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)

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)

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)

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)

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)

Spasticity and muscle weakness is common in children with cerebral palsy (CP). Spasticity can be treated with Botulinum Neurotoxin-A (BoNT-A), but this drug has also been reported to induce muscle weakness. Our purpose was to describe the effect on muscle strength in the lower extremities after BoNT-A injections in children with cerebral palsy. A secondary aim was to relate the effect of BoNT-A to gait pattern and range of motion. Twenty children with spastic cerebral palsy were included in the study, (...) eight girls and 12 boys (mean age 7.7 years). All were able to walk without support, but with increased muscle tone interfering with motor function and gait pattern. Sixteen children had unilateral spastic CP and four bilateral spastic CP. Twenty-four legs received injections with BoNT-A in the plantar flexor muscles. The children were tested before treatment, around six weeks after at the peak effect of BoNT-A, and at six months after treatment, with measurement of muscle strength, gait analysis and range of motion. There were no differences in muscle strength in plantar flexors of treated legs at peak effect compared to baseline. Six months after treatment, there was still no change in untreated plantar flexor muscles, but an increasing trend in plantar flexor strength in legs treated with BoNT-A. Parents reported positive effects in all children, graded as: small in three children, moderate in eight, and large in nine children. The gait analysis showed a small improvement in knee extension at initial contact, and there was a small increase in passive range of motion for ankle dorsiflexion. Two children had a period with transient weakness and pain. We found that voluntary force production in plantar flexor muscles did not decrease after BoNT-A, instead there was a trend to increased muscle strength at follow-up. The increase may be explained as an effect of the blocking of involuntary nerve impulses, leading to an opportunity to using and training the muscles with voluntary control. Adequate muscle strength is important for maintaining the ability to walk and knowledge of how a treatment affects muscle strength is useful when selecting interventions. (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)

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)

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.

Facial muscle activities are essential for the appearance and communication of human beings. Therefore, exploring the activation patterns of facial muscles can help understand facial neuromuscular disorders such as Bell’s palsy. Given the irregular shape of the facial muscles as well as their different locations, it should be difficult to detect the activities of whole facial muscles with a few electrodes. In this study, a high-density surface electromyogram system with 90 electrodes was used to record EMG signals (...) of facial muscles in both healthy and Bell’s palsy subjects when they did different facial movements. The electrodes were arranged in rectangular arrays covering the forehead and cheek regions of the face. The muscle activation patterns were shown on maps, which were constructed from the Root Mean Square values of all the 90-channel EMG recordings. The experimental results showed that the activation patterns of facial muscles were distinct during doing different facial movements and the activated muscle regions could be clearly observed. Moreover, two features of the activation patterns, 2D correlation coefficient and Centre of Gravity were extracted to quantify the spatial symmetry and the location of activated muscle regions respectively. Furthermore, the deviation of activated muscle regions on the paralyzed side of a face compared to the healthy side was quantified by calculating the distance between two sides of CGs. The results revealed that corr2 of the activated facial muscle region in Bell’s palsy subjects was significantly lower than that in healthy subjects, while CG distance of activated facial region in Bell’s palsy subjects was significantly higher than that in healthy subjects. The correlation between corr2 of these regions and Bell’s palsy [assessed by the Facial Nerve Grading Scale 2.0] was also significant in Bell’s palsy subjects. The spatial information on activated muscle regions may be useful in the diagnosis and treatment of Bell’s palsy in the future. (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)

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)

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)

Proprioception is a crucial sensory modality involved in the control and regulation of coordinated movements and in motor learning. However, the extent to which proprioceptive acuity is influenced by local muscle fatigue is obscured by methodological differences in proprioceptive and fatiguing protocols. In this study, we used high resolution kinematic measurements provided by a robotic device, as well as both frequency and time domain analysis of signals captured via surface electromyography (sEMG) to examine the effects of local muscle fatigue on (...) wrist proprioceptive acuity in 16 physically and neurologically healthy young adults. To this end, participants performed a flexion/extension ipsilateral joint position matching test (JPM), after which a high-resistive robotic task was used to induce muscle fatigue of the flexor carpi radialis (FCR) muscle. The JPM test was then repeated in order to analyze potential changes in proprioceptive acuity. Results indicated that the fatigue protocol had a significant effect on movements performed in flexion direction, with participants exhibiting a tendency to undershoot the target before the fatigue protocol (-1.218°), but overshooting after the fatigue protocol (0.587°). In contrast, in the extension direction error bias values were similar before and after the fatigue protocol as expected (pre = -1.852°, post = -1.237°) and reflected a tendency to undershoot the target. Moreover, statistical analysis indicated that movement variability was not influenced by the fatigue protocol or movement direction. In sum, results of the present study demonstrate that an individual’s estimation of wrist joint displacement (i.e., error bias), but not precision (i.e., variability), is affected by muscular fatigue in a sample of neurologically and physically healthy adults. (shrink)

IntroductionThe pelvic floor muscles are active during motor tasks that increase intra-abdominal pressure, but little is known about how the PFM respond to dynamic activities, such as gait. The purpose of this study was to characterize and compare PFM activity during walking and jogging in continent adults across the entire gait cycle.Methods17 able-bodied individuals with no history of incontinence participated in this study. We recorded electromyography from the abdominal muscles, gluteus maximus, and PFM while participants performed attempted maximum (...) voluntary contractions of all muscles and completed 60–70 strides in four gait conditions: slow walk ; regular walk ; transition walk ; jog. We quantified activity throughout the whole gait cycle and during periods of bursting for each participant, and analyzed the timing of PFM bursting periods to explore when the PFM were most active in the gait cycle. We also conducted a phase metric analysis on the PFM and GM burst timings. We performed a Spearman's rank-order correlation to examine the effect of speed on %aMVCGC, %aMVCBR, and phase metric score, and used the Wilcoxon Signed-Rank test to evaluate the effect of gait modality, matched for speed, on these variables.ResultsThe PFM were active throughout the gait cycle, with bursts typically occurring during single-leg support. The PFM and GM were in phase for 44–69% of the gait cycle, depending on condition. There was a positive correlation between gait speed and both %aMVCGC and %aMVCBR. Phase metric scores were significantly higher during jogging than transition walking, but there was no difference between gait modality on %aMVCGC or %aMVCBR. Where possible we disaggregated data by sex, although were unable to make statistical comparisons due to low sample sizes.ConclusionThe PFM are active during walking and jogging, with greater activity at faster speeds and with bursts in activity around single-leg support. The PFM and GM co-activate during gait, but are not completely in phase with each other. (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.

Skeletal myoblasts have their origin early in embryogenesis within specific somites. Determined myoblasts are committed to a myogenic fate; however, they only differentiate and express a muscle‐specific phenotype after they have received the appropriate environmental signals. Once proliferating myoblasts enter the differentiation programme they withdraw from the cell cycle and form post‐mitotic multinucleated myofibres (myogenesis); this transformation is accompanied by muscle‐specific gene expression. Muscle development is associated with complex and diverse protein isoform transitions, generated by differential gene expression and mRNA (...) splicing. The myofibres are in a state of dynamic adaptation in response to hormones, mechanical activity and motor innervation, which modulate differential gene expression and splicing during this functional acclimatisation. This review will focus on the profound effects of thyroid hormone on skeletal muscle, which produce alterations in gene and isoform expression, biochemical properties and morphological features that precipitate in modified contractile/mechanical characteristics. Insight into the molecular events that control these events was provided by the recent characterisation of the MyoD gene family, which encodes helix‐loop‐helix proteins; these activate muscle‐specific transcription and serve as targets for a variety of physiological stimuli. The current hypothesis on hormonal regulation of myogenesis is that thyroid hormones (1) directly regulate the myoD and contractile protein gene families, and (2) induce thyroid hormone receptor‐transcription factor interactions critical to gene expression. (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.

Plamondon & Alimi's model will gain substantially in credibility when it is able to come up with predictions for new (rather than old) experimental results that discriminate between various models. Moreover, the present model is nothing more than a descriptive not an explanation for motor performance. A link to the contribution of various neuronal mechanisms involved in motor control and of muscle properties to the performance of the model is crucial.

The Motoneurone and its Muscle Fibres presents a state-of-the-art summary of knowledge concerning the motoneurones, vital for innervating and commanding skeletal muscles. No muscle action would be possible without motoneurones. These cells are therefore absolutely essential for the execution of normal behaviour and for life support. It is their degeneration that leads to various kinds of motoneurone disease that are often ultimately lethal. However, the study of motoneurones is also important for general insights as to how neurones work, because (...) the motoneurone is probably the best understood kind of nerve cell so far in neuroscience. Motoneurones of the spinal cord were the first type of central nerve cell to be subjected to detailed physiological measurements, and much is known about how their activity is regulated by synapses from other central neurones. For most of the individual neurones within the central nervous system, the precise functional tasks are difficult to define. However, for motoneurones much is now known about their short- and long-term interactions with their main targets, the skeletal muscle fibres. Functions of neurones must be analyzed in relation to the response properties of their target cells. Therefore, this book deals with both, summarizing classical as well as recent knowledge concerning the motoneurone and its muscle fibres. This is the first time that so many aspects of this broad subject matter are treated in one comprehensive monograph. (shrink)

Cytoskeletal organization in embryonic skeletal muscle cells has been examined by transmission electron microscopy; the technique involves preparation using the platinum replication method of freezedried samples, with and without cryofracture. The cytoskeletons in developing muscle cells appear to play a role in preserving cell shape as well as in anchoring myofibrils to cell membrane.