Muscle fatigue encompasses a class of acute effects that impair motor performance. The mechanisms that can produce fatigue involve all elements of the motor system, from a failure of the formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of the contractile proteins. We propose four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms: 1) task dependency to identify the conditions that activate the various mechanisms; 2) force-fatigability relationship to explore the interaction between the mechanisms that results in a hyperbolic relationship between force and endurance time; 3) muscle wisdom to examine the association among a concurrent decline in force, relaxation rate, and motor neuron discharge that results in an optimization of force; and 4) sense of effort to determine the role of effort in the impairment of performance. On the basis of this perspective with an emphasis on neural mechanisms, we suggest a number of experiments to advance our understanding of the neurobiology of muscle fatigue.

Neurobiology of muscle fatigue.

An electromyographic (EMG) activity pattern for individual muscles in the gait cycle exhibits a great deal of intersubject, intermuscle and context-dependent variability. Here we examined the issue of common underlying patterns by applying factor analysis to the set of EMG records obtained at different walking speeds and gravitational loads. To this end healthy subjects were asked to walk on a treadmill at speeds of 1, 2, 3 and 5kmh−1 as well as when 35–95% of the body weight was supported using a harness. We recorded from 12–16 ipsilateral leg and trunk muscles using both surface and intramuscular recording and determined the average, normalized EMG of each record for 10–15 consecutive step cycles. We identified five basic underlying factors or component waveforms that can account for about 90% of the total waveform variance across different muscles during normal gait. Furthermore, while activation patterns of individual muscles could vary dramatically with speed and gravitational load, both the limb kinematics and the basic EMG components displayed only limited changes. Thus, we found a systematic phase shift of all five factors with speed in the same direction as the shift in the onset of the swing phase. This tendency for the factors to be timed according to the lift-off event supports the idea that the origin of the gait cycle generation is the propulsion rather than heel strike event. The basic invariance of the factors with walking speed and with body weight unloading implies that a few oscillating circuits drive the active muscles to produce the locomotion kinematics. A flexible and dynamic distribution of these basic components to the muscles may result from various descending and proprioceptive signals that depend on the kinematic and kinetic demands of the movements.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2003.057174

Five basic muscle activation patterns account for muscle activity during human locomotion

This book is about the processing of information in face-to-face communication when a speaker makes both audible and visible information available to a perceiver. Both auditory and visual sources of information are evaluated and integrated to achieve speech perception. The evaluation of the information source provides information about the strength of alternative interpretations, rather than just all-or-none categorical information, as claimed by “categorical perception” theory. Information sources are evaluated independently; the integration process insures that the least ambiguous sources have the most influences on the judgment. Similar processes occur in a variety of other behaviors, ranging from personality judgments and categorization to sentence interpretation and decision making. The experimental results are consistent with a fuzzy logical model of perception, positing three operations in perceptual (primary) recognition: feature evaluation, feature integration, and pattern classification. Continuously valued features are first evaluated, then integrated and matched against prototype descriptions in memory; finally, an identification decision is made on the basis of the relative goodness-of-match of the stimulus information with the relevant prototype descriptions.

Speech Perception By Ear and Eye: A Paradigm for Psychological Inquiry

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and ex...

/pdf/sarcopenia-aging-related-loss-of-muscle-mass-and-function-2u9nyjfrxt.pdf

Sarcopenia: Aging-Related Loss of Muscle Mass and Function.

Chronic obstructive pulmonary disease (COPD) is a major health care problem. Formerly mainly a disease of men, women are increasingly frequently afflicted. In many of these patients, exercise intolerance is the chief complaint. Few effective therapies are available. In recent years, dysfunction of the muscles of ambulation has been identified as a source of a portion of the exercise intolerance these patients experience, and this dysfunction has been shown to be, at least in part, remediable. Mechanisms inducing muscle dysfunction include disuse atrophy, malnutrition, low levels of anabolic steroids, and myopathy from corticosteroid use. Endurance exercise training has been conclusively demonstrated to improve exercise tolerance in COPD. Recent studies suggest that strength training is beneficial as well. A new frontier of therapy for muscle dysfunction in COPD is the use of anabolic hormones. Testosterone supplementation has been shown to increase muscle mass and strength in both hypogonadal and eugonadal healthy men. Low-dose testosterone supplementation is being considered for use in postmenopausal women. Though short-term administration of testosterone in moderate doses seems to be well tolerated in both men and women, further studies are required before safety and effectiveness can be established for routine use in COPD patients.

Skeletal muscle dysfunction in chronic obstructive pulmonary disease.

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.

On the function of muscle and reflex partitioning

Evidence is presented for the existence of a localization of monosynaptic Ia excitatory post-synaptic potentials (e.p.s.p.s) in the motor nucleus of a cat hind limb muscle. Intracellular recordings from biceps femoris motoneurones were made in anaesthetized low spinal cats of the effects of stimuli to the nerve branches supplying the anterior, middle, and posterior portions of the biceps femoris muscle. Recordings were also made during stimulation of nerves to semimembranosus and semitendinosus in order to provide a means of categorizing middle biceps cells as 'extensors' (middle biceps-extensor; i.e. like anterior biceps cells) or as 'flexors' (middle biceps-flexor; like posterior biceps). Homonymous nerve-branch (i.e. from anterior, middle or posterior biceps) monosynaptic Ia e.p.s.p.s were compared within unifunctional (flexor or extensor) groups of motoneurones. In three of four comparisons (anterior biceps nerve branch onto anterior and middle biceps-extensor cells, middle biceps onto middle biceps-flexor and posterior biceps, posterior biceps onto middle biceps-flexor and posterior biceps) the anterior, middle and posterior biceps nerve branches contributed larger e.p.s.p.s to their 'own' motoneurones than to motoneurones supplying other 'compartments' of the muscle. In the fourth case, middle biceps's input appeared to have similar effects onto anterior biceps and middle biceps-extensor cells. A normalization was performed to eliminate the possibility that the differences in e.p.s.p. sizes were due to differences in cell type within the four cell groupings (i.e. differences in the number of cells supplying FF, F(int.), FR and S muscle units). This normalization confirmed that the localization in the first three comparisons was not a consequence of differences in motoneurone type and, in addition, suggested that middle biceps may indeed have greater effects on middle biceps-extensor than anterior biceps cells. In addition to the asymmetrical effects of anterior and middle biceps nerve branches onto anterior biceps and middle biceps-extensor motoneurones, it was shown that while semitendinosus and posterior biceps contributed larger e.p.s.p.s to middle biceps-flexor than to middle biceps-extensor cells, the anterior biceps nerve branch and semimembranosus nerve contributed equally to the two middle biceps groups. Analysis of cell location in the spinal cord and rostro-caudal differences in group I volley sizes gave evidence of a topographic organization of the biceps femoris motor nucleus which could contribute to the observed localization. However, localization was also evident when comparing e.p.s.p. amplitudes in pairs of neighbouring cells of different category, indicating a role for neuronal recognition factors.

Localization of monosynaptic Ia excitatory post‐synaptic potentials in the motor nucleus of the cat biceps femoris muscle.

Partitioning of monosynaptic Ia EPSP connections with motoneurons according to neuromuscular topography: generality and functional implications

1. Signal averaging was used in forty experiments on low-spinal cats to measure and characterize the oligosynaptic responses of seventy-six motoneurons supplying the medial gastrocnemius muscle to the single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. In thirteen experiments on chloralose-urethane anaesthetized preparations, twelve (43%) of the tested twenty-eight motoneurones exhibited a single-axon recurrent inhibitory post-synaptic potential (recurrent i.p.s.p.), as compared to sixty-four (62%) of the 103 motoneurones tested in twenty-seven animals in the absence of anaesthetic after ischaemic decapitation. 3. Single-axon recurrent i.p.s.p.s most often consisted of a single, long-lasting hyperpolarization. Ten of the recurrent i.p.s.p.s contained a second late peak of hyperpolarization. In another eight of the i.p.s.p.s, a small late depolarization was evident. 4. The distinct profiles of the recurrent i.p.s.p.s were readily distinguished from the relatively flat profiles with low noise levels in the averages of the fifty-five 'no-response' cells. The transmembrane and post-synaptic nature of the i.p.s.p.s was confirmed by extracellular control recordings taken immediately outside seven of the cells with positive responses. In addition, ten cells with positive responses were subjected to current passage during the averaging procedure. In all cases, depolarization increased and hyperpolarization reduced the amplitude of their single-axon recurrent i.p.s.p.s. 5. The mean amplitude of the responses was 12.0 microV in chloralose-urethane preparations as compared to a peak-to-peak noise level less than 6.0 microV in the no-response averages. Corresponding values in ischaemic-decapitate preparations were 46.2 microV and less than 7.5 microV, respectively. 6. Latency, rise-time and half-width (i.e. duration at half-amplitude) values of the i.p.s.p.s were similar for chloralose-urethane and ischaemic-decapitate preparations. The average values in both preparations were 2.5, 5.6 and 19.3 ms, respectively. The latency values indicated both disynaptic and, perhaps, longer components in the recurrent i.p.s.p.s. The rise-time and half-width values were relatively similar to those reported or measured from published records for analogous composite recurrent i.p.s.p.s (i.e. responses to antidromic stimulation of the whole muscle nerve rather than single motor axons). A weak, but significant, correlation between rise-time and half-width was observed for the sixty-six single-axon recurrent i.p.s.p.s with a single negative-going peak.(ABSTRACT TRUNCATED AT 400 WORDS)

The measurement of single motor-axon recurrent inhibitory post-synaptic potentials in the cat.

Experiments were conducted to test the hypothesis that a partitioning of Ia monosynaptic excitatory postsynaptic potentials (Ia EPSPs) is present in motor nuclei supplying muscles with regions capable of different mechanical actions. Intracellular recordings of synaptic potentials were made in lateral gastrocnemius (LG) motoneurons in anesthetized low-spinal cats. The effects were tested of stimuli (group I range) to the four primary nerve branches of the LG nerve supplying muscle compartments LGm, LG1, LG2, and LG3 (terminology of English, Ref. 26) and the nerve to a heteronymous muscle, soleus. Stimulation of a given LG nerve branch produced monosynaptic Ia EPSPs of greater amplitude in "own-branch" motoneurons than "other-branch" cells. A significant partitioning of mean Ia EPSPs was found in three (LG1, LG2, LG3) out of the four homonymous pathways studied. An EPSP normalization (7) was performed to eliminate potential differences in cell type that might affect the amplitudes of the EPSPs between these four cell groups (e.g., differences in the number of cells supplying FF, FR, and S muscle units). This normalization confirmed that the partitioning of monosynaptic Ia inputs upon stimulation of LG1, LG2, and LG3 could not be attributed to differences in cell type. In addition, the effects of LGm stimulation were found to be significantly greater in the LGm motoneurons compared with the other cell groups. Heteronymous input (from soleus) to the LG motor nucleus showed some partitioned effects. Motoneurons innervating compartment LG2 received larger EPSPs from soleus than did the cells supplying compartments LG1, LG3, and LGm. The contributions of location specificity and species specificity (terminology of Scott and Mendell, Ref. 55) in the establishment of these Ia-afferent-motoneuronal connections were examined. Cell location sites within the spinal cord were consistent with location specificity making some contribution to the observed pattern of homonymous Ia connections. A more prominent role for species specificity was indicated by species-dependent differences in EPSP amplitude in pairs of LG motoneurons (e.g., LGm vs. LG2) at similar rostrocaudal locations upon stimulation of a given homonymous or heteronymous nerve/branch.

T. M. Hamm

Papers

On the function of muscle and reflex partitioning

Localization of monosynaptic Ia excitatory post‐synaptic potentials in the motor nucleus of the cat biceps femoris muscle.

Partitioning of monosynaptic Ia EPSP connections with motoneurons according to neuromuscular topography: generality and functional implications

The measurement of single motor-axon recurrent inhibitory post-synaptic potentials in the cat.

Partitioning of monosynaptic Ia excitatory postsynaptic potentials in the motor nucleus of the cat lateral gastrocnemius muscle.