Kinematic and dynamic aspects of chimpanzee knuckle walking: finger flexors likely do not buffer ground impact forces.

Chimpanzees are knuckle walkers, with forelimbs contacting the ground by the dorsum of the finger's middle phalanges. As these muscular apes are given to high-velocity motions, the question arises of how the ground reaction forces are buffered so that no damage ensues in the load-bearing fingers. In the literature, it was hypothesized that the finger flexors help buffer impacts because in knuckle stance the metacarpophalangeal joints (MCPJs) are strongly hyperextended, which would elongate the finger flexors. This stretching of the finger flexor muscle-tendon units would absorb impact energy. However, EMG studies did not report significant finger flexor activity in knuckle walking. Although these data by themselves question the finger flexor impact buffering hypothesis, the present study aimed to critically investigate the hypothesis from a biomechanical point of view. Therefore, various aspects of knuckle walking were modeled and the finger flexor tendon displacements in the load-bearing fingers were measured in a chimpanzee cadaver hand, of which also an MRI was taken in knuckle stance. The biomechanics do not support the finger flexor impact buffering hypothesis. In knuckle walking, the finger flexors are not elongated to lengths where passive strain forces would become important. Impact buffering by large flexion moments at the MCPJs from active finger flexors would result in impacts at the knuckles themselves, which is dysfunctional for various biomechanical reasons and does not occur in real knuckle walking. In conclusion, the current biomechanical analysis in accumulation of previous EMG findings suggests that finger flexors play no role in impact buffering in knuckle walking.

Revisiting the segmental organization of the human spinal cord.

In classic anatomic atlases, the spinal cord is standardly represented in its anatomical form with symmetrically emerging anterior and posterior roots, which at the level of the intervertebral foramen combine into the spinal nerves. The parts of the cord delimited by the boundaries of the roots are called segments or myelomeres. Associated with their regular repetitive appearance is the notion that the cord is segmentally organized. This segmental view is reinforced by clinical practice. Spinal cord roots innervate specific body parts. The level of cord trauma is diagnosed by the de-innervation symptoms of these parts. However, systemically, the case for a segmentally organized cord is not so clear. To date, developmental and genetic research points to a regionally rather than a segmentally organized cord. In the present study, to what degree the fila radicularia are segmentally implanted along the cord was investigated. The research hypothesis was that if the fila radicularia were non-segmentally implanted at the cord surface, it would be unlikely that the internal neuron stratum would be segmented. The visual segmented aspect of the myelomeres would then be the consequence of the necessary bundling of axons towards the vertebral foramen as the only exits of the vertebral canal, rather than of an underlying segment organization of the cord itself. To investigate the research hypothesis, the fila radicularia in the cervical-upper thoracic part of five spinal cords were detached from their spinal nerves and dissected in detail. The principal research question was if the fila radicularia are separated from their spinal nerves and dissected from their connective tissues up to the cord, would it be possible to reconstruct the original spinal segments from the morphology and interspaces of the fila? The dissections revealed that the anterior fila radicularia emerge from the cord at regular regionally modulated interspaces without systematic segmental delineations. The posterior fila radicularia are somewhat more segmentally implanted, but the pattern is individually inconsistent. The posterior and anterior roots have notable morphological differences, and hypotheses are presented to help explain these. The macroscopic observations are consistent with a regionally but not a segmentally organized cord. This conclusion was visually summarized in photographs of spinal cords with ipsilateral intact roots and contralateral individually dissected fila radicularia. It was suggested that this dual view of the spinal cord be added to the standard anatomic textbooks to counterbalance the current possibly biased view of a segmented cord.

A multifactorial conceptual model of peripheral neuromusculoskeletal predisposing factors in task-specific focal hand dystonia in musicians: etiologic and therapeutic implications.

A model is presented showing how peripheral factors may cause a process of movement adaptation that leads to task-specific focal hand dystonia in musicians (FHDM). To acquire a playing technique, the hand must find effective and physiologically sustainable movements within a complex set of functional demands and anatomic, ergonomic, and physiological constraints. In doing so, individually discriminating constraints may become effective, such as limited anatomic independence of finger muscles/tendons, limited joint ranges of motion, or (subclinical) neuromusculoskeletal defects. These factors may, depending on the instrument-specific playing requirements, compromise or exclude functional playing movements. The controller (i.e., the brain) then needs to develop alternative motions to execute the task, which is called compensation. We hypothesize that, if this compensation process does not converge to physiologically sustainable muscle activation patterns that satisfy all constraints, compensation could increase indefinitely under the pressure of practice. Dystonic symptoms would become manifest when overcompensation occurs, resulting in motor patterns that fail in proper task execution. The model presented in this paper only concerns the compensatory processes preceding such overcompensations and does not aim to explain the nature of the dystonic motions themselves. While the model considers normal learning processes in the development of compensations, neurological predispositions could facilitate developing overcompensations or further abnormal motor programs. The model predicts that if peripheral factors are involved, FHDM symptoms would be preceded by long-term gradual changes in playing movements, which could be validated by prospective studies. Furthermore, the model implies that treatment success might be enhanced by addressing the conflict between peripheral factors and playing tasks before decompensating/retraining the affected movements.

Focal hand dystonia in musicians: a synopsis.

Focal hand dystonia in musicians (FHDM), also known as 'musicians' cramp', is a relatively rare, task-specific, pain-free disorder of control, causing unintentional, abnormal movements and/or positions in a part of the body directly involved in playing a musical instrument. Few physicians are familiar with the diagnosis, yet the exact cause of the disorder remains unknown and there is no generally effective therapy. In this synopsis, the authors present their experience with the diagnosis and treatment of FHDM and their aetiology hypothesis that musicians' cramp is caused by a loss of central motor control initiated by a failure of coping mechanisms, which (try to) compensate for the effects of peripheral local movement disturbing factors in the hand. Recent publications focus on the role of the central nervous system and on motor pattern relearning. We recommend further (prospective) research of the results of operative (peripheral) therapy, followed by (central) motor pattern relearning, and of neuropsychological contributions.

Left shoulder pain in a violinist, related to extensor tendon adhesions in a small scar on the back of the wrist.

A female professional orchestra violin player, age 54, with an 8-year history of severe left shoulder problems, presented with reproducible, acute, incapacitating left shoulder pain when playing the lowest violin string. This complaint was found caused by compensatory left arm positions for unnoticed finger extensor excursion limitations in a well-healed scar bed from two dorsal wrist ganglion operations 11 and 13 years before. Immediately after extensor tendon mobilization in the scar bed, the patient could assume a normal playing position, which was pain free, and could return to orchestral duties without further major shoulder complaints (follow-up of 10 years). The case study presents finger extensor excursion limitations at the wrist as an unusual extra-regional risk factor for a shoulder complaint and analyses the biomechanics linking these limitations to the complaint. The case illustrates the importance of long-term post-operative hand surgery rehabilitation in musicians.

Reverse engineering finger extensor apparatus morphology from measured coupled interphalangeal joint angle trajectories - a generic 2D kinematic model.

The interphalangeal (IP) finger joints coordinate as a mechanism when the deep flexor is active. This mechanism is created by the complex finger extensor apparatus (EA) - a confluence of end tendons of one or two extensors, radial and ulnar interossei, and lumbrical - which inserts as a single structure into both the middle and distal phalanges. Although the IP-coupling principle was well demonstrated more than half a century ago, the detailed relationship between EA morphology and IP coupling remains not well described. Main reasons are that by dissection the EA's fiber network loses functional consistency, while fibers becoming taut or slack beyond measuring resolutions complicate measuring functional fiber motions. To circumvent these difficulties, we present a two dimensional kinematic multi tendon-string EA model of fiber slackness and tautness through IP motion, including the retinacular and oblique retinacular EA ligaments. The model parameters were the strings' lengths and attachment points. The model's functional redundancies were resolved by individually interactively fitting model IP trajectories to previously measured IP trajectories of 68 fingers. All model trajectories accurately fitted their target IP trajectories for proximal interphalangeal (PIP) joint ranges smaller than 25° to 45°; about half accurately fitted over the entire IP range with the remaining half having maximum approximation errors between 3° to 12°, while all models again converged to target trajectories for full IP flexion. These accuracies suggest the model reflects real functional EA principles, with potential applications in biomechanical modeling, surgical reconstruction, rehabilitation, and prosthetic EA replacements.

Kinematic evaluation of the finger's interphalangeal joints coupling mechanism--variability, flexion-extension differences, triggers, locking swanneck deformities, anthropometric correlations.

The human finger contains tendon/ligament mechanisms essential for proper control. One mechanism couples the movements of the interphalangeal joints when the (unloaded) finger is flexed with active deep flexor. This study's aim was to accurately determine in a large finger sample the kinematics and variability of the coupled interphalangeal joint motions, for potential clinical and finger model validation applications. The data could also be applied to humanoid robotic hands. Sixty-eight fingers were measured in seventeen hands in nine subjects. Fingers exhibited great joint mobility variability, with passive proximal interphalangeal hyperextension ranging from zero to almost fifty degrees. Increased measurement accuracy was obtained by using marker frames to amplify finger segment motions. Gravitational forces on the marker frames were not found to invalidate measurements. The recorded interphalangeal joint trajectories were highly consistent, demonstrating the underlying coupling mechanism. The increased accuracy and large sample size allowed for evaluation of detailed trajectory variability, systematic differences between flexion and extension trajectories, and three trigger types, distinct from flexor tendon triggers, involving initial flexion deficits in either proximal or distal interphalangeal joint. The experimental methods, data and analysis should advance insight into normal and pathological finger biomechanics (e.g., swanneck deformities), and could help improve clinical differential diagnostics of trigger finger causes. The marker frame measuring method may be useful to quantify interphalangeal joints trajectories in surgical/rehabilitative outcome studies. The data as a whole provide the most comprehensive collection of interphalangeal joint trajectories for clinical reference and model validation known to us to date.

Morphology of deltoid origin and end tendons--a generic model.

This study provides a model of the complex deltoid origin and end tendons, as a basis for further anatomical, biomechanical and clinical research. Although the deltoid is used in transpositions with upper limb paralysis, its detailed morphology and segmentation has not been object of much study. Morphologically, the deltoid faces two distinct challenges. It closely envelops a ball joint, and it reduces its width over a short distance from a very wide origin along clavicle, acromion and spina scapula, to an insertion as narrow as the humerus. These challenges necessitate specific morphological tendon adaptations. A qualitative model for these tendons is developed by the stepwise transformation of a unipennate muscle model into a functional deltoid muscle. Each step is the solution to one of the mentioned morphological challenges. The final model is of an end tendon consisting of a continuous succession of bipennate end tendon blades centrally interspaced by unipennate tendon parts. The origin tendon consists of lamellae that interdigitate with the end tendon blades, creating a natural segmentation. The model is illustrated by qualitative dissection results. In addition, in view of a proliferation of terms found in the literature to describe deltoid tendons, tendon concepts are reviewed and the systematic use of the unique and simple terminology of 'origin and end tendons' is proposed.

Assessment of individual finger muscle activity in the extensor digitorum communis by surface EMG.

The extensor digitorum communis (ED) is a slender muscle group in the dorsal forearm from which tendons arise to the index (D2), medius (D3), ring (D4), and little (D5) fingers. Limited independence has been attributed to the parts that actuate the individual fingers. However, in a detailed anatomical analysis, it was found that the ED parts to the different fingers have constant and widely spaced anatomical locations that promote independent function. These observations and the superficial muscle belly locations prompted the hypothesis that these ED parts would be individually assessable by small anatomically placed surface EMG electrodes. In the present study, this hypothesis was evaluated by measuring electromyography (EMG) from the ED parts and surrounding muscles during individual finger tapping tasks with the forearm resting on a flat surface. It was found that individual ED activity can be well measured in ED2, ED3, ED4, and extensor digiti minimi (EDM). ED3 did not give nor did its electrodes receive significant crosstalk from other ED parts. ED4 electrodes recorded an EMG level of 30 +/- 19% (mean +/- SD) ED2 EMG in D2 tapping and ED2 electrodes a level of 53 +/- 22% ED4 EMG in D4 tapping, by hypothesis mostly crosstalk. EDM electrodes may record EMG at the level of ED4 EMG in D4 tapping. In D2 tapping, the mutual ED2 and extensor indicis redundancy reflected in large intersubject EMG differences with sometimes one or the other almost silent. The results may expand the possibilities of EMG analysis and finger muscle electrostimulation in ergonomic and clinical applications.

Anatomic basis for individuated surface EMG and homogeneous electrostimulation with neuroprostheses of the extensor digitorum communis.

The extensor digitorum communis (ED) is generally regarded as a fairly undiversified muscle that gives extensor tendons to all fingers. Some fine wire electromyographic (EMG) investigations have been carried out to study individuation of the muscle parts to the different fingers. However, individuated surface EMG of the ED has not been investigated. This study analyses the anatomy of the ED muscle parts to the different fingers in detail and proposes optimal locations for surface or indwelling electrodes for individuated EMG and for electrostimulation with neuroprostheses. The dissections show that the ED arises from extensive origin tendons (OT), which originate at the lateral epicondyle and reach far in the forearm. The ED OT is V-shaped with shorter central tendon fibers but with a long radial and an even longer ulnar slip. The ED parts to the individual fingers consistently arise from distinct OT locations: the ED3 (medius) arises proximally, the ED2 (index) from the radial slip distal to ED3, the ED4 (ring finger) from the ulnar slip distal to ED3, and the ED5 (to ring/little finger) from the ulnar slip distal to ED4. This lengthwise widely spaced arrangement of ED parts compensates to some degree for the narrow ED width and suggests that ED parts should be individually assessable by indwelling and even by surface EMG electrodes, albeit in the latter case with variable mutual cross-talk. Conversely, the anatomic spacing of ED parts warrants that electromyographic stimulation with neuroprostheses by a single implanted electrode cannot likely homogeneously activate all ED parts.

The minimum number of muscles to control a chain of joints with and without tenodeses, arthrodeses, or braces--application to the human finger.

While the underlying principles of controlling a single joint have been well described, the principles of simultaneously controlling multiple joints have not been comprehensively addressed in the literature of reconstructive hand surgery. This article analyzes (1) how many muscles are minimally required to fully control a chain of joints with in total N Degrees of Freedom (DoF), and (2) to what degree tenodeses, arthrodeses or braces can reduce the required number of muscles. It is demonstrated by mathematical analysis and illustrated by examples that the minimal number of muscles to control a chain of N DoF is N + 1. The number of muscles required for control can be reduced by mechanisms that reduce the number of DoF in the chain. (i) An arthrodesis is a permanent surgical fixation of a joint. An arthrodesis eliminates as many DoF in the chain as the arthrodized joints contributed. (ii) Tenodeses are coordinative tendon constructions. Each independent tenodesis eliminates one DoF from the chain. (iii) Braces are removable external supports. They eliminate as many DoF for muscle control as they immobilize. These principles are applied to illustrate the fundamental importance of tendinous structures in control in the human finger. Being able to determine the minimum number of muscles needed for multiarticular control gives additional knowledge in the design of functional reconstruction.

A method and device for measuring force transfers between the deep flexors in the musician's hand.

A device is presented to measure the range of independence, and the force transfers between the deep flexor tendons of different fingers. These force transfers result from the coactivation of the individual deep flexors, or the stretching of anatomical interconnections between the individual muscle bellies or tendons. In the quantification of the forces in the deep flexor end tendons, errors may occur due to (involuntary) forces in the extensors and superficial flexors. These errors are investigated by modelling the distal interphalangeal joint equilibrium equation, and illustrated by measuring results. They can be avoided by appropriate instrumental design, and proper finger positioning during measurement.

Measuring force transfers in the deep flexors of the musician's hand: theoretical analysis, clinical examples.

In the present paper the anatomical and functional interdependencies which regularly exist between the deep flexor tendons of the different fingers are modelled. The model results are validated by measurements on real hands. The results show that intertendinous force transfers may be caused by (i) coactivation of muscle fibres inserting in different tendons, and (ii) passive connections between tendons or muscle bellies. The coactivation is validated by the measuring results of a hand in which all intertendinous connections were surgically removed. The present models and measurements are currently used for diagnosis of hand problems in musicians at our hand clinic.

Anatomical factors predisposing to focal dystonia in the musician's hand--principles, theoretical examples, clinical significance.

In this paper, anatomical interconnections between tendons, between tendons and their environment, and anatomical constraints on joint mobility are considered as possible causes of focal dystonia in the hand of the musician. By hypothesis, focal dystonias arise when the constraints on movement resulting from these anatomic limitations impede playing movements with sufficiently low energy expenditure. This hypothesis is modelled for connections between the deep finger flexors. The displacements, forces, stresses, and work per volume in the finger motors in some common piano playing movements are calculated. The results indicate that with mentioned connections, in certain movements the extensor and lumbrical, and in others the lumbrical and interossei are most strained, while the interossei may become the main power source in loaded instrumental movements. Also discussed are compensatory movements. The biomechanical principles of surgical and conservative treatment are summarised.

Morphology of holes in aponeuroses as caused by perforating nerves or vessels at the medial epicondyle of the elbow.

In the present paper the morphology of holes in aponeuroses of origin of muscles is generally modelled. These holes result from perforations of the aponeuroses by nerves or vessels, or from joints which are crossed by the line of origin of the aponeurosis. The concept of aponeurotic holes as morphological entities comprehensively explains frequent small-scale anatomic variations in muscles, such as muscles with multiple separate origins in parallel. It also contributes to a better understanding of nerve compressions and their surgical treatment. The model is illustrated by dissection results of the aponeuroses at the medial epicondyle of the elbow, which are typically perforated by the n. medianus, the a. brachialis, and the n. ulnaris.

A generic morphological model of the anatomic variability in the m. flexor digitorum profundus, m. flexor pollicis longus and mm. lumbricales complex.

In the present study a generic model is presented of the anatomic variability in the muscle group formed by the m. flexor digitorum profundus, m. flexor pollicis longus and mm. lumbricales. This model provides a hypothesis about the structural causes of the frequent interdependence of tendons and muscle bellies in this muscle group. The model considers the muscle group as composed of two simple elementary building blocks: the monogastric contractile units of the FDP-FPL, and the digastric contractile elements of the lumbrical, and shows that these units can be assembled into complex entities, to which in reality a third structural element, the synovial membranes, not discussed in the present paper, adds a further complexity. The model allows to generate homologues of the existing anatomical variants, which are illustrated by typical dissection results. The present study should be of relevance to the morphologist, embryologist, surgeon, and musician/pedagogue. To the morphologist, it presents an alternative method of description or understanding of anatomic variability, based on (i) the 'atomary' concept that the anatomic structure is assembled from simple basic elements, and (ii) the local spatial constraints. To the embryologist, it raises the question to what degree the 'atomary' anatomical components of this model, which describes the macroscopic anatomy of the muscle group in detail, have an embryological basis. To the surgeon, the study presents detailed information about the scope of the variability in the deep flexor group, and the nature of its intertendinous connections. To the musician/pedagogue, it presents a visual illustration of the congenital interdependence of the muscles and tendons of an important finger motor group, as a possible cause of lack in finger independence which may hamper a fluent instrumental technique.

A generic morphological model of a muscle group--application to the muscles of the forearm.

In the present paper some aspects of the morphology of muscles in muscle groups are investigated modelwise. The analysis is initiated by the observation that the skeletal surfaces of the body are too small to provide the origin area for all muscles, and that therefore they must be enlarged by aponeuroses of muscular origin. These aponeuroses provide flexible surfaces of origin which allow to adapt the individual muscle shape to the various spaces available in the body, and to optimize their configuration in muscle groups. The shape of these origin surfaces determines the shape of the muscles and of their end tendons; this relationship is closely investigated. The models are illustrated by dissection examples of the muscle groups at the elbow,and may find an application in the teaching of clinical anatomy.

Connections between the tendons of the musculus flexor digitorum profundus involving the synovial sheaths in the carpal tunnel.

In the carpal tunnel anatomical interconnections between the tendons of the musculus flexor digitorum profundus are systematically present. These interconnections limit the mutual tendon displacements, which decreases finger independence and may be problematic in a musician's hand. The present study investigates a possible role of the synovial sheaths in the formation of these intertendinous connections in the carpal tunnel. To this end a morphological model is provided which correlates the often distinctly fibrous structure of the deep flexor tendons in the carpal tunnel and the frequent exchange of tendon fibres between the tendons to the different fingers, with the tendency of the synovial membranes to strongly adhere to the tendons. This model is validated by gross dissection results, and by cross sections of the flexor tendons in the carpal tunnel. In agreement with the model, the anatomic data show that the synovial membranes tend to invade and become trapped in tendons made up from individualised tendon strands, and also strongly adhere to the substantial amounts of tendon fibres which may be exchanged between the flexor tendons proximal to the lumbrical origins. These fibres and the synovial membranes may form a strong fabric able to withstand substantial stretching forces of interconnected oppositely pulled flexor tendons.

Why the lumbrical muscle should not be bigger--a force model of the lumbrical in the unloaded human finger.

The present paper investigates the forces and the stresses in the lumbrical and the other finger motors in an unloaded human finger model, with and without the ab-adduction degree of freedom of the MCP joint. Unique solutions are obtained by minimization of the maximal muscle stress calculated with a normal and a variable lumbrical physiological cross-sectional area. It is concluded that in the model with biaxial MCP joint, a stronger than normal lumbrical is not useful in unloaded finger control, and will merely result in spare lumbrical capacity. Also the natural synergism of the lumbrical and the ulnar interosseus in the control of the finger in the sagittal plane is pointed out.

The controllability of the unloaded human finger with superficial or deep flexor.

The unloaded human finger with a taut deep flexor functions as a bi-articular chain, because the angulations of the distal two joints are mechanically coupled. In this chain two flexors exist: the superficial and deep flexor. However, for elementary control of the unloaded finger, only one flexor is required. This puts forward the question of which flexor is most suited for unloaded finger control. In the present paper it is argued that due to the chiasma tendinum and the coupled rotations of the distal two joints, the deep flexor is anatomically better positioned than the superficial flexor for optimal unloaded finger control.