Sensitivity analyses of probabilistic and deterministic DTI tractography methodologies for studying arm muscle architecture.

PURPOSE: To determine the sensitivity profiles of probabilistic and deterministic DTI tractography methods in estimating geometric properties in arm muscle anatomy. METHODS: Spin-echo diffusion-weighted MR images were acquired in the dominant arm of 10 participants. Both deterministic and probabilistic tractography were performed in two different muscle architectures of the parallel-structured biceps brachii (and the pennate-structured flexor carpi ulnaris. Muscle fascicle geometry estimates and number of fascicles were evaluated with respect to tractography turning angle, polynomial fitting order, and SNR. The DTI tractography estimated fascicle lengths were compared with measurements obtained from conventional cadaveric dissection and ultrasound modalities. RESULTS: The probabilistic method generally estimated fascicle lengths closer to ranges reported by conventional methods than the deterministic method, most evident in the biceps brachii (p > 0.05), consisting of longer, arc-like fascicles. For both methods, a wide turning angle (50º-90°) generated fascicle lengths that were in close agreement with conventional methods, most evident in the flexor carpi ulnaris (p > 0.05), consisting of shorter, feather-like fascicles. The probabilistic approach produced at least two times more fascicles than the deterministic approach. For both approaches, second-order fitting yielded about double the complete tracts as third-order fitting. In both muscles, as SNR decreased, deterministic tractography produced less fascicles but consistent geometry (p > 0.05), whereas probabilistic tractography produced a consistent number but altered geometry of fascicles (p < 0.001). CONCLUSION: Findings from this study provide best practice recommendations for implementing DTI tractography in skeletal muscle and will inform future in vivo studies of healthy and pathological muscle structure.

Serial sarcomere number is substantially decreased within the paretic biceps brachii in individuals with chronic hemiparetic stroke.

A muscle's structure, or architecture, is indicative of its function and is plastic; changes in input to or use of the muscle alter its architecture. Stroke-induced neural deficits substantially alter both input to and usage of individual muscles. We combined in vivo imaging methods (second-harmonic generation microendoscopy, extended field-of-view ultrasound, and fat-suppression MRI) to quantify functionally meaningful architecture parameters in the biceps brachii of both limbs of individuals with chronic hemiparetic stroke and in age-matched, unimpaired controls. Specifically, serial sarcomere number (SSN) and physiological cross-sectional area (PCSA) were calculated from data collected at three anatomical scales: sarcomere length, fascicle length, and muscle volume. The interlimb differences in SSN and PCSA were significantly larger for stroke participants than for participants without stroke (P = 0.0126 and P = 0.0042, respectively), suggesting we observed muscle adaptations associated with stroke rather than natural interlimb variability. The paretic biceps brachii had ∼8,200 fewer serial sarcomeres and ∼2 cm2 smaller PCSA on average than the contralateral limb (both P < 0.0001). This was manifested by substantially smaller muscle volumes (112 versus 163 cm3), significantly shorter fascicles (11.0 versus 14.0 cm; P < 0.0001), and comparable sarcomere lengths (3.55 versus 3.59 µm; P = 0.6151) between limbs. Most notably, this study provides direct evidence of the loss of serial sarcomeres in human muscle observed in a population with neural impairments that lead to disuse and chronically place the affected muscle at a shortened position. This adaptation is consistent with functional consequences (increased passive resistance to elbow extension) that would amplify already problematic, neurally driven motor impairments.

The Biomechanical Basis of the Claw Finger Deformity: A Computational Simulation Study.

PURPOSE: Claw finger deformity occurs during attempted finger extension in patients whose intrinsic finger muscles are weakened or paralyzed by neural impairments. The deformity is generally not acutely present after intrinsic muscle palsy. The delayed onset, with severity progressing over time, suggests soft tissue changes that affect the passive biomechanics of the hand exacerbate and advance the deformity. Clinical interventions may be more effective if such secondary biomechanical changes are effectively addressed. Using a computational model, we simulated these altered soft tissue biomechanical properties to quantify their effects on coordinated finger extension. METHODS: To evaluate the effects of maladaptive changes in soft tissue biomechanical properties on the development and progression of the claw finger deformity after intrinsic muscle palsy, we completed 45 biomechanical simulations of cyclic index finger flexion and extension, varying the muscle excitation level, clinically relevant biomechanical factors, and wrist position. We evaluated to what extent (1) increased joint laxity, (2) decreased mechanical advantage of the extensors about the proximal interphalangeal joint, and (3) shortening of the flexor muscles contributed to the development of claw finger deformity in an intrinsic-minus hand model. RESULTS: Of the mechanisms studied, shortening (or contracture) of the extrinsic finger flexors was the factor most associated with the development of claw finger deformity in simulation. CONCLUSIONS: These simulations suggest that adaptive shortening of the extrinsic finger flexors is required for the development of claw finger deformity. Increased joint laxity and decreased extensor mechanical advantage only contributed to the severity of the deformity in simulations when shortening of the flexor muscles was present. CLINICAL RELEVANCE: In both the acute and chronic stages of intrinsic finger paralysis, maintaining extrinsic finger flexor length should be an area of focus in rehabilitation to prevent formation of the claw finger deformity and achieve optimal outcomes after surgical interventions.

Decoupling the Wrist: A Cadaveric Experiment Examining Wrist Kinematics Following Midcarpal Fusion and Scaphoid Excision.

At the wrist, kinematic coupling (the relationship between flexion-extension and radial-ulnar deviation) facilitates function. Although the midcarpal joint is critical for kinematic coupling, many surgeries, such as 4-corner fusion (4CF) and scaphoidexcision 4-corner fusion (SE4CF), modify the midcarpal joint. This study examines how 4CF and SE4CF influence kinematic coupling by quantifying wrist axes of rotation. Wrist axes of rotation were quantified in 8 cadaveric specimens using an optimization algorithm, which fit a 2-revolute joint model to experimental data. In each specimen, data measuring the motion of the third metacarpal relative to the radius was collected for 3 conditions (nonimpaired, 4CF, SE4CF). The calculated axes of rotation were compared using spherical statistics. The angle between the axes of rotation was used to assess coupling, as the nonimpaired wrist has skew axes (ie, angle between axes approximately 60°). Following 4CF and SE4CF, the axes are closer to orthogonal than those of the nonimpaired wrist. The mean angle (±95% confidence interval) between the axes was 92.6° ± 25.2° and 99.8° ± 22.0° for 4CF and SE4CF, respectively. The axes of rotation defined in this study can be used to define joint models, which will facilitate more accurate computational and experimental studies of these procedures.

Multicenter Survey of the Effects of Rehabilitation Practices on Pinch Force Strength After Tendon Transfer to Restore Pinch in Tetraplegia.

OBJECTIVE: To identify key components of conventional therapy after brachioradialis (BR) to flexor pollicis longus (FPL) transfer, a common procedure to restore pinch strength, and evaluate whether any of the key components of therapy were associated with pinch strength outcomes. DESIGN: Rehabilitation protocols were surveyed in 7 spinal cord injury (SCI) centers after BR to FPL tendon transfer. Key components of therapy, including duration of immobilization, participation, and date of initiating therapy activities (mobilization, strengthening, muscle reeducation, functional activities, and home exercise), were recorded by the patient's therapist. Pinch outcomes were recorded with identical equipment at 1-year follow-up. SETTING: Seven SCI rehabilitation centers where the BR to FPL surgery is performed on a routine basis. PARTICIPANTS: Thirty-eight arms from individuals with C5-7 level SCI injury who underwent BR to FPL transfer surgery (N=34). INTERVENTION: Conventional therapy according to established protocol in each center. MAIN OUTCOME MEASURES: The frequency of specific activities and their time of initiation (relative to surgery) were expressed as means and 95% confidence intervals. Outcome measures included pinch strength and the Canadian Occupational Performance Measure (COPM). Spearman rank-order correlations determined significant relations between pinch strength and components of therapy. RESULTS: There was similarity in the key components of therapy and in the progression of activities. Early cast removal was associated with pinch force (Spearman ρ=-.40, P=.0269). Pinch force was associated with improved COPM performance (Spearman ρ=.48, P=.0048) and satisfaction (Spearman ρ=.45, P=.0083) scores. CONCLUSIONS: Initiating therapy early after surgery is beneficial after BR to FPL surgery. Postoperative therapy protocols have the potential to significantly influence the outcome of tendon transfers after tetraplegia.

Upper Limb Strength and Muscle Volume in Healthy Middle-Aged Adults.

Our purpose was to characterize shoulder muscle volume and isometric moment, as well as their relationship, for healthy middle- aged adults. Muscle volume and maximum isometric joint moment were assessed for 6 functional muscle groups of the shoulder, elbow, and wrist in 10 middle-aged adults (46–60 y, 5M, 5F). Compared with young adults, shoulder abductors composed a smaller percentage of total muscle volume (P = .0009) and there was a reduction in shoulder adductor strength relative to elbow flexors (P = .012). We observed a consistent ordering of moment-generating capacity among functional groups across subjects. Although total muscle volume spanned a 2.3-fold range, muscle volume was distributed among functional groups in a consistent manner across subjects. On average, 72% of the variation in joint moment could be explained by the corresponding functional group muscle volume. These data are useful for improved modeling of upper limb musculoskeletal performance in middle-aged subjects, and may improve computational predictions of function for this group.

Withdrawal reflexes in the upper limb adapt to arm posture and stimulus location.

INTRODUCTION: Withdrawal reflexes in the leg adapt in a context-appropriate manner to remove the limb from noxious stimuli, but the extent to which withdrawal reflexes adapt in the arm remains unknown. METHODS: We examined the adaptability of withdrawal reflexes in response to nociceptive stimuli applied in different arm postures and to different digits. Reflexes were elicited at rest, and kinetic and electromyographic responses were recorded under isometric conditions, thereby allowing motorneuron pool excitability to be controlled. RESULTS: Endpoint force changed from a posterior-lateral direction in a flexed posture to predominantly a posterior direction in a more extended posture [change in force angle (mean ± standard deviation) 35.6 ± 5.0°], and the force direction changed similarly with digit I stimulation compared with digit V (change = 22.9 ± 2.9°). CONCLUSIONS: The withdrawal reflex in the human upper limb adapts in a functionally relevant manner when elicited at rest.

Evaluation of hand motion capture protocol using static computed tomography images: application to an instrumented glove.

There has been a marked increase in the use of hand motion capture protocols in the past 20 yr. However, their absolute accuracies and precisions remain unclear. The purpose of this technical brief was to present a method for evaluating the accuracy and precision of the joint angles determined by a hand motion capture protocol using simultaneously collected static computed tomography (CT) images. The method consists of: (i) recording seven functional postures using both the motion capture protocol and a CT scanner; (ii) obtaining principal axes of the bones in each method; (iii) calculating the flexion angle at each joint for each method as the roll angle of the composite, sequential, roll-pitch-yaw rotations relating the orientation of the distal bone to the proximal bone; and (iv) comparing corresponding joint angle measurements. For demonstration, we applied the method to a Cyberglove protocol. Accuracy and precision of the instrumented-glove protocol were calculated as the mean and standard deviation, respectively, of the differences between the angles determined from the Cyberglove output and the CT images across the seven postures. Implementation in one subject highlighted substantial errors, especially for the distal joints of the fingers. This technical note both clearly demonstrates the need for future work and introduces a solid, technical approach with the potential to improve the current state of such assessments in our field.

Multi-sweep 3-dimensional ultrasound is accurate for in vivo muscle volume quantification, expanding use to larger muscles.

Muscle volume is an important parameter in analyzing three-dimensional structure of muscle-tendon units. Three-dimensional ultrasound (3DUS) enables excellent quantification of muscle volume in small muscles; however, when a muscle's cross sectional area is larger than the field of view of the ultrasound transducer at any point along its length, more than one sweep is necessary to reconstruct muscle anatomy. Confounding image registration errors have been reported between multiple sweeps. Here, we detail imaging phantom studies used to (1) define an acquisition protocol that reduces misalignment in 3D reconstruction caused by muscle deformation, and (2) quantify accuracy of 3DUS for measures of volume when phantoms are too large to be fully imaged via a single transducer sweep. Finally, we (3) establish the feasibility of our protocol for in vivo measures by comparing biceps brachii muscle volumes using 3DUS and magnetic resonance imaging (MRI). Phantom studies indicate operator intent to use constant pressure across multiple sweeps effectively mitigates image misalignment, yielding minimal volume error (1.70 ± 1.30%). Intentional application of different pressure between sweeps replicated discontinuity observed previously, leading to larger errors (5.30 ± 0.94%). Based on these findings, we adopted a gel bag standoff and acquired in vivo images of biceps brachii muscles using 3DUS and compared this volume to MRI. We did not observe misalignment errors and there were no significant differences between imaging modalities (-0.71 ± 5.03 %), indicating 3DUS can reliably be used to quantify muscle volume in larger muscles requiring multiple transducer sweeps.

A Musculoskeletal Model of the Hand and Wrist Capable of Simulating Functional Tasks.

OBJECTIVE: The purpose of this work was to develop an open-source musculoskeletal model of the hand and wrist and to evaluate its performance during simulations of functional tasks. METHODS: The current model was developed by adapting and expanding upon existing models. An optimal control theory framework that combines forward-dynamics simulations with a simulated-annealing optimization was used to simulate maximum grip and pinch force. Active and passive hand opening were simulated to evaluate coordinated kinematic hand movements. RESULTS: The model's maximum grip force production matched experimental measures of grip force, force distribution amongst the digits, and displayed sensitivity to wrist flexion. Simulated lateral pinch strength replicated in vivo palmar pinch strength data. Additionally, predicted activations for 7 of 8 muscles fell within variability of EMG data during palmar pinch. The active and passive hand opening simulations predicted reasonable activations and demonstrated passive motion mimicking tenodesis, respectively. CONCLUSION: This work advances simulation capabilities of hand and wrist models and provides a foundation for future work to build upon. SIGNIFICANCE: This is the first open-source musculoskeletal model of the hand and wrist to be implemented during both functional kinetic and kinematic tasks. We provide a novel simulation framework to predict maximal grip and pinch force which can be used to evaluate how potential surgical and rehabilitation interventions influence these functional outcomes while requiring minimal experimental data.

Biomechanical constraints on the feedforward regulation of endpoint stiffness.

Although many daily tasks tend to destabilize arm posture, it is still possible to have stable interactions with the environment by regulating the multijoint mechanics of the arm in a task-appropriate manner. For postural tasks, this regulation involves the appropriate control of endpoint stiffness, which represents the stiffness of the arm at the hand. Although experimental studies have been used to evaluate endpoint stiffness control, including the orientation of maximal stiffness, the underlying neural strategies remain unknown. Specifically, the relative importance of feedforward and feedback mechanisms has yet to be determined due to the difficulty separately identifying the contributions of these mechanisms in human experiments. This study used a previously validated three-dimensional musculoskeletal model of the arm to quantify the degree to which the orientation of maximal endpoint stiffness could be changed using only steady-state muscle activations, used to represent feedforward motor commands. Our hypothesis was that the feedforward control of endpoint stiffness orientation would be significantly constrained by the biomechanical properties of the musculoskeletal system. Our results supported this hypothesis, demonstrating substantial biomechanical constraints on the ability to regulate endpoint stiffness throughout the workspace. The ability to regulate stiffness orientation was further constrained by additional task requirements, such as the need to support the arm against gravity or exert forces on the environment. Together, these results bound the degree to which slowly varying feedforward motor commands can be used to regulate the orientation of maximum arm stiffness and provide a context for better understanding conditions in which feedback control may be needed.

Computational Development of Jacobian Matrices for Complex Spatial Manipulators.

Current methods for developing manipulator Jacobian matrices are based on traditional kinematic descriptions such as Denavit and Hartenberg parameters. The resulting symbolic equations for these matrices become cumbersome and computationally inefficient when dealing with more complex spatial manipulators, such as those seen in the field of biomechanics. This paper develops a modified method for Jacobian development based on generalized kinematic equations that incorporates partial derivatives of matrices with Leibniz's Law (the product rule). It is shown that a set of symbolic matrix functions can be derived that improve computational efficiency when used in MATLAB(®) M-Files and are applicable to any spatial manipulator. An articulated arm subassembly and a musculoskeletal model of the hand are used as examples.

Muscle short-range stiffness can be used to estimate the endpoint stiffness of the human arm.

The mechanical properties of the human arm are regulated to maintain stability across many tasks. The static mechanics of the arm can be characterized by estimates of endpoint stiffness, considered especially relevant for the maintenance of posture. At a fixed posture, endpoint stiffness can be regulated by changes in muscle activation, but which activation-dependent muscle properties contribute to this global measure of limb mechanics remains unclear. We evaluated the role of muscle properties in the regulation of endpoint stiffness by incorporating scalable models of muscle stiffness into a three-dimensional musculoskeletal model of the human arm. Two classes of muscle models were tested: one characterizing short-range stiffness and two estimating stiffness from the slope of the force-length curve. All models were compared with previously collected experimental data describing how endpoint stiffness varies with changes in voluntary force. Importantly, muscle properties were not fit to the experimental data but scaled only by the geometry of individual muscles in the model. We found that force-dependent variations in endpoint stiffness were accurately described by the short-range stiffness of active arm muscles. Over the wide range of evaluated arm postures and voluntary forces, the musculoskeletal model incorporating short-range stiffness accounted for 98 ± 2, 91 ± 4, and 82 ± 12% of the variance in stiffness orientation, shape, and area, respectively, across all simulated subjects. In contrast, estimates based on muscle force-length curves were less accurate in all measures, especially stiffness area. These results suggest that muscle short-range stiffness is a major contributor to endpoint stiffness of the human arm. Furthermore, the developed model provides an important tool for assessing how the nervous system may regulate endpoint stiffness via changes in muscle activation.

Comparison of wrist and elbow stabilization following pinch reconstruction in tetraplegia.

PURPOSE: Individuals with spinal cord injuries resulting in tetraplegia may receive tendon transfer surgery to restore grasp and pinch function. These procedures often involve rerouting the brachioradialis (Br) and the extensor carpi radialis longus tendons volar to the flexion-extension axis of the wrist, leaving the extensor carpi radialis brevis (ECRB) muscle to provide wrist extension strength. The purpose of this study was to determine whether externally stabilizing the wrist after transfer procedures would improve the ability to activate the transferred Br and resulting pinch force, similar to the effect observed when the elbow is externally stabilized. METHODS: We used a one-way repeated-measures study design to determine the effect of 3 support conditions on muscle activation and lateral pinch force magnitude in 8 individuals with tetraplegia and previous tendon transfer surgeries. Muscle activation was recorded from Br and ECRB with intramuscular electrodes and from biceps and triceps muscles with surface electrodes. We quantified pinch strength with a 6-axis force sensor and custom grip. We recorded measurements in 3 support conditions: with the arm self-stabilized, with elbow stabilization, and with elbow and wrist stabilization. Pairwise differences were tested using Wilcoxon signed-rank tests. RESULTS: Maximum effort pinch force magnitude and Br activation were significantly increased in both supported conditions compared with the self-supported trials. The addition of wrist stabilization had no significant effect compared with elbow stabilization alone. CONCLUSIONS: A strong ECRB has adequate strength to extend the wrist, even after multiple transfers that contribute an additional flexion moment from strong activation of donor muscles. Anatomical and functional differences between the wrist and elbow musculature are important determinants for self-stabilizing joints proximal to the tendon transfer. The ability to increase Br activation and resulting pinch force may be determined, in part, by the individual's ability to develop new coordination strategies.

Passive Properties of the Wrist and Fingers Following Chronic Hemiparetic Stroke: Interlimb Comparisons in Persons With and Without a Clinical Treatment History That Includes Botulinum Neurotoxin.

Background: Neural impairments that follow hemiparetic stroke may negatively affect passive muscle properties, further limiting recovery. However, factors such as hypertonia, spasticity, and botulinum neurotoxin (BoNT), a common clinical intervention, confound our understanding of muscle properties in chronic stroke. Objective: To determine if muscle passive biomechanical properties are different following prolonged, stroke-induced, altered muscle activation and disuse. Methods: Torques about the metacarpophalangeal and wrist joints were measured in different joint postures in both limbs of participants with hemiparetic stroke. First, we evaluated 27 participants with no history of BoNT; hand impairments ranged from mild to severe. Subsequently, seven participants with a history of BoNT injections were evaluated. To mitigate muscle hypertonia, torques were quantified after an extensive stretching protocol and under conditions that encouraged participants to sleep. EMGs were monitored throughout data collection. Results: Among participants who never received BoNT, no significant differences in passive torques between limbs were observed. Among participants who previously received BoNT injections, passive flexion torques about their paretic wrist and finger joints were larger than their non-paretic limb (average interlimb differences = +42.0 ± 7.6SEM Ncm, +26.9 ± 3.9SEM Ncm, respectively), and the range of motion for passive finger extension was significantly smaller (average interlimb difference = -36.3° ± 4.5°SEM; degrees). Conclusion: Our results suggest that neural impairments that follow chronic, hemiparetic stroke do not lead to passive mechanical changes within the wrist and finger muscles. Rather, consistent with animal studies, the data points to potential adverse effects of BoNT on passive muscle properties post-stroke, which warrant further consideration.

Variability of in vivo Sarcomere Length Measures in the Upper Limb Obtained With Second Harmonic Generation Microendoscopy.

The lengths of a muscle's sarcomeres are a primary determinant of its ability to contract and produce force. In addition, sarcomere length is a critical parameter that is required to make meaningful comparisons of both the force-generating and excursion capacities of different muscles. Until recently, in vivo sarcomere length data have been limited to invasive or intraoperative measurement techniques. With the advent of second harmonic generation microendoscopy, minimally invasive measures of sarcomere length can be made for the first time. This imaging technique expands our ability to study muscle adaptation due to changes in stimulus, use, or disease. However, due to past inability to measure sarcomeres outside of surgery or biopsy, little is known about the natural, anatomical variability in sarcomere length in living human subjects. To develop robust experimental protocols that ensure data provide accurate representations of a muscle's sarcomere lengths, we sought to quantify experimental uncertainty associated with in vivo measures of sarcomere lengths. Specifically, we assessed the variability in sarcomere length measured (1) within a single image, along a muscle fiber, (2) across images captured within a single trial, across trials, and across days, as well as (3) across locations in the muscle using second harmonic generation in two upper limb muscles with different muscle architectures, functions, and sizes. Across all of our measures of variability we estimate that the magnitude of the uncertainty for in vivo sarcomere length is on the order of ∼0.25 µm. In the two upper limb muscles studied we found larger variability in sarcomere lengths within a single insertion than across locations. We also developed custom code to make measures of sarcomere length variability across a single fiber and determined that this codes' accuracy is an order of magnitude smaller than our measurement uncertainty due to sarcomere variability. Together, our findings provide guidance for the development of robust experimental design and analysis of in vivo sarcomere lengths in the upper limb.

Obtaining Quality Extended Field-of-View Ultrasound Images of Skeletal Muscle to Measure Muscle Fascicle Length.

Muscle fascicle length, which is commonly measured in vivo using traditional ultrasound, is an important parameter defining a muscle's force generating capacity. However, over 90% of all upper limb muscles and 85% of all lower limb muscles have optimal fascicle lengths longer than the field-of-view of common traditional ultrasound (T-US) probes. A newer, less frequently adopted method called extended field-of-view ultrasound (EFOV-US) can enable direct measurement of fascicles longer than the field-of-view of a single T-US image. This method, which automatically fits together a sequence of T-US images from a dynamic scan, has been demonstrated to be valid and reliable for obtaining muscle fascicle lengths in vivo. Despite the numerous skeletal muscles with long fascicles and the validity of the EFOV-US method for making measurements of such fascicles, few published studies have utilized this method. In this study, we demonstrate both how to implement the EFOV-US method to obtain high quality musculoskeletal images and how to quantify fascicle lengths from those images. We expect that this demonstration will encourage the use of the EFOV-US method to increase the pool of muscles, both in healthy and impaired populations, for which we have in vivo muscle fascicle length data.

Model-Based Control of Individual Finger Movements for Prosthetic Hand Function.

Prosthetic devices for hand difference have advanced considerably in recent years, to the point where the mechanical dexterity of a state-of-the-art prosthetic hand approaches that of the natural hand. Control options for users, however, have not kept pace, meaning that the new devices are not used to their full potential. Promising developments in control technology reported in the literature have met with limited commercial and clinical success. We have previously described a biomechanical model of the hand that could be used for prosthesis control. The goal of this study was to evaluate the feasibility of this approach in terms of kinematic fidelity of model-predicted finger movement and the computational performance of the model. We show the performance of the model in replicating recorded hand and finger kinematics and find average correlations of 0.89 between modelled and recorded motions; we show that the computational performance of the simulations is fast enough to achieve real-time control with a robotic hand in the loop; and we describe the use of the model for controlling object gripping. Despite some limitations in accessing sufficient driving signals, the model performance shows promise as a controller for prosthetic hands when driven with recorded EMG signals. User-in-the-loop testing with amputees is necessary in future work to evaluate the suitability of available driving signals, and to examine translation of offline results to online performance.

Effect of biceps-to-triceps transfer on rotator cuff stress during upper limb weight-bearing lift in tetraplegia: A modeling and simulation analysis.

Rotator cuff stress during upper limb weight-bearing lifts presumably contribute to rotator cuff disease, which is the most common cause of shoulder pain in individuals with tetraplegia. Elbow extension strength appears to be a key determinant of rotator cuff stress during upper limb weight-bearing lifts since individuals with paraplegia who generate greater elbow extensor moments experience lower rotator cuff stress relative to individuals with tetraplegia. Biceps-to-triceps transfer surgery can increase elbow extension strength in individuals with tetraplegia. The purpose of this study was to determine whether active elbow extension via biceps transfer decreases rotator cuff stress during weight-bearing lifts in individuals with tetraplegia. A forward dynamics computational framework was used to estimate muscle stress during the lift; stress was computed as muscle force divided by the peak isometric muscle force. We hypothesized that rotator cuff stresses would be lower in simulated lifting with biceps transfer relative to simulated lifting without biceps transfer. We found that limited elbow extension strength in individuals with tetraplegia, regardless of whether elbow strength is enabled via biceps transfer or is residual after spinal cord injury, results in muscle stresses exceeding 85% of the peak isometric muscle stress in the supraspinatus, infraspinatus, and teres minor. The rotator cuff stresses we estimated suggest that performance of weight-bearing activities should be minimized or assisted in order to reduce the risk for shoulder pain. Our results also indicate that biceps transfer is unlikely to decrease rotator cuff stress during weight-bearing lifts in individuals with tetraplegia.

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.