Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

Traditional approaches to human information processing tend to deal with perception and action planning in isolation, so that an adequate account of the perception-action interface is still missing On the perceptual side, the dominant cognitive view largely underestimates, and thus fails to account for, the impact of action-related processes on both the processing of perceptual information and on perceptual learning On the action side, most approaches conceive of action planning as a mere continuation of stimulus processing, thus failing to account for the goal-directedness of even the simplest reaction in an experimental task We propose a new framework for a more adequate theoretical treatment of perception and action planning, in which perceptual contents and action plans are coded in a common representational medium by feature codes with distal reference Perceived events (perceptions) and to-be-produced events (actions) are equally represented by integrated, task-tuned networks of feature codes – cognitive structures we call event codes We give an overview of evidence from a wide variety of empirical domains, such as spatial stimulus-response compatibility, sensorimotor synchronization, and ideomotor action, showing that our main assumptions are well supported by the data

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X01000103

The Theory of Event Coding (TEC): a framework for perception and action planning.

This paper concerns how motor actions are neurally represented and coded. Action planning and motor preparation can be studied using a specific type of representational activity, motor imagery. A close functional equivalence between motor imagery and motor preparation is suggested by the positive effects of imagining movements on motor learning, the similarity between the neural structures involved, and the similar physiological correlates observed in both imaging and preparing. The content of motor representations can be inferred from motor images at a macroscopic level, based on global aspects of the action (the duration and amount of effort involved) and the motor rules and constraints which predict the spatial path and kinematics of movements. A more microscopic neural account calls for a representation of object-oriented action. Object attributes are processed in different neural pathways depending on the kind of task the subject is performing. During object-oriented action, a pragmatic representation is activated in which object affordances are transformed into specific motor schemas (independently of other tasks such as object recognition). Animal as well as human clinical data implicate the posterior parietal and premotor cortical areas in schema instantiation. A mechanism is proposed that is able to encode the desired goal of the action and is applicable to different levels of representational organization.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00034026

The representing brain: Neural correlates of motor intention and imagery

Voluntary acts are preceded by electrophysiological “ readiness potentials” (RPs). With spontaneous acts involving no preplanning, the main negative RP shift begins at about -550 ms. Such RPs were used to indicate the minimum onset times for the cerebral activity that precedes a fully endogenous voluntary act. The time of conscious intention to act was obtained from the subject’s recall of the spatial clock position of a revolving spot at the time of his initial awareness of intending or wanting to move (W). W occurred at about -200 ms. Control experiments, in which a skin stimulus was timed (S), helped evaluate each subject’s error in reporting the clock times for awareness of any perceived event.

Unconscious cerebral initiative and the role of conscious will in voluntary action

Do we consciously cause our actions, or do they happen to us? Philosophers, psychologists, neuroscientists, theologians, and lawyers have long debated the existence of free will versus determinism. In this book Daniel Wegner offers a novel understanding of the issue. Like actions, he argues, the feeling of conscious will is created by the mind and brain. Yet if psychological and neural mechanisms are responsible for all human behavior, how could we have conscious will? The feeling of conscious will, Wegner shows, helps us to appreciate and remember our authorship of the things our minds and bodies do. Yes, we feel that we consciously will our actions, Wegner says, but at the same time, our actions happen to us. Although conscious will is an illusion, it serves as a guide to understanding ourselves and to developing a sense of responsibility and morality. Approaching conscious will as a topic of psychological study, Wegner examines the issue from a variety of angles. He looks at illusions of the will -- those cases where people feel that they are willing an act that they are not doing or, conversely, are not willing an act that they in fact are doing. He explores conscious will in hypnosis, Ouija board spelling, automatic writing, and facilitated communication, as well as in such phenomena as spirit possession, dissociative identity disorder, and trance channeling. The result is a book that sidesteps endless debates to focus, more fruitfully, on the impact on our lives of the illusion of conscious will.

The Illusion of Conscious Will

1. An anatomical peculiarity allows the hand to be positioned so that the terminal phalanx of the middle finger cannot be moved by voluntary effort. When positioned in this way only joint and cutaneous mechanisms subserve position sense. By altering the position of the hand the muscles are again engaged and able to move the finger. Moving the joint then also excites muscular afferents. 2. The position sense of twelve subjects was assessed with and without engagement of the muscles at the joint. Three tests were used in which either angular displacement, angular velocity or duration of displacement were held constant. 3. When muscular attachment was restored, performance in all tests was greatly enhanced. As engagement of the muscles caused little change in the 'stiffness' of the joint, it is unlikely that the improved performance resulted from increased discharges from the joint receptors. Cutaneous mechanisms are unlikely to mediate this improvement as they are likely to have been unaffected by engagement of muscles. It is concluded that intramuscular receptors are partly responsible for normal position sense. 4. In seven of the twelve subjects the test finger was anaesthetized to isolate the contribution of intramuscular receptors. This muscle sense was variable. In some subjects it provided accurate kinaesthetic information but in others the information was crude. If with the test finger anaesthetized subjects exerted voluntary tension with the muscles that move the joint the muscle sense was improved.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1976.sp011521

Joint sense, muscle sense, and their combination as position sense, measured at the distal interphalangeal joint of the middle finger.

Judgements about onset of rapid voluntary movements in man

1. Kinaesthetic acuity was tested at the distal interphalangeal joint of the middle finger when the hand was postured so that the joint was effectively disengaged from its muscular attachments. Subjects were required to detect the direction of 5 deg movements applied at different angular velocities from a mid-position under control conditions, after intra-articular injection of a plasma expander and after intra-articular injection of a local anaesthetic. 2. Kinaesthetic performance was enhanced after the injection of a plasma expander and deteriorated after injection of local anaesthetic. This deterioration could not be explained by spread of the local anaesthesic from the injection site on the dorsum of the joint. 3. The results suggest that the discharge of joint receptors can produce perceived signals of joint movement. Under normal conditions these receptors may duplicate the kinaesthetic input from muscle spindle endings.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1987.sp016522

The role of joint receptors in human kinaesthesia when intramuscular receptors cannot contribute.

Differences between the senses of movement and position shown by the effects of loading and vibration of muscles in man

1. Visual stimuli were presented to normal human subjects to test simple and more complex voluntary motor responses. Large and small visual stimuli were presented. In some trials, the small stimulus was followed 50 ms later by the large stimulus, so that the small stimulus was not perceived; this is the phenomenon of "backward masking." 2. Although subjects were not able to detect the masked, visual stimulus on forced-choice testing, they performed motor, reaction-time (RT) tasks in response to it. The RTs for responses to the masked stimulus were the same as those for responses to the easily perceived, nonmasked stimulus. 3. This result confirms and extends the findings of Fehrer and Biederman and was demonstrated with both simple and more complex motor responses. 4. Discussion of the findings focuses on their implications for motor control, particularly with respect to the preprogramming of voluntary movement.

D I McCloskey

Papers

Joint sense, muscle sense, and their combination as position sense, measured at the distal interphalangeal joint of the middle finger.

Judgements about onset of rapid voluntary movements in man

The role of joint receptors in human kinaesthesia when intramuscular receptors cannot contribute.

Differences between the senses of movement and position shown by the effects of loading and vibration of muscles in man

Triggering of preprogrammed movements as reactions to masked stimuli