The Effects of Boxing Glove Design on Thumb Position When Making a Fist for Striking.

ABSTRACT: Peveler, WW, Schoffstall, J, Coots, J, Kilian, J, and Glauser, J. The effects of boxing glove design on thumb position when making a fist for striking. J Strength Cond Res 38(5): 948-950, 2024-It has been suggested that boxing glove design alters thumb position increasing the risk of injury. The purpose of this study was to determine the effects of boxing glove design on thumb joint angles when making a fist. Ten experienced fighters participated in this study. A DEXA scan was used to produce an x-ray image of thumb position for all conditions (no gloves and 10-oz boxing gloves). Mean values for dependent measures were compared using a paired-sample T test and an alpha of 0.05. The carpometacarpal (CMC) joint angle was significantly different between no glove (14.1 ± 6.54°) and boxing glove (34.2 ± 7.60°) at p ≤ 0.001. The metacarpophalangeal (MP) joint angle was significantly different between no glove (132.6 ± 12.74°) and boxing glove (149.40 ± 8.15°) at p ≤ 0.001. The IP joint angle was not significantly different between no glove (135.50 ± 19.12°) and boxing glove (144.40 ± 17.39°) at p = 0.269. The perpendicular distance from the second metacarpal of the hand to the center of the MP joint was significantly different between no glove (0.48 ± 0.54 cm) and boxing glove (1.84 ± 0.29 cm) at p ≤ 0.001. Use of a boxing glove resulted in abduction of the thumb away from the hand and increased CMC and MP joint angles that were significantly different in relation to making a fist without a glove. Information from this study may provide insight into the high rate of thumb injury and provide insight for future boxing glove design.

The effect of trapeziometacarpal joint passive stiffness on mechanical loadings of cartilages.

Hypermobility of the trapeziometacarpal joint is commonly considered to be a potential risk factor for osteoarthritis. Nevertheless, the results remain controversial due to a lack of quantitative validation. The objective of this study was to evaluate the effect of joint laxity on the mechanical loadings of cartilage. A patient-specific finite element model of trapeziometacarpal joint passive stiffness was developed. The joint passive stiffness was modeled by creating linear springs all around the joint. The linear spring stiffness was determined by using an optimization process to fit force-displacement data measured during laxity tests performed on eight healthy volunteers. The estimated passive stiffness parameters were then included in a full thumb finite element simulation of a pinch grip task driven by muscle forces to evaluate the effect on trapeziometacarpal loading. The correlation between stiffness and the loading of cartilage in terms of joint contact pressure and maximum shear strain was analyzed. A significant negative correlation was found between the trapeziometacarpal joint passive stiffness and the contact pressure on trapezium cartilage during the simulated pinch grip task. These results therefore suggest that the hypermobility of the trapeziometacarpal joint could affect the contact pressure on trapezium cartilage and support the existence of an increased risk associated with hypermobility.

The first and second phases of the muscle compound action potential in the thumb are differently affected by electrical stimulation trains.

Sarcolemmal membrane excitability is often evaluated by considering the peak-to-peak amplitude of the compound muscle action potential (M wave). However, the first and second M-wave phases represent distinct properties of the muscle action potential, which are differentially affected by sarcolemma properties and other factors such as muscle architecture. Contrasting with previous studies in which voluntary contractions have been used to induce muscle fatigue, we used repeated electrically induced tetanic contractions of the adductor pollicis muscle and assessed the kinetics of M-wave properties during the course of the contractions. Eighteen participants (24 ± 6 yr; means ± SD) underwent 30 electrically evoked tetanic contractions delivered at 30 Hz, each lasting 3 s with 1 s intervals. We recorded the amplitudes of the first and second M-wave phases for each stimulation. During the initial stimulation train, the first and second M-wave phases exhibited distinct kinetics. The first phase amplitude showed a rapid decrease to reach ∼59% of its initial value (P < 0.001), whereas the second phase amplitude displayed an initial transient increase of ∼19% (P = 0.007). Within subsequent trains, both the first and second phase amplitudes consistently decreased as fatigue developed with a reduction during the last train reaching ∼47% of its initial value (P < 0.001). Analyzing the first M wave of each stimulation train unveiled different kinetics for the first and second phases during the initial trains, but these distinctions disappeared as fatigue progressed. These findings underscore the interplay of factors affecting the M wave and emphasize the significance of separately scrutinizing its first and second phases when assessing membrane excitability adjustments during muscle contractions.NEW & NOTEWORTHY Our understanding of how the first and second phases of the compound muscle action potential (M wave) behave during fatigue remains incomplete. Using electrically evoked repeated tetanic contractions of the adductor pollicis, we showed that the first and second phases of the M wave followed distinct kinetics only during the early stages of fatigue development. This suggests that the factors affecting the M-wave first and second phases may change as fatigue develops.

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.

Predictions of thumb, hand, and arm muscle parameters derived using force measurements of varying complexity and neural networks.

Subject-specific musculoskeletal models are a promising avenue for personalized healthcare. However, current methods for producing personalized models require dense, biomechanical datasets that include expensive and time-consuming physiological measurements. For personalized models to be clinically useful, we must be able to rapidly generate models from simple, easy to collect data. In this context, the objective of this paper is to evaluate if and how simple data, namely height/weight and pinch force data, can be used to achieve model personalization via machine learning. Using simulated lateral pinch force measurements from a synthetic population of 40,000 randomly generated subjects, we train neural networks to estimate four Hill-type muscle model parameters and bone density. We compare parameter estimates to the true parameters of 10,000 additional synthetic subjects. We also generate new personalized models using the parameter estimates and perform new lateral pinch simulations to compare predicted forces using these personalized models to those generated using a baseline model. We demonstrate that increasing force measurement complexity reduces the root-mean-square error in the majority of parameter estimates. Additionally, musculoskeletal models using neural network-based parameter estimates provide up to an 80% reduction in absolute error in simulated forces when compared to a generic model. Thus, easily obtained force measurements may be suitable for personalizing models of the thumb, although extending the method to more tasks and models involving other joints likely requires additional measurements.

Measuring fascicle lengths of extrinsic and intrinsic thumb muscles using extended field-of-view ultrasound.

Complex motion of the human thumb is enabled by the balanced architectural design of the extrinsic and intrinsic thumb muscles. Given that recent imaging advances have not yet been applied to enhance our understanding of the in vivo properties of thumb muscles, the objective of this study was to test the reliability and validity of measuring thumb muscle fascicle lengths using extended field of view ultrasound (EFOV-US). Three muscles (FPL: flexor pollicis longus, APB: abductor pollicis brevis, and ECU: extensor carpi ulnaris) were imaged in eight healthy adults (4 female; age, 21.6 ± 1.3 years; height, 175.9 ± 8.3 cm)[mean ± SD]. Measured fascicle lengths were compared to cadaveric data (all muscles) and ultrasound data (ECU only). Additionally, to evaluate how fascicle lengths scale with anthropometric measurements, height, forearm length, hand length, and hand width were recorded. The EFOV-US method obtained precise fascicle length measurements [mean ± SD] for the FPL (6.2 ± 0.5 cm), APB (5.1 ± 0.3 cm), and ECU (4.0 ± 0.4 cm). However, our EFOV-US measurements were consistently different (p < 0.05) than prior cadaveric data, highlighting the need to better understand differences between in vivo and ex vivo fascicle length measurements. Fascicle length was significantly related to only hand length (r2 = 0.56, p = 0.03) for APB, highlighting that anthropometric scaling may not accurately estimate thumb muscle length. As the first study to apply EFOV-US to measure thumb muscle fascicle lengths, this study expands the utility of this imaging technology within the upper limb.

Using a finite element model of the thumb to study Trapeziometacarpal joint contact during lateral pinch.

BACKGROUND: Finite element (FE) analysis is widely used in different fields of orthopaedic surgery, however, its application to the trapeziometacarpal joint has been limited due to the small size, complex biconcave-convex joint geometry, and complex musculature. The goal of this study was to improve upon existing models by creating a muscle-driven FE thumb model and use the model to simulate the biomechanical effect of hand therapy exercises and ligament reconstructive surgeries. METHODS: Bone and cartilage geometry were based on a CT dataset of a subject performing a static lateral pinch task. A previously validated musculoskeletal model was utilized to extract electromyography (EMG)-driven muscle forces. Five ligaments with biomechanical significance were modeled as springs using literature values and attached according to their anatomical landmarks. FINDINGS: The biomechanical consequence of various interventions was proxied as a change in the maximum cartilage stress. The result shows tightening the dorsal ligament complex (dorsal radial ligament, dorsal central ligament, posterior oblique ligament) is the most effective, achieving a stress reduction of 4.8%. Five exercises used in hand therapies were modeled, among which thenar eminence strengthening showed the most prominent stress reduction of 4.0%. Four ligament reconstructive surgeries were modeled, with Eaton-Littler reconstruction showed the most significant stress reduction of 25.0%. INTERPRETATION: Among the routinely utilized treatment options for early thumb osteoarthritis, we found that three methods: dorsal ligament imbrication, thenar eminence exercise, and the Eaton-Littler method may confer biomechanical advantages cartilage loading. These advantages align with the clinically observed favorable outcomes.

Morphometry of thenar muscles by water bath ultrasonography in trapeziometacarpal osteoarthritis: intra- and inter-rater reliability.

We studied the intra- and inter-rater reliability of muscle thickness and cross-sectional area measurements of thenar muscles with a water bath ultrasonography technique in eight healthy volunteers and 16 patients with trapeziometacarpal osteoarthritis. Thickness and cross-sectional area of the opponens pollicis, abductor pollicis brevis, flexor pollicis brevis, first dorsal interosseous and adductor pollicis muscle were measured. The results showed changes in the morphometric properties of the thenar muscles in patients with trapeziometacarpal osteoarthritis (TMC OA) compared with the healthy volunteers. In the dominant-sided patients (n = 14), there were lower cross-sectional area values for the abductor pollicis brevis and opponens pollicis muscles. In the non-dominant-sided patients (n = 10), there were lower cross-sectional area values for the abductor pollicis brevis and flexor pollicis brevis and lower muscle thickness of the abductor pollicis brevis. The water bath ultrasonography technique could be used to diagnose and treat diseases where changes in thenar muscle thickness and cross-sectional area can be expected.Level of evidence: III.

Alteration of Ligamento-Muscular Reflex Patterns After Cutaneous and Periarticular Desensitization of the Basal Thumb Joint: An Electromyographic Study.

PURPOSE: Stimulation of the dorsoradial ligament (DRL) of the first carpometacarpal joint (CMC-1) has shown a ligamento-muscular reflex pathway between the DRL and CMC-1 stabilizing muscles in healthy volunteers. However, it remains unclear how this ligamento-muscular reflex pattern is altered after anesthetizing sensory skin receptors and administering a further periarticular block around the CMC-1 joint, which may influence the dynamic aspects of joint stability. METHODS: Ligamento-muscular reflexes were obtained from the extensor pollicis longus, abductor pollicis longus, abductor pollicis brevis, and the first dorsal interosseous muscles in 10 healthy participants after establishing superficial anesthesia of the skin around the CMC-1. The DRL was stimulated with a fine wire electrode while EMG activities were recorded during isometric tip, key, and palmar pinch. The measurements were repeated after an additional periarticular CMC-1 block using 5 ml of 1% lidocaine. Average EMG values were analyzed to compare the prestimulus and poststimulus activity. RESULTS: Statistically significant changes in poststimulus EMG activity were observed in all 4 muscles and all 3 tested thumb positions. A markedly reduced activity in all 4 muscles was observed in the palmar position, followed by the tip and key pinch positions. Almost no reactions were observed in the first 20 ms poststimulus for all muscles in all positions. CONCLUSIONS: Superficial skin anesthesia and an additional periarticular CMC-1 block anesthesia resulted in a reduced ligamento-muscular reflex pattern in all 4 muscles. CLINICAL RELEVANCE: Ligamento-muscular reflexes play an important role in dynamic CMC-1 joint stability. The elimination of early reactions, those considered joint-protective reflexes, is a potential risk factor for developing osteoarthritis or injury because it results in an inability to adequately protect and stabilize the joint in sudden movements.

The effect of different grasping types on strain distributions in the trapezium of bonobos (Pan paniscus).

The thumb has played a key role in primate evolution due to its involvement in grasping and manipulation. A large component of this wide functionality is by virtue of the uniquely shaped trapeziometacarpal (TMC) joint. This TMC joint allows for a broad range of functional positions, but how its bone structure is adapted to withstand such a large variety of loading regimes is poorly understood. Here, we outline a novel, integrated finite element - micro finite element (FE-µFE) workflow to analyse strain distributions across the internal bony architecture. We have applied this modelling approach to study functional adaptation in the bonobo thumb. More specifically, the approach allows us to evaluate how strain is distributed through the trapezium upon loading of its distal articular facet. As loading conditions, we use pressure distributions for different types of grasping that were estimated in a previous study. Model evaluation shows that the simulated strain values fall within realistic boundaries of the mechanical response of bone. The results show that the strain distributions between the simulated grasps are highly similar, with dissipation towards the proximo-ulnar cluster of trabeculae regardless of trapezial bone architecture. This study presents an innovative FE-µFE approach to simulating strain distributions, and yields insight in the functional adaptation of the TMC joint in bonobos.

Anatomy and Biomechanics of the Thumb Carpometacarpal Joint.

This review discusses the anatomy and biomechanics of the thumb carpometacarpal (CMC) joint. This articulation between the trapezium and first metacarpal is integral for opposition and other complex movements necessary for pinch and grasp maneuvers. Fortunately, this joint is well equipped to handle the extreme forces imposed by these movements, as it is stabilized by an elaborate arrangement of ligaments and muscles. Without this stability, thumb subluxation would occur with loading during pinch and grasp, and human prehension would be impossible. Understanding the interactions occurring within this joint is essential for adequately treating pathology arising in this crucial joint.

Visual form of ASL verb signs predicts non-signer judgment of transitivity.

Longstanding cross-linguistic work on event representations in spoken languages have argued for a robust mapping between an event's underlying representation and its syntactic encoding, such that-for example-the agent of an event is most frequently mapped to subject position. In the same vein, sign languages have long been claimed to construct signs that visually represent their meaning, i.e., signs that are iconic. Experimental research on linguistic parameters such as plurality and aspect has recently shown some of them to be visually universal in sign, i.e. recognized by non-signers as well as signers, and have identified specific visual cues that achieve this mapping. However, little is known about what makes action representations in sign language iconic, or whether and how the mapping of underlying event representations to syntactic encoding is visually apparent in the form of a verb sign. To this end, we asked what visual cues non-signers may use in evaluating transitivity (i.e., the number of entities involved in an action). To do this, we correlated non-signer judgments about transitivity of verb signs from American Sign Language (ASL) with phonological characteristics of these signs. We found that non-signers did not accurately guess the transitivity of the signs, but that non-signer transitivity judgments can nevertheless be predicted from the signs' visual characteristics. Further, non-signers cue in on just those features that code event representations across sign languages, despite interpreting them differently. This suggests the existence of visual biases that underlie detection of linguistic categories, such as transitivity, which may uncouple from underlying conceptual representations over time in mature sign languages due to lexicalization processes.

Effect of the position of the interphalangeal joint on movements of the trapeziometacarpal joint during thumb opposition.

The Kapandji test is a simple method to score thumb opposition; however, the position of the interphalangeal joint of the thumb during this test has not been described. We aimed to quantitatively examine the effect of the thumb interphalangeal joint position on movements of the trapeziometacarpal joint during thumb opposition using the Kapandji test. The Kapandji test was carried out in 20 healthy participants during thumb interphalangeal joint extension and flexion. Movements of the joints and the activity of thenar muscles were recorded using motion capture and electromyography, respectively. We found that interphalangeal joint extension increased the trapeziometacarpal joint movement and thenar muscle activity compared with interphalangeal joint flexion, which contributed to thumb opposition at Kapandji Positions 0-6. These findings suggest the position of the thumb interphalangeal joint affects the trapeziometacarpal joint during thumb opposition, and assessment of thumb opposition using the Kapandji test is best done with the thumb interphalangeal joint in extension.

A Simplified Modification of the Henderson Extensor Carpi Ulnaris Opposition Transfer.

PURPOSE: There are several potential donor muscle-tendon units for a thumb opposition transfer. The extensor carpi ulnaris (ECU) is useful when the more usual donor units are not available. The technique and results of a simplified ECU opposition transfer elongated with a free tendon graft are described. METHODS: Ten ECU opposition transfers were performed using this modification of Henderson technique in 5 adults after complex trauma, 3 adults with median and ulnar nerve palsies, and 2 children with congenital hand differences. RESULTS: Seven patients achieved a Kapandji opposition score of 6 to the distal phalanx of the small finger, and 3 patients achieved a Kapandji score of 5 to the distal phalanx of the ring finger. None of the patients required a secondary tenolysis or developed a radial deviation imbalance of their wrist. CONCLUSIONS: This modification of the Henderson technique using ECU elongated with a free tendon graft and inserted directly and only into the abductor pollicis brevis tendon is an effective method of restoring opposition to the thumb, especially when other conventional donor muscle-tendon units are not available. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic V.

Multijointed Pneumatic Soft Hand with Flexible Thenar.

Soft robotic hands provide better safety and adaptability than rigid robotic hands. Furthermore, a multijointed structure that imitates the movement of a human hand represents significant progress in realizing its anthropomorphism. In this study, we present a multijointed pneumatic soft anthropomorphic hand that is capable of expressing letters through sign language and grasping different objects using three grasping modes, namely thumb grasping, precision grasping, and power grasping. This novel soft hand is composed of multijointed soft fingers, a thumb, thenar, and 3D-printed palm. Tests were performed to characterize the displacement track and force performance of the fingers, thumb, and thenar, which was made by mold casting silicone rubber. In addition, a dedicated pneumatic control system was designed and built to enable the soft hand to automatically perform the tasks set by specific programs. This new multijointed hand with a flexible thenar represents significant progress in the development of anthropomorphic bionic hands, offering the benefits of fast response, low cost, as well as ease of fabrication, assembly, and replacement.

Distinct behavior of the little finger during the vertical translation of an unsteady thumb platform while grasping.

Object stabilization while grasping is a common topic of research in motor control and robotics. Forces produced by the peripheral fingers (index and little) play a crucial role in sustaining the rotational equilibrium of a handheld object. In this study, we examined the contribution of the peripheral fingers towards object stabilization when the rotational equilibrium is disturbed. For this purpose, the thumb was placed over an unsteady platform and vertically translated. The task was to trace a trapezoid or an inverted trapezoid pattern by moving the thumb platform in the vertical direction. The thumb displacement data served as visual feedback to trace the pattern displayed. Participants were instructed to maintain the handle in static equilibrium at all times. We observed that the change in the normal force of the little finger due to the downward translation of the thumb was significantly greater than the change in the normal force of the index finger due to the upward translation. We speculate that morphological correlations (between thumb and little finger) during the displacement of the thumb might be a reason for such large increases in the little finger forces.

Classifying muscle parameters with artificial neural networks and simulated lateral pinch data.

OBJECTIVE: Hill-type muscle models are widely employed in simulations of human movement. Yet, the parameters underlying these models are difficult or impossible to measure in vivo. Prior studies demonstrate that Hill-type muscle parameters are encoded within dynamometric data. But, a generalizable approach for estimating these parameters from dynamometric data has not been realized. We aimed to leverage musculoskeletal models and artificial neural networks to classify one Hill-type muscle parameter (maximum isometric force) from easily measurable dynamometric data (simulated lateral pinch force). We tested two neural networks (feedforward and long short-term memory) to identify if accounting for dynamic behavior improved accuracy. METHODS: We generated four datasets via forward dynamics, each with increasing complexity from adjustments to more muscles. Simulations were grouped and evaluated to show how varying the maximum isometric force of thumb muscles affects lateral pinch force. Both neural networks classified these groups from lateral pinch force alone. RESULTS: Both neural networks achieved accuracies above 80% for datasets which varied only the flexor pollicis longus and/or the abductor pollicis longus. The inclusion of muscles with redundant functions dropped model accuracies to below 30%. While both neural networks were consistently more accurate than random guess, the long short-term memory model was not consistently more accurate than the feedforward model. CONCLUSION: Our investigations demonstrate that artificial neural networks provide an inexpensive, data-driven approach for approximating Hill-type muscle-tendon parameters from easily measurable data. However, muscles of redundant function or of little impact to force production make parameter classification more challenging.

Emerging of new bioartificial corticospinal motor synergies using a robotic additional thumb.

It is likely that when using an artificially augmented hand with six fingers, the natural five plus a robotic one, corticospinal motor synergies controlling grasping actions might be different. However, no direct neurophysiological evidence for this reasonable assumption is available yet. We used transcranial magnetic stimulation of the primary motor cortex to directly address this issue during motor imagery of objects' grasping actions performed with or without the Soft Sixth Finger (SSF). The SSF is a wearable robotic additional thumb patented for helping patients with hand paresis and inherent loss of thumb opposition abilities. To this aim, we capitalized from the solid notion that neural circuits and mechanisms underlying motor imagery overlap those of physiological voluntary actions. After a few minutes of training, healthy humans wearing the SSF rapidly reshaped the pattern of corticospinal outputs towards forearm and hand muscles governing imagined grasping actions of different objects, suggesting the possibility that the extra finger might rapidly be encoded into the user's body schema, which is integral part of the frontal-parietal grasping network. Such neural signatures might explain how the motor system of human beings is open to very quickly welcoming emerging augmentative bioartificial corticospinal grasping strategies. Such an ability might represent the functional substrate of a final common pathway the brain might count on towards new interactions with the surrounding objects within the peripersonal space. Findings provide a neurophysiological framework for implementing augmentative robotic tools in humans and for the exploitation of the SSF in conceptually new rehabilitation settings.

Comparison of the on-line effects of different motor simulation conditions on corticospinal excitability in healthy participants.

In healthy participants, corticospinal excitability is known to increase during motor simulations such as motor imagery (MI), action observation (AO) and mirror therapy (MT), suggesting their interest to promote plasticity in neurorehabilitation. Further comparing these methods and investigating their combination may potentially provide clues to optimize their use in patients. To this end, we compared in 18 healthy participants abductor pollicis brevis (APB) corticospinal excitability during MI, AO or MT, as well as MI combined with either AO or MT. In each condition, 15 motor-evoked potentials (MEPs) and three maximal M-wave were elicited in the right APB. Compared to the control condition, mean normalized MEP amplitude (i.e. MEP/M) increased during MI (P = .003), MT (P < .001) and MT + MI (P < .001), without any difference between the three conditions. No MEP modulation was evidenced during AO or AO + MI. Because MI provided no additional influence when combined with AO or MT, our results may suggest that, in healthy subjects, visual feedback and unilateral movement with a mirror may provide the greatest effects among all the tested motor simulations.