To controlforceaccurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable (...) in normal limb movements.Morton's hypothesis, thatlengthis controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.The idea that feedback is used to compensate for changes in musclestiffnesshas received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems forvelocityorviscosityare considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint. (shrink)

The idea that the brain is a representational organ has roots in the nineteenth century, when neurologists began drawing conclusions about what the brain represents from clinical and experimental studies. One of the earliest controversies surrounding representation in the brain was the “muscles versus movements” debate, which concerned whether the motor cortex represents complex movements or rather fractional components of movement. Prominent thinkers weighed in on each side: neurologists John Hughlings Jackson and F.M.R. Walshe in favor of complex movements, neurophysiologist (...) Charles Sherrington and neurosurgeon Wilder Penfield in favor of movement components. This essay examines these and other brain scientists’ evolving notions of representation during the first eighty years of the muscles versus movements debate (c. 1873–1954). Although participants agreed about many of the superficial features of representation, their inferences reveal deep-seated disagreements about its inferential role. Divergent epistemological commitments stoked conflicting conceptions of what representational attributions imply and what evidence supports them. (shrink)

In the wake of Clark and Chalmers famous argument for extended cognition some people have argued that willpower equally can extend into the environment (e.g. Heath and Anderson in The thief of time: philosophical essays on procrastination. Oxford University Press, New York, pp 233–252, 2010). In a recent paper Fabio Paglieri (Consciousness in interaction: the role of the natural and social context in shaping consciousness. John Benjamins, Amsterdam, pp 179–206, 2012) provides an interesting argument to the effect that there might (...) well be extended self control, but that willpower does not lend itself to extension. This paper argues that Paglieri is right in claiming that previous attempts to extend the will are flawed. It then provides an argument for extending the will that does not fall foul of Paglieri’s argument and actually provides us with an even stronger case for extension than the one that Clark and Chalmers provide for cognition. (shrink)

Muscle synergies reflect the presence of a common neural input to multiple muscles. Steering small sets of synergies is commonly believed to simplify the control of complex motor tasks like walking and running. When these locomotor patterns emerge, it is likely that synergies emerge as well. We hence hypothesized that in children learning to run the number of accompanying synergies increases and that some of the synergies’ activities display a temporal shift related to a reduced stance phase as observed (...) in adults. We investigated the development of locomotion in 23 children aged 2–9 years of age and compared them with seven young adults. Muscle activity of 15 bilateral leg, trunk, and arm muscles, ground reaction forces, and kinematics were recorded during comfortable treadmill walking and running, followed by a muscle synergy analysis. We found that toddlers and preschoolers utilize a “walk-run strategy” when learning to run: they managed the fastest speeds on the treadmill by combining double support and flight phases. In particular the activity duration of the medial gastrocnemius muscle was weakly correlated with age. The number of synergies across groups and conditions needed to cover sufficient data variation ranged between four and eight. The number of synergies tended to be smaller in toddlers than it did in preschoolers and school-age children but the adults had the lowest number for both conditions. Against our expectations, the age groups did not differ significantly in the timing or duration of synergies. We believe that the increase in the number of muscle synergies in older children relates to motor learning and exploration. The ability to run with a FP is clearly associated with an increase in the number of muscle synergies. (shrink)

Cellular mechanisms whereby quiescent stem cells sense tissue injury and transition to an activated state are largely unknown. Quiescent skeletal muscle stem cells (MuSCs, also called satellite cells) have elaborate, heterogeneous projections that rapidly retract in response to muscle injury. They may therefore act as direct sensors of their niche environment. Retraction is driven by a Rac‐to‐Rho GTPase activity switch that promotes downstream MuSC activation events. These and other observations lead to several hypotheses: (1) projections are morphologically dynamic (...) at quiescence, providing a surveillance function for muscle damage; (2) quiescent projection dynamics are regulated by the relative balance of Rac and Rho activities promoted by niche‐derived cues; (3) projections, particularly their associated filopodia, sense tissue damage via changes to the biomechanical properties of the niche and/or detection of signaling cues released by damaged myofibers; and (4) the dynamic nature of projections results in a population of MuSCs with heterogeneous functional properties. These concepts may extend to other types of quiescent stem cells, as well as prove useful in translational research settings. (shrink)

We hypothesise that the sarcolemma of an actively growing myofibre has different properties to the sarcolemma of a mature adult myofibre. Such fundamentally different properties have clinical consequences for the onset, and potential therapeutic targets, of various skeletal muscle diseases that first manifest either during childhood (e.g. Duchenne muscular dystrophy, DMD) or after cessation of the main growth phase (e.g. dysferlinopathies). These characteristics are also relevant to the selection of both tissue culture and in vivo models employed to study (...) such myopathies and the molecular regulation of adult myofibres. During growth, multinucleated myofibres increase enormously in size and volume with dramatic increases in length (up to ∼600 mm). This is in striking contrast with most mononucleated cells such as fibroblasts, that remain at a relatively small size (∼10–20 µm diameter). The consequences of a dynamic, expanding sarcolemma during growth, compared with that of an adult myofibre of a fixed length, are discussed with respect to various aspects of muscle biology. (shrink)

For patients with multiple sclerosis, deficits in gait significantly reduce the quality of life. Using the concept of muscle synergies, this study investigated the modular organization of motor control during level and inclined walking in MS patients compared with healthy participants to identify the potential demand-specific adjustments in motor control in MSP. We hypothesized a widening of the time-dependent activation patterns in MSP to increase the overlap of temporally-adjacent muscle synergies, especially during inclined walking, as a strategy to (...) increase the robustness of motor control, thus compensating pathology-related deficits. We analyzed temporal gait parameters and muscle synergies from myoelectric signals of 13 ipsilateral leg muscles using non-negative matrix factorization. Compared with HP, MSP demonstrated a widening in the time-dependent coefficients, as well as altered relative muscle contribution, in certain synergies during level and inclined walking. Moreover, inclined walking revealed a demand-specific adjustment in the modular organization in MSP, resulting in an extra synergy compared with HP. This further increased the overlap of temporally-adjacent muscle synergies to provide sufficient robustness in motor control to accomplish the more demanding motor task while coping with pathology-related motor deficits during walking. (shrink)

If, as Anthony Elliot argues, `the [symbolic] law of the father triumphs over the loss of the maternal body' in the making of men, how is the masculine body possible? The answer would appear to be, on condition that it becomes implacably hard, disciplined, an object of work. On the other hand, excessive interest in the body, as in the case of bodybuilding, would appear also to betoken narcissism and femininity. Drawing on the notions of the `hard man', the significance (...) of muscle, bodybuilding and the case of `Iron' Mike Tyson, this article attempts to come to grips with the complex and contradictory relations among desire, anxiety and the embodiment of masculinity. Specifically, this entails focusing on the deconstruction of discourses relating to muscle and `hardness', and on how identifications with such discourses are made and sustained. (shrink)

In recent years, live imaging has been adopted to study stem cells in their native environment at cellular resolution. In the skeletal muscle field, this has led to visualising the initial events of muscle repair in mouse, and the entire regenerative response in zebrafish. Here, we review recent discoveries in this field obtained from live imaging studies. Tracking of tissue resident stem cells, the satellite cells, following injury has captured the morphogenetic dynamics of stem/progenitor cells as they facilitate (...) repair. Asymmetric satellite cell division generated a clonogenic progenitor pool, providing in vivo validation for this mechanism. Furthermore, there is an emerging role of stem/progenitor cell guidance at the injury site by cellular protrusions. This review concludes that live imaging is a critical tool for discovering the distinct processes that occur during regeneration, emphasising the importance of imaging in diverse animal models to capture the entire scope of stem cell functions. (shrink)

The paper considers apprenticeship as a model of education that both teaches technical skills and provides the grounding for personal formation. The research presented is based on long-term anthropological fieldwork with minaret builders in Yemen, mud masons in Mali and fine-woodwork trainees in London. These case studies of on-site learning and practice support an expanded notion of knowledge that exceeds propositional thinking and language and centrally includes the body and skilled performance. Crafts -- like sport, dance and other skilled physical (...) activities -- are largely communicated, understood and negotiated between practitioners without words, and learning is achieved through observation, mimesis and repeated exercise. The need for an interdisciplinary study of communication and understanding from the body is therefore underlined, and the paper suggests a way forward drawing on linguistic theory and recent neurological findings. It is argued that the validation and promotion of skilled practice as 'intelligent' is necessary for raising the status and credibility of apprenticestyle learning within our Western systems of education. (shrink)

MacNeilage's target article develops a theory for the evolution of human speech articulation along the lines of “slot- filler” structure. His content/frame schema commits him to the tenets of embodiment, muscle sense, and a memory for speech. My commentary ties these aspects together in their historical and current perspective.

Autism spectrum disorder (ASD) is a neuro-developmental condition, which requires a multi-disciplinary matrix of treatments, including functional, educational, and emotional interventions. The latter mode of treatment entails particular difficulties, inasmuch as the core deficits of this condition seem to challenge the very premises of traditional psychotherapy. Reciprocity, verbal, and symbolic expression and inter-subjective dynamics are often difficult to attain with clients diagnosed with ASD, and emotional treatment thus often turns out to be a frustrating process, which may well elicit questions (...) as to the efficacy of psychotherapeutic emotional interventions. These core challenges, described in the literature, become particularly acute in view of the increasing number of clients diagnosed on the autistic spectrum in recent years, and the growing need for qualified therapists who have trained for working specifically with this condition. It seems, therefore, that it is high time for systematic research into the lived experience of therapists working with these clients in order to attain a better clinical and theoretical understanding of the condition itself and broaden the range of effective interventions. This study, informed by a phenomenological-hermeneutic approach which guided both the collection of data and its subsequent analysis, aims to address these issues by exploring the particular challenges faced by therapists in this field, the questions that come up in the process, modes of personal and professional coping, and the insights elicited by the therapeutic encounter. The research consisted of in-depth interviews with 28 practicing therapists from a broad range of clinical orientations, including dance/movement, arts, music, and drama therapists, clinical psychologists, and clinical social workers. The essential themes that emerged from the participants’ responses and the analysis of the findings lend support to theoretical and developmental approaches, which focus on the primacy and the foundational role of the concrete body in inter-subjective relationships and in the therapeutic process, and indicate the potential efficacy of somatic and kinetic interventions. The clinical implications of this study are thus highly relevant to the training and support of therapists working with ASD, who should be encouraged to develop greater receptivity to non-verbal modes of interaction in the therapeutic process. (shrink)

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A (...) partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems. (shrink)

In Hypatia's 3, issue, Xinyan Jiang describes a failed experiment in sexual equality conducted during the Chinese Cultural Revolution. She believes the lesson to be drawn from it is that males will continue to have an advantage in societies requiring much physical strength. In contrast, I argue here that this failed experiment shows that the Maoist attempt to force women into men's roles was not feminist. American pioneers are cited as a counterexample.

In Hypatia's 3, issue, Xinyan Jiang describes a failed experiment in sexual equality conducted during the Chinese Cultural Revolution. She believes the lesson to be drawn from it is that males will continue to have an advantage in societies requiring much physical strength. In contrast, I argue here that this failed experiment shows that the Maoist attempt to force women into men's roles was not feminist. American pioneers are cited as a counterexample.

In Hypatia's 3, issue, Xinyan Jiang describes a failed experiment in sexual equality conducted during the Chinese Cultural Revolution. She believes the lesson to be drawn from it is that males will continue to have an advantage in societies requiring much physical strength. In contrast, I argue here that this failed experiment shows that the Maoist attempt to force women into men's roles was not feminist. American pioneers are cited as a counterexample.

In Hypatia's 3, issue, Xinyan Jiang describes a failed experiment in sexual equality conducted during the Chinese Cultural Revolution. She believes the lesson to be drawn from it is that males will continue to have an advantage in societies requiring much physical strength. In contrast, I argue here that this failed experiment shows that the Maoist attempt to force women into men's roles was not feminist. American pioneers are cited as a counterexample.

In humans, there are two skeletal muscle α‐actinins, encoded by ACTN2 and ACTN3, and the ACTN3 genotype is associated with human athletic performance. Remarkably, approximately 1 billion people worldwide are deficient in α‐actinin‐3 due to the common ACTN3 R577X polymorphism. The α‐actinins are an ancient family of actin‐binding proteins with structural, signalling and metabolic functions. The skeletal muscle α‐actinins diverged ∼250–300 million years ago, and ACTN3 has since developed restricted expression in fast muscle fibres. Despite ACTN2 and (...) ACTN3 retaining considerable sequence similarity, it is likely that following duplication there was a divergence in function explaining why α‐actinin‐2 cannot completely compensate for the absence of α‐actinin‐3. This paper focuses on the role of skeletal muscle α‐actinins, and how possible changes in functions between these duplicates fit in the context of gene duplication paradigms. (shrink)