The Sensitivity of the Endpoint Forces of Thumb Extrinsic and Intrinsic Muscles to Changes in Joint Angles, Muscle Moment Arms and Bone Lengths in the Flexed Thumb.

A previous study showed in situ measurements of thumb-tip forces produced by muscles vary substantially among cadaveric specimens. Potential sources of variability include inter-specimen anatomic differences and postural deviations from the nominal posture in which the specimens were tested. This study aimed to theoretically determine the variation in thumb-tip force caused by inter-specimen differences in thumb anatomy and posture. We developed a two-dimensional mathematical model of force production at the thumb tip based on published estimates of muscle moment arms, bone length, and joint angle measurements from nine cadaveric specimens. The model was placed in a flexed posture. Using the model, we calculated variations in magnitude and direction of each muscle's thumb-tip force induced by a ±1 standard deviation (or equivalent) variation in each bone length, the moment arm of the muscle (i.e., anatomic factors), and each joint angle (i.e., postural factor). For most muscles, inter-specimen differences in the metacarpophalangeal (MP) joint angle produced at least a 75% larger variation in thumb-tip force magnitude than that produced by other factors. For all muscles, differences in the interphalangeal joint angle among specimens produced the largest variation in thumb-tip force direction. For some muscles, inter-specimen differences in bone lengths, moment arms, and MP joint angles also produced large variations in thumb-tip force direction. This study suggests deviation from the nominal flexed thumb posture and large measurement variability in muscle moment arms are primary and secondary sources, respectively, of variability in thumb-tip forces produced by the majority of thumb muscles. Further, this study suggests a more careful approach to standardizing the thumb posture would likely improve current measurements of thumb-tip forces.Clinical Relevance- This work describes the influence of anatomic and postural factors on thumb-tip forces that thumb muscles produce. The results of this work have implications for musculoskeletal modeling and surgical reconstruction of grasp.

Measurement of the Three-Dimensional Muscle Endpoint Forces in the Extended Thumb and Its Application to Determining Muscle Combinations that Enable Lateral Pinch Force Production Throughout the Plane of Flexion-Extension.

Functional outcomes of tendon transfer surgeries, designed to restore lateral pinch grasp to persons following cervical spinal cord injury, have been mixed. That is, pinch force magnitudes have varied by 10-fold and have been reported to be as low as low as tenths of a pound. We believe a novel tendon transfer approach in which the donor muscle actuates a small group of paralyzed thumb muscles, instead of just the flexor pollicis longus (FPL) muscle (the current approach), will enable endpoint forces that are better directed and therefore a consistently stronger pinch force following surgery. We further believe that such surgeries can be better designed to account for grasp force production throughout the entire plane of flexion-extension if muscle endpoint forces in the extended thumb are known. Consequently, we measured muscle endpoint forces in the extended thumb in 6 cadaveric specimens after a force of 10 N was applied to each muscle. Further, we simulated a tendon transfer surgery in which the donor muscle applied equal force to each muscle in 246 small groups of muscles, calculated the direction of the resulting endpoint force throughout the flexion-extension plane, and determined if those groups of muscles produced a better directed force than FPL's. While we found that 3 individual muscles and 52 muscle groups could produce desirably directed endpoint forces in parts of the flexion-extension plane, no muscle or muscle group could produce well-directed endpoint forces throughout the flexion-extension plane. We concluded that a group of muscles could likely be found if the donor muscle provided different levels of force to each of the muscles in a muscle group. This would be possible through intentional geometric manipulation of the donor-to-recipient muscle attachment to allow for unequal splitting of donor muscle force.Clinical Relevance-This work aims to determine whether the same combination of thumb muscles can produce well-directed endpoint forces throughout the flexion-extension plane. If so, then this work informs surgeons which muscle groups could be involved in a tendon transfer to restore lateral pinch grasp ability throughout the plane of flexion-extension in person with cervical spinal cord injury.

Multiaxis grip characteristics for varying handle diameters and effort.

OBJECTIVE: A multiaxis dynamometer was used to quantify grip force vector angles and longitudinal centers of pressure (COPs) while varying handle size and effort used. BACKGROUND: Authors of many studies have examined maximum grip strength using scalar instruments; a few have measured two-axis forces limited to one or more finger contact. This novel dynamometer uses two instrumented beams that are grasped by the distal fingers and proximal palm to compute two orthogonal components of force and the longitudinal COP through which the force acts. METHOD: Sixteen healthy, right-handed participants grasped the multiaxis dynamometer with plastic handles ranging in diameter from 3.81 to 7.62 cm. They were required to scale their effort to 25%, 50%, 75%, and 100% of maximum. RESULTS: Grip force vector angles were affected by both handle diameter and effort level, with angles increasing an average of 8.1° from the least to greatest effort. Longitudinal COP, averaged among the two beams, shifted 1.75 cm radially as handle diameter increased from 3.81 cm to 7.62 cm. Average COP along the beam in contact with the distal finger segments shifted 0.75 cm ulnarly as effort level increased from 25% to 100% of maximum. CONCLUSION: Grip force characteristics changed with handle diameter and effort level. Overall grip force magnitude comprised both force components measured. APPLICATION: Understanding grip characteristics should be important for handle and grip design and for evaluating hand function.

Finger-thumb coupling contributes to exaggerated thumb flexion in stroke survivors.

The purpose of this study was to investigate altered finger-thumb coupling in individuals with chronic hemiparesis poststroke. First, an external device stretched finger flexor muscles by passively rotating the metacarpophalangeal (MCP) joints. Subjects then performed isometric finger or thumb force generation. Forces/torques and electromyographic signals were recorded for both the thumb and finger muscles. Stroke survivors with moderate (n = 9) and severe (n = 9) chronic hand impairment participated, along with neurologically intact individuals (n = 9). Stroke survivors exhibited strong interactions between finger and thumb flexors. The stretch reflex evoked by stretch of the finger flexors of stroke survivors led to heteronymous reflex activity in the thumb, while attempts to produce isolated voluntary finger MCP flexion torque/thumb flexion force led to increased and undesired thumb force/finger MCP torque production poststroke with a striking asymmetry between voluntary flexion and extension. Coherence between the long finger and thumb flexors estimated using intermuscular electromyographic correlations, however, was small. Coactivation of thumb and finger flexor muscles was common in stroke survivors, whether activation was evoked by passive stretch or voluntary activation. The coupling appears to arise from subcortical or spinal sources. Flexor coupling between the thumb and fingers seems to contribute to undesired thumb flexor activity after stroke and may impact rehabilitation outcomes.

Capacity of small groups of muscles to accomplish precision grasping tasks.

An understanding of the capacity or ability of various muscle groups to generate endpoint forces that enable grasping tasks could provide a stronger biomechanical basis for the design of reconstructive surgery or rehabilitation for the treatment of the paralyzed or paretic hand. We quantified two-dimensional endpoint force distributions for every combination of the muscles of the index finger, in cadaveric specimens, to understand the capability of muscle groups to produce endpoint forces that accomplish three common types of grasps-tripod, tip and lateral pinch-characterized by a representative level of Coulomb friction. We found that muscle groups of 4 or fewer muscles were capable of generating endpoint forces that enabled performance of each of the grasping tasks examined. We also found that flexor muscles were crucial to accomplish tripod pinch; intrinsic muscles, tip pinch; and the dorsal interosseus muscle, lateral pinch. The results of this study provide a basis for decision making in the design of reconstructive surgeries and rehabilitation approaches that attempt to restore the ability to perform grasping tasks with small groups of muscles.

Diminished capacity to modulate motor activation patterns according to task contributes to thumb deficits following stroke.

The objective of this study was to explore motor impairment of the thumb following stroke. More specifically, we quantitatively examined kinetic deficits of the thumb. We anticipated that force deficits would be nonuniformly distributed across the kinetic workspace, due in part to varying levels of difficulty in altering the motor activation pattern to meet the task. Eighteen stroke survivors with chronic hemiparesis participated in the trials, along with nine age-matched controls. Of the stroke-survivor group, nine subjects had moderate hand impairment, and the other nine subjects had severe hand impairment. Subjects were instructed to generate maximal isometric thumb-tip force, as measured with a load cell, in each of six orthogonal directions with respect to the thumb tip. Activity of three representative thumb muscles was monitored through intramuscular and surface electrodes. Univariate split-plot analysis of variance revealed that clinical impairment level had a significant effect on measured force (P < 0.001), with the severely impaired group producing only 13% of the control forces, and the moderately impaired group generating 32% of control forces, on average. Weakness in the moderately impaired group exhibited a dependence on force direction (P = 0.015), with the least-relative weakness in the medial direction. Electromyographic recordings revealed that stroke survivors exhibited limited modulation of thumb-muscle activity with intended force direction. The difference in activation presented by the control group for a given muscle was equal to 40% of its full activation range across force directions, whereas this difference was only 26% for the moderately impaired group and 15% for the severely impaired group. This diminished ability to modify voluntary activation patterns, which we observed previously in index-finger muscles as well, appears to be a primary factor in hand impairment following stroke.

Lack of hypertonia in thumb muscles after stroke.

Despite the importance of the thumb to hand function, little is known about the origins of thumb impairment poststroke. Accordingly, the primary purpose of this study was to assess whether thumb flexors have heightened stretch reflexes (SRs) following stroke-induced hand impairment. The secondary purpose was to compare SR characteristics of thumb flexors in relation to those of finger flexors since it is unclear whether SR properties of both muscle groups are similarly affected poststroke. Stretch reflexes in thumb and finger flexors were assessed at rest on the paretic side in each of 12 individuals with chronic, severe, stroke-induced hand impairment and in the dominant thumb in each of eight control subjects also at rest. Muscle activity and passive joint flexion torques were measured during imposed slow (SS) and fast stretches (FS) of the flexors that span the metacarpophalangeal joints. Putative spasticity was then quantified in terms of the peak difference between FS and SS joint torques and electromyographic changes. For both the hemiparetic and control groups, the mean normalized peak torque differences (PTDs) measured in thumb flexors were statistically indistinguishable (P = 0.57). In both groups, flexor muscles were primarily unresponsive to rapid stretching. For 10 of 12 hemiparetic subjects, PTDs in thumb flexors were less than those in finger flexors (P = 0.03). Paretic finger flexor muscle reflex activity was consistently elicited during rapid stretching. These results may reflect an important difference between thumb and finger flexors relating to properties of the involved muscle afferents and spinal motoneurons.

Effect of finger posture on the tendon force distribution within the finger extensor mechanism.

Understanding the transformation of tendon forces into joint torques would greatly aid in the investigation of the complex temporal and spatial coordination of multiple muscles in finger movements. In this study, the effects of the finger posture on the tendon force transmission within the finger extensor apparatus were investigated. In five cadaver specimens, a constant force was applied sequentially to the two extrinsic extensor tendons in the index finger, extensor digitorum communis and extensor indicis proprius. The responses to this loading, i.e., fingertip force/moment and regional strains of the extensor apparatus, were measured and analyzed to estimate the tendon force transmission into the terminal and central slips of the extensor hood. Repeated measures analysis of variance revealed that the amount of tendon force transmitted to each tendon slip was significantly affected by finger posture, specifically by the interphalangeal (IP) joint angles (p<0.01). Tendon force transmitted to each of the tendon slips was found to decrease with the IP flexion. The main effect of the metacarpophalangeal (MCP) joint angle was not as consistent as the IP angle, but there was a strong interaction effect for which MCP flexion led to large decreases in the slip forces (>30%) when the IP joints were extended. The ratio of terminal slip force:central slip force remained relatively constant across postures at approximately 1.7:1. Force dissipation into surrounding structures was found to be largely responsible for the observed force-posture relationship. Due to the significance of posture in the force transmission to the tendon slips, the impact of finger posture should be carefully considered when studying finger motor control or examining injury mechanisms in the extensor apparatus.

Estimation of the effective static moment arms of the tendons in the index finger extensor mechanism.

A novel technique to estimate the contribution of finger extensor tendons to joint moment generation was proposed. Effective static moment arms (ESMAs), which represent the net effects of the tendon force on joint moments in static finger postures, were estimated for the 4 degrees of freedom (DOFs) in the index finger. Specifically, the ESMAs for the five tendons contributing to the finger extensor apparatus were estimated by directly correlating the applied tendon force to the measured resultant joint moments in cadaveric hand specimens. Repeated measures analysis of variance revealed that the finger posture, specifically interphalangeal joint angles, had significant effects on the measured ESMA values in 7 out of 20 conditions (four DOFs for each of the five muscles). Extensor digitorum communis and extensor indicis proprius tendons were found to have greater MCP ESMA values when IP joints are flexed, whereas abduction ESMAs of all muscles except extensor digitorum profundus were mainly affected by MCP flexion. The ESMAs were generally smaller than the moment arms estimated in previous studies that employed kinematic measurement techniques. Tendon force distribution within the extensor hood and dissipation into adjacent structures are believed to contribute to the joint moment reductions, which result in smaller ESMA values.

Use of intrinsic thumb muscles may help to improve lateral pinch function restored by tendon transfer.

BACKGROUND: For surgical reconstruction of lateral pinch following tetraplegia, the function of the paralyzed flexor pollicis longus is commonly restored. The purpose of this study was to investigate if one of the intrinsic muscles could generate a more suitably directed thumb-tip force during lateral pinch than that of flexor pollicis longus. METHODS: Endpoint force resulting from 10 N applied to each thumb muscle was measured in eleven upper extremity cadaveric specimens. We utilized the Kruskal-Wallis test (alpha=0.05) to determine whether thumb-tip forces of intrinsic muscles were less directed toward the base of the thumb, i.e., proximally directed, than the thumb-tip force produced by flexor pollicis longus. Additionally, a biomechanical model was used to assess the effect of an increase in tendon force on intrinsic muscle endpoint forces. FINDINGS: All of the intrinsic muscles produced thumb-tip force vectors, ranging from 127 degrees to 156 degrees , that were significantly (P<0.009) less proximally directed than that of flexor pollicis longus (66 degrees (46 degrees )). A biomechanical model predicted that intrinsic muscle thumb-tip forces would vary non-linearly with tendon force. A 2-fold increase in tendon force produced, on average, a 2.3-fold increase in force magnitude and an 8 degrees shift in force direction across all intrinsic muscles. INTERPRETATION: This study suggests the possibility of using an intrinsic muscle, e.g., the flexor pollicis brevis (ulnar head), instead of flexor pollicis longus, to produce a more advantageously directed thumb-tip force during lateral pinch in the surgically-reconstructed tetraplegic thumb and thus potentially enhance function.

The effect of percutaneous pin fixation of the interphalangeal joint on the thumb-tip force produced by the flexor pollicis longus: a cadaver study.

PURPOSE: Interphalangeal joint stabilization often is performed concomitantly with tendon transfers that restore key pinch (lateral pinch) to the paralyzed thumb. The goal of this study was to measure the effect of interphalangeal joint stabilization via percutaneous pin fixation on the thumb-tip force produced by the flexor pollicis longus (FPL). METHODS: We applied 10 N of force to the tendon of the FPL in 7 cadaveric specimens and measured the resulting thumb-tip force in the intact thumb and after stabilization of the interphalangeal joint. RESULTS: The nominal thumb-tip force was approximately 6 times less than the applied force and was directed primarily in the thumb's plane of flexion-extension at an oblique angle of 44 degrees relative to the palmar direction (the direction that is perpendicular to the thumb tip in the plane). Joint stabilization increased significantly the nominal force and oriented the force more toward the palmar direction (ie, decreased the obliqueness of the force). CONCLUSIONS: After paralysis and a tendon transfer to the paralyzed FPL the FPL is often the only muscle actuating the thumb. We conclude that the oblique nominal force direction is prone to cause the thumb to slip during pinch. Joint stabilization, however, has the capacity to reduce the tendency for slippage because it rotates the force toward the palmar direction.

Towards a realistic biomechanical model of the thumb: the choice of kinematic description may be more critical than the solution method or the variability/uncertainty of musculoskeletal parameters.

A biomechanical model of the thumb can help researchers and clinicians understand the clinical problem of how anatomical variability contributes to the variability of outcomes of surgeries to restore thumb function. We lack a realistic biomechanical model of the thumb because of the variability/uncertainty of musculoskeletal parameters, the multiple proposed kinematic descriptions and methods to solve the muscle redundancy problem, and the paucity of data to validate the model with in vivo coordination patterns and force output. We performed a multi-stage validation of a biomechanical computer model against our measurements of maximal static thumbtip force and fine-wire electromyograms (EMG) from 8 thumb muscles in each of five orthogonal directions in key and opposition pinch postures. A low-friction point-contact at the thumbtip ensured that subjects did not produce thumbtip torques during force production. The 3-D, 8-muscle biomechanical thumb model uses a 5-axis kinematic description with orthogonal and intersecting axes of rotation at the carpometacarpal and metacarpophalangeal joints. We represented the 50 musculoskeletal parameters of the model as stochastic variables based on experimental data, and ran Monte Carlo simulations in the "inverse" and "forward" directions for 5000 random instantiations of the model. Two inverse simulations (predicting the distribution of maximal static thumbtip forces and the muscle activations that maximized force) showed that: the model reproduces at most 50% of the 80 EMG distributions recorded (eight muscle excitations in 5 force directions in two postures); and well-directed thumbtip forces of adequate magnitude are predicted only if accompanied by unrealistically large thumbtip torques (0.64+/-0.28Nm). The forward simulation (which fed the experimental distributions of EMG through random instantiations of the model) resulted in misdirected thumbtip force vectors (within 74.3+/-24.5 degrees from the desired direction) accompanied by doubly large thumbtip torques (1.32+/-0.95Nm). Taken together, our results suggest that the variability and uncertainty of musculoskeletal parameters and the choice of solution method are not the likely reason for the unrealistic predictions obtained. Rather, the kinematic description of the thumb we used is not representative of the transformation of net joint torques into thumbtip forces/torques in the human thumb. Future efforts should focus on validating alternative kinematic descriptions of the thumb.