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)

Morsella et al. claim that consciousness functions to integrate incompatible skeletal muscle intentions. Their primary cases of conscious skeletomotor integration involve action suppression, such as holding one's breath underwater, which requires suppressing the urge to inhale. This indicates that the authors seem to think action inhibition counts as the relevant kind of integration. I suggest that there is reason to think this kind of inhibition can occur unconsciously via unconscious inhibitory mechanisms. Unconscious inhibition therefore potentially undermines the (...) authors' thesis. If cases of unconscious inhibition are not instances of integration, then the authors should clarify exactly what counts as integration. (shrink)

Frustration in traffic is one of the causes of aggressive driving. Knowledge whether a driver is frustrated may be utilized by future advanced driver assistance systems to counteract this source of crashes. One possibility to achieve this is to automatically recognize facial expressions of drivers. However, only little is known about the facial expressions of frustrated drivers. Here, we report the results of a driving simulator study investigating the facial muscle activity that comes along with frustration. Twenty-eight participants were (...) video-taped during frustrated and non-frustrated driving situations. Their facial muscle activity was manually coded according to the Facial Action Coding System. Participants showed significantly more facial muscle activity in the mouth region. Thus, recording facial muscle behavior potentially provides traffic researchers and assistance system developers with the possibility to recognize frustration while driving. (shrink)

Functional electrical stimulation (FES), a method for inducing muscle contraction, has been successfully used in gait rehabilitation for patients with deficits after neurological disorders and several clinical studies have found that it can improve gait function after stroke and spinal cord injury. However, FES gait training is not suitable for patients with walking difficulty, such as those with severe motor paralysis of the lower limbs. We have previously shown that action observation combined with motor imagery (AO + MI) (...) of walking induces walking-related cortical activity. Therefore, we combined FES, which alternately generates dorsiflexion and plantar flexion, with AO + MI as an alternative to gait training. The present study investigates the transient effects of 20-min of FES simultaneously with and without AO + MI of walking on corticospinal and spinal reflex excitability in able-bodied participants. We measured motor evoked potentials and Hoffmann-reflexes to assess corticospinal and spinal reflex excitability at rest before and after the 20-min FES with and without the AO + MI. Our results show that FES without AO + MI did not change excitability (p > 0.05), while FES with AO + MI facilitated corticospinal excitability (p < 0.05). This facilitation likely occurred due to the synchronization of sensory inputs from FES and cortical activity during AO + MI. Facilitation was observed only in the dorsiflexor but not the plantar flexor muscle (p < 0.05), suggesting muscle specificity of the facilitation. These results demonstrate the effectiveness of combining FES with AO + MI and pave the way for novel neurorehabilitation strategies for patients with neurological gait deficits. (shrink)

Hodgkin and Huxley’s 1952 model of the action potential is an apparent dream case of covering-law explanation. The model appeals to general laws of physics and chemistry (specifically, Ohm’s law and the Nernst equation), and the laws, coupled with details about antecedent and background conditions, entail many of the significant properties of the action potential. However, Hodgkin and Huxley insist that their model falls short of an explanation. This historical fact suggests either that there is more to explaining (...) the action potential than subsuming it under a general laws or that Hodgkin and Huxley were wrong about the explanatory import of their model. In this paper, I defend Hodgkin and Huxley’s view that their model alone does not explain the action potential (contra Weber 2005). I argue further that neuroscientists lacked crucial explanatory details about the action potential until they could describe the molecular and ionic mechanisms by virtue of which their model holds (see Bogen 2005). Mathematical generalizations are important epistemic tools for assessing mechanistic explanations, but they are neither necessary nor sufficient for adequate explanations, even at the lowest levels of organization where biological phenomena are integrated with physics and chemistry. (shrink)

The mechanism underlying action potentials is routinely used to explicate the mechanistic model of explanation in the philosophy of science. However, characterisations of action potentials often fixate on neurons, mentioning plant cells in passing or ignoring them entirely. The plant sciences are also prone to neglecting non-neuronal action potentials and their role in plant biology. This oversight is significant because plant action potentials bear instructive similarities to those generated by neurons. This paper (...) helps correct the imbalance in representations of action potentials by offering an overview of the mechanism for plant action potentials and highlighting their similarity to those in neurons. Furthermore, it affirms the role of plant action potentials in discovering the evolution and function of mechanisms of action potentials more broadly. We stress the potential of plants for producing generalisations about action potentials and the possible role of plants as experimental organisms. (shrink)

A pianist’s movements are fundamental to music-making by producing the musical sounds and the expressive movements of the trunk and arms which communicate the music’s structural and emotional information making it valuable for this review to examine upper-body movement in the performance process in combination with the factors important in skill acquisition. The underpinning playing technique must be efficient with economic muscle use by using body segments according to their design and movement potential with the arm segments mechanically linked (...) to produce coordinated and fluent movement. Two physiologically and pianistically important actions proposed by early music scientists to deliver the keystroke involve dropping the hand from the shoulders towards the keys via a wave action with the joints activated sequentially, and forearm rotation to position the fingers for the keystroke, an action followed by the elbow/upper-arm rotating in the opposite direction. Both actions spare the forearm muscles by generating the energy needed in the larger shoulder muscles. The hand in the playing position has a curved palm through action of the metacarpal (knuckle) joints and curved fingers. Palm/finger posture controls sound quality from loud, high tempo sounds to a more mellow legato articulation, and to perform effectively the forearms should slope down towards the keyboard. The technique must be automatic through systematic practice which develops the motor skills for proficient playing, with practice duration tempered to reduce the risk of causing injury through overuse of the forearm muscles. Efficient movement patterns and strategic muscle relaxation which results in faster movement are realized only through extensive training. The constant movements of the head and trunk, and flowing arm movement with frequent hand lifts and rotational elbow movements, although generated in producing the playing technique, resonate with audience members who perceive them as expressive and thereby creating in them an empathic engagement with the music. It was proposed that music students be trained in the mechanical aspects of upper-body use in the playing technique, and practice strategies, with specialist pedagogy for children to develop motor skills for efficient playing, and training methods fostering an appreciation of the communicative aspects of music performance. (shrink)

Hodgkin and Huxley’s model of the action potential is an apparent dream case of covering‐law explanation in biology. The model includes laws of physics and chemistry that, coupled with details about antecedent and background conditions, can be used to derive features of the action potential. Hodgkin and Huxley insist that their model is not an explanation. This suggests either that subsuming a phenomenon under physical laws is insufficient to explain it or that Hodgkin and Huxley were wrong. I (...) defend Hodgkin and Huxley against Weber’s heteronomy thesis and argue that explanations are descriptions of mechanisms. †To contact the author, please write to: Department of Philosophy, Philosophy‐Neuroscience‐Psychology Program, Washington University in St. Louis, One Brookings Drive, Wilson Hall, St. Louis, MO 63130; e‐mail: [email protected]. (shrink)

Parvalbumins (PVs) are acidic, intracellular Ca2+‐binding proteins of low molecular weight. They are associated with several Ca2+‐mediated cellular activities and physiological processes. It has been suggested that PV might function as a “Ca2+ shuttle” transporting Ca2+ from troponin‐C (TnC) to the sarcoplasmic reticulum (SR) Ca2+ pump during muscle relaxation. Thus, PV may contribute to the performance of rapid, phasic movements by accelerating the contraction–relaxation cycle of fast‐twitch muscle fibers. Interestingly, PVs promote the generation of power stroke in fish (...) by speeding up the rate of relaxation and thus provide impetus to attain maximal sustainable speeds. However, immunological monitoring of diverse tissues demonstrated that PVs are also present in non‐muscle cells. The axoplasmic transport and various intracellular secretory mechanisms including the endocrine secretions seem to be controlled by the Ca2+ regulation machinery. Any defect in the Ca2+ handling apparatus may cause several clinical problems; for instance, PV deficiency alters the neuronal activity, a key mechanism leading to epileptic seizures. Moreover, atypical relaxation of the heart results in diastolic dysfunction, which is a major cause of heart failure predominantly among the aged people. PV may offer a unique potential to correct defective relaxation in energetically compromised failing hearts through PV gene transfer. Consequently, PV gene transfer may present a new therapeutic approach to correct cellular disturbances in Ca2+ signaling pathways of diseased organs. Hence, PVs appear to be amazingly useful candidate proteins regulating a variety of cellular functions through action on Ca2+ flux management. (shrink)

The present theory section deals with learned helplessness produced by pervasive experiences of failure or negative events, leading to decreased motivation and risk for depression. In their target article, Boddez, van Dessel, and de Houwer apply this concept to different forms of psychological suffering and propose a goal-directed mechanism –generalisation over similar goals. Duda and Joormann define goal similarity by action-outcome contingencies and highlight individual differences in attribution styles. Brandstätter proposes incentive classes as the organising principle for goal similarity (...) and explores outcomes such as deterioration of cognitive ability. Mikulincer and Lifshin focus on differences between goals and introduce the notion of motivated helplessness, serving as an anxiety buffer in uncontrollable life settings. Finally, potential contributions from emotion science are outlined, in particular individual differences and biases in appraising goals and coping potential likely to produce negative emotion dispositions. In conclusion, the need to elaborate a more comprehensive theoretical framework for helplessness that can guide empirical studies using multifactorial and longitudinal designs is emphasised. Given the strong increase in the prevalence of anxiety and depression worldwide, better understanding of predispositions and eliciting factors is needed for early diagnosis and the development of intervention programmes to reduce psychological suffering. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Notes and Fragments ENJOYING NEGATIVE EMOTIONS IN FICTIONS by John Morreall There is a puzzle going back to Aristotle and Augustine that has sometimes been called the "paradox of tragedy": how is it that nonmasochistic, nonsadistic people are able to enjoy watching or reading about fictional situations which are filled with suffering? The problem here actually extends beyond tragedy to our enjoyment of horror films and other fictional depictions (...) of situations which we would not enjoy in real life. What is it about fiction that turns intensely unpleasant situations into situations we can enjoy? Ofthe many solutions proposed to this puzzle, I shall mentionjust four. Aristotle held that in viewing tragedy we experience negative emotions — pity and fear — but get pleasure from the experience nonetheless because we are thereby purged of excess pity and fear. For Aristotle, too, there is a pleasure which tragedy shares with fiction in general, the delight we take in representations.1 Hume put more weight on this latter delight. Our pleasure in tragedy, he said, is based on our appreciation of the skill with which the literary or other artist has depicted the unpleasant scenes.2 Susan Feagin has recently located the pleasure we take in tragic art not in our direct responses to the work, which are "unpleasant experiences," she says, but in our meta-response of moral self-congratulation at these responses.3 The fourth solution to our puzzle is really a dissolution: it consists of denying that we actually feel fear, pity, and other negative emotions in our experience of fiction. The guiding principle here is that in order to have some emotion toward a situation, we must believe that it is a real situation. This principle is articulated in an influential article by Kendall Walton, though he does not apply it to our puzzle.4 95 96Philosophy and Literature I believe that this last solution is no solution at all because we do in fact feel emotions in our appreciation of fiction (a point I will develop later). The other three solutions above address the puzzle, and the pleasures they appeal to all have some explanatory power. But these solutions seem to me to overlook perhaps the most basic pleasure we take in tragedy, horror films, and the like — the direct pleasure of feeling (ear, pity, and similar emotions. The idea of enjoying negative emotions seems counterintuitive at first; indeed it may seem analytically false. But let us look more closely at those emotions to see what there might be to enjoy in them. Because the idea that we experience real emotions in response to fiction is controversial, we will begin not with fiction but with real life. After seeing how negative emotions can be enjoyed in real life, we can then consider their enjoyment in response to fiction. Fear is perhaps the most basic negative emotion, and at first glance one of the least likely to be enjoyed. To feel fear, many have claimed, we have tojudge that we are in danger, and danger is disturbing to us. And so it is natural that in fear there is the motivation to eliminate the danger, by fleeing, protecting ourselves, or attacking. Changes in the autonomic nervous system and the endocrine system equip us forjust such actions. They produce an increase in alertness and muscle tension, a faster and stronger heartbeat, a redistribution of the blood from the skin and internal organs to the voluntary muscles and brain, and a release of stored sugar from the liver into the bloodstream. Now although we may not be able to identify all these changes when we are in a state of fear, we can feel many of them directly, and we certainly feel the overall excitement which they produce. It is this excitement, I think, that makes fear potentially enjoyable. Especially for someone who leads a relatively dull life, the stimulation provided by fear can be pleasurable by contrast with the ordinary lack of stimulation. Many people, indeed, go to considerable trouble to put themselves into dangerous situations, in part, at least, for the thrill of fear that the danger will provide. Consider mountain climbers and skydivers, for instance. Now there are many sources... (shrink)

Smiles that vary in muscular configuration also vary in how they are perceived. Previous research suggests that “Duchenne smiles,” indicated by the combined actions of the orbicularis oculi and the zygomaticus major muscles, signal enjoyment. This research has compared perceptions of Duchenne smiles with non-Duchenne smiles among individuals voluntarily innervating or inhibiting the orbicularis oculi muscle. Here we used a novel set of highly controlled stimuli: photographs of patients taken before and after receiving botulinum toxin treatment for crow’s feet (...) lines that selectively paralyzed the lateral orbicularis oculi muscle and removed visible lateral eye wrinkles, to test perception of smiles. Smiles in which the orbicularis muscle was active were rated as more felt, spontaneous, intense, and happier. Post treatment patients looked younger, although not more attractive. We discuss the potential implications of these findings within the context of emotion science and clinical research on botulinum toxin. (shrink)

Global challenge-led research seeks to contribute to solution-generation for complex problems. Multicultural, multidisciplinary, and multisectoral teams must be capable of operating in highly demanding contexts. This brings with it a swathe of ethical conflicts that require quick and effective solutions that respect both international conventions and cultural diversity. The objective of this article is to describe the process of creating a toolkit designed to support global researchers in navigating these ethical challenges. The process of creating the toolkit embodied the model (...) of ethical research practice that it advocates. Specifically, at the heart of ethical decision-making is consideration of the following: Place, solutions must be relevant to the context in which they are to be used; People, those impacted by the outcomes must be partners in co-creation; Principles, ethical projects must be guided by clear values; and Precedent, the existing evidence-base should guide the project and, in turn, the project should extend the evidence-base. It is the thesis underlying the toolkit that consideration of these 4Ps provides a strong basis for understanding ethical conflicts and allows for the generation of potential solutions. This toolkit has been designed in two phases of collaborative work. More than 200 researchers participated from more than 30 countries and more than 60 different disciplines. This allowed us to develop a model for contextual, dynamic analysis of ethical conflicts in global research that is complementary to traditional codes of ethics. It emphasizes the need to consider ethical analysis as an iterative, reflective, process relevant at all stages of the research journey, including, ultimately, in evaluating the legacy of a project. The toolkit is presented as an open access website to promote universal access. A downloadable “pocket guide” version is also now available in 11 languages. (shrink)

We recently demonstrated, by means of short latency afferent inhibition (SAI), that before an imagined movement, during the reaction time (RT), SAI decreases only in the movement-related muscle (sensorimotor modulation) and that a correlation exists between sensorimotor modulation and motor imagery (MI) ability. Excitatory anodal transcranial direct current stimulation (a-tDCS) on M1 could enhance the MI outcome; however, mechanisms of action are not completely known. Here, we assessed if a-tDCS on M1 prior to an MI task could affect (...) sensorimotor modulation. Participants imagined abducting the index or little finger in response to an acoustic signal. SAI was evaluated from the first dorsal interosseus after the “go” signal, before the expected electromyographic (EMG) activity. Participants received 20-min 1.5 mA a-tDCS or sham-tDCS on M1 on two different days, in random order. Results showed that a-tDCS on M1 increases the sensorimotor modulation consisting of a weakening of SAI after the Go signal with respect to sham-tDCS, in the movement-related muscle right before the beginning of MI. These results suggest that a-tDCS on M1 further potentiate those circuits responsible for sensorimotor modulation in the RT phase of MI. Increased sensorimotor modulation during MI may be one of the mechanisms involved in MI improvement after a-tDCS over M1. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Kennedy Institute of Ethics Journal 11.1 (2001) 91-112 [Access article in PDF] Scope Note 40 Animals in Research and Education: Ethical Issues Laura Jane Bishop and Anita Lonnes Nolen Scientific enquiry is inexorably tied to animal experimentation in the popular imagination and human history. Many, if not most, of the spectacular innovations in the medical understanding and treatment of today's human maladies have been based on research using animals. (...) However, the use of animals in research and experimentation has been debated, defended, and protested by both individuals and organizations at various levels. Responses range from personal lifestyle decisions and fervent philosophical treatises to strident arguments, violent demonstrations, and direct action. The continuum of attitudes about animals and the human relationship with animals spans the range between those who support no regulation of the human use of animals and those who advocate absolute animal liberation from all human use (see II, Orlans 1993, p. 22). History The first recorded experimentation on animals occurred in ancient Rome, but not until the Renaissance did scholars begin serious study of how the body works. Leonardo da Vinci (1452-1519) and other artists and anatomists pursued anatomical investigations of muscle and bone structure. William Harvey (1578-1657) discovered the circulation of the blood via his experiments on live deer. During this period, much live animal experimentation both in England and France was based on the view of French philosopher René Descartes (1596-1650) that animals are incapable of feeling pain. Jeremy Bentham (1748-1832), the English utilitarian philosopher, thought otherwise. In his "Introduction to the Principles of Morals and Legislation" in 1789, Bentham declared, "The question is not, can they reason? Nor, can they talk? But can they suffer? Despite Bentham and others, the belief that animals could not perceive pain persisted in many quarters into the twentieth century. Nineteenth century French physiologist Claude Bernard [End Page 91] (1813-1878)-and his teacher, François Magendie (1783-1855)-conducted wide-ranging animal experiments including surgery, use of drugs, and removal of body parts from many species. Bernard argued that while no amount of benefit could justify any harm to human research subjects, even extreme harm and pain for animal research subjects could be justified by the potential benefit to human beings. Although he did use anesthetics in his work after their discovery in 1847, even Bernard's later work was controversial because of the numbers of animals used and the repetitive nature of his research. The experiments of Magendie and Bernard both laid the foundations for animal experimentation as a practice for scientific advance and contributed in large measure to the emergence of the anti-vivisection movement. Public protests over animal experiments conducted in France and the fear that these might come to England led to the passage of the first law controlling animal experimentation, the "1876 Cruelty to Animals Act" in England. This history and the dynamic tension between scientific inquiry and public concern set the stage for the activism and scholarship of the twentieth century.Since the 1960s, the amount of attention, activism, and scholarship related to animal use has increased at a rapid pace. The modern animal protection movement, led by the Australian philosopher Peter Singer in his book, Animal Liberation (II, 1975), based its advocacy on animals' ability to experience pain and suffering (Bentham's argument). Singer's book and other investigations into animal research, such as LIFE magazine's photojournalism piece on pet theft, animal cruelty, and animal experimentation (Concentration Camps for Dogs. LIFE (4 February 1966), pp. 22-29), brought the use of animals in research, testing, and education to the attention of the general public. Activists, advocates, laypersons, scientists, lawmakers, and animals themselves, have created the interesting, complicated, and complex history of animal rights and animal welfare over the last several decades. Current Statistics Worldwide, approximately 35 million animals are used in research each year; the United States alone uses 12 million animals annually--more than any other country. In 1998, the official number of research animals recorded in the United States was 1... (shrink)

This article argues against Benjamin Libet's claim that his experiment has shown that our actions are caused by brain events which begin before we consciously undertake the action. It clarifies what exactly should be meant by saying that the readiness potential causes, initiates, or prepares an action. It shows why Libet's experiment does not support his claim and why the experiments by Herrmann et al. and by Trevena \& Miller provide evidence against it. The empirical evidence is compatible (...) with strong libertarian free will. Neither the readiness potential nor the lateralized readiness potential cause our actions. (shrink)

Einstein's linear Lagrangian is replaced by a Lagrangian which is quadratic in the curvature quantities (gauge invariance). The hypothesis is made that the basic metrical field is highly agitated (due to periodic boundary conditions) thus establishing a submicroscopic basic lattice structure of the space-time world which, however, is macroscopically isotropic. All consequences follow from these assumptions. The “free vector” of Einstein's theory (void of physical significance and used for the normalization of the reference system) is no longer free but of (...) physical significance. It becomes subject to the wave equation and the Lorentz condition. It can thus be identified with the electromagnetic vector potential. Although the present investigation does not go beyond the linear approximation, the contours of auniverse of maxium rationality emerge. (shrink)

Rolls shares important data on hunger, thirst, sexuality, and learned behaviors, but is it pertinent to understanding the fundamental nature of emotionality? Important as such work is for understanding the motivated behaviors of animals, Rolls builds a constructivist theory of emotions and primary-process affective consciousness without considering past evidence on specific types of emotional tendencies and their diverse neural substrates.

It is well established that neural grafts can exert functional effects on the host animal by a multiplicity of different mechanisms – by diffuse release of trophic molecules, neurohormones, and deficient neurotransmitters, as well as by growth and reformation of neural circuits. Our challenge is to understand how these different mechanisms complement each other.

ObjectiveThe time interval between transcranial magnetic stimulation pulses affects evoked muscle responses when the targeted muscle is resting. This necessitates using sufficiently long inter-pulse intervals. However, there is some evidence that the IPI has no effect on the responses evoked in active muscles. Thus, we tested whether voluntary contraction could remove the effect of the IPI on TMS motor evoked potentials.MethodsIn our study, we delivered sets of 30 TMS pulses with three different IPIs to the left primary (...) motor cortex. These measurements were performed with the resting and active right hand first dorsal interosseous muscle in healthy participants. MEP amplitudes were recorded through electromyography.ResultsWe found that the IPI had no significant effect on the MEP amplitudes in the active muscle, whereas in the resting muscle, the IPI significantly affected the MEP amplitudes, decreasing the MEP amplitude of the 2 s IPI.ConclusionsThese results show that active muscle contraction removes the effect of the IPI on the MEP amplitude. Therefore, using active muscles in TMS motor mapping enables faster delivery of TMS pulses, reducing measurement time in novel TMS motor mapping studies. (shrink)

This article applies reflexive and dialogue oriented approaches to municipal planning. Experience from the dialogical development process in Vennesla is discussed, highlighting the potential of collaborative work in a development coalition. Dialogue and democracy in the coalition are discussed, emphasising the social construction of meaning and knowledge.

This article examines the potential of Action Research informed by Dewey’s pragmatism as a research methodology in the social sciences. Not only a philosophical orientation, pragmatism is also a powerful mode of inquiry. When combined with the democratic research approach of Action Research, Deweyan pragmatism has great potential to shed light on educational and other social science questions, forward social change, and enact Dewey’s vision of radical social democracy. Although Dewey’s philosophy, one could argue, has never been mainstream (...) in education and in research, the combination of Deweyan philosophy and Action Research has the potential to revive interest in Dewey’s work and serve as an example of Deweyan .. (shrink)

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)