit discharge times when young and old adults performed isometric contractions with a hand muscle. Single motor units (n = 32) in the first dorsal interosseus muscle were recorded as subjects [16 young (18-35 years old) and 16 old (≥ 70 years old)] exerted a constant abduction force with the index finger during 60 s isometric contractions. The target force was displayed on a monitor in front of the subjects, and they were asked to match the index finger force to a target force. The amount of visual feedback, or gain of the signal, was varied between 24 (low gain) and 1175 pixels N(-1) (high gain). In addition, some trials were performed in the absence of visual feedback. The dependent variables were the variability (standard deviation and coefficient of variation) and the regularity (detrended fluctuation analysis self-similarity parameter, α) of motor unit discharge times and the abduction force. Motor unit discharge times became less regular with an increase in visual feedback gain for both young and old adults, whereas motor unit discharge variability was not influenced by changes in visual gain. The regularity of motor unit discharge times was less for old adults than for young adults, but the variability was greater for old adults. However, there was a significant association between the regularity of motor unit discharge times and the regularity of force for the old adults, but not the young adults. These observations suggest that adjustments in the synaptic inputs delivered to motor neurons with changes in the visual gain differed for young and old adults.

Dynamical degrees of freedom and correlations in isometric finger force production

Prior research has concluded that the correlations of isometric finger forces represent the extent to which the fingers are controlled as a single unit. If this is the case, finger force correlations should be consistent with estimates of the controlled (dynamical) degrees of freedom in finger forces. The present study examined the finger force correlations and the dynamical degrees of freedom in four isometric force tasks. The tasks were to produce a preferred level of force with the (a) Index, (b) Ring, (c) Both fingers and also to (d) Rest the fingers on the load cells. Dynamical degrees of freedom in finger forces were lowest in the Both finger force task and progressively higher in the Ring, Index and Resting finger force tasks. The finger force correlations were highest in the Resting and lowest in the Index and Ring finger tasks. The results for the dynamical degrees of freedom in finger forces were consistent with a reduction in degrees of freedom in response to the degrees of freedom problem and the task constraints. The results for the finger force correlations were inconsistent with a reduction in the dynamical degrees of freedom. These findings indicate that finger force correlations do not necessarily reflect the coupling of finger forces. The findings also highlight the value of time-domain analyses to reveal the organization of control in isometric finger forces.

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

BackgroundThe 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. MethodsThe hands of subjects were scanned using a 3.0T 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). ResultsThe 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. InterpretationMoment arms of the flexor digitorum superficialis change due to both changes in joint angle and muscle activation; they scale with various anthropometric measures.