Muscle fatigue encompasses a class of acute effects which impair motor performance. The mechanisms that produce fatigue may involve all elements of the motor system, from a formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of muscle contractile proteins. Recently our laboratory has proposed four themes that provide a basis for the systematic evaluation of the neural and neuromuscular mechanisms that can contribute to fatigue: (1) task dependency -- to identify the conditions that activate the various fatigue mechanisms; (2) force-fatigability relationship -- to explore the interactions between the mechanisms that result in a hyperbolic relationship between force and endurance time; (3) muscle wisdom -- to examine the association between a concurrent decline in muscle relaxation rate and motor neuron discharge during fatigue that results in an optimization of force; and (4) sense of effort -- to determine the role of perceived effort in the impairment of performance. This project addresses the first three of these themes, and several of our Aim 1 techniques permit a cellular neurophysiological approach to the sense of effort. Thus, this project is an essential component of a broadly based four-theme approach to understanding the neurobiology of muscle fatigue. Segmental motor mechanisms is a term used to describe the integrated (systems) operation of brainstem/spinal motor circuitry, motor units, muscle receptors and the segmental actions of muscle-, joint- and cutaneous afferents. This project links study of the fatigue of segmental motor mechanisms to the quadripartite approach described above.
Aim 1 addresses task dependency and muscle wisdom; it involves quantifying the adaptation of motor neuron discharge during sustained and intermittent stimulation when the spinal cord is passive (i.e., not generating a rhythmic motor output) or active (i.e., when generating a rhythmic motor output).
Aim 2 addresses task dependency and the force-fatigability relationship and examines how the CNS might exploit the catch-like property of skeletal muscle and the stimulus frequency-force relationship to reduce and delay fatigue. Both sets of experiments will be undertaken on decapitated turtles, an animal species which is particularly well suited for the study of spinal cord-muscle relationships. It is anticipated that these studies will contribute information on fundamental issues in motor control that have not yet been resolved. Furthermore, the work has profound implications for orthopedics, sports medicine, the fatigability that is evident in various effort syndromes that accompany higher brain malfunctions, motor recovery after brain damage, many neuromuscular diseases, peripheral neuromuscular damage, functional electrical stimulation, and the emerging field of motor prosthetics.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
2R01NS025077-06
Application #
3410187
Study Section
Orthopedics and Musculoskeletal Study Section (ORTH)
Project Start
1987-09-01
Project End
1996-08-31
Budget Start
1992-09-01
Budget End
1993-08-31
Support Year
6
Fiscal Year
1992
Total Cost
Indirect Cost
Name
University of Arizona
Department
Type
Schools of Medicine
DUNS #
City
Tucson
State
AZ
Country
United States
Zip Code
85721
Laouris, Y; Bevan, L; Reinking, R M et al. (2004) Associations between force and fatigue in fast-twitch motor units of a cat hindlimb muscle. Can J Physiol Pharmacol 82:577-88
Callister, Robert J; Sesodia, Sanjay; Enoka, Roger M et al. (2004) Fatigue of rat hindlimb motor units: biochemical-physiological associations. Muscle Nerve 30:714-26
Callister, R J; Donnelly, R P; Pierce, P A et al. (1996) Motor pool organization of the external gastrocnemius muscle in the turtle, Pseudemys (Trachemys) scripta elegans. J Morphol 227:171-83
Callister, R J; Laidlaw, D H; Stuart, D G (1995) A commentary on the segmental motor system of the turtle: implications for the study of its cellular mechanisms and interactions. J Morphol 225:213-27
Laidlaw, D H; Callister, R J; Stuart, D G (1995) Fiber-type composition of hindlimb muscles in the turtle, Pseudemys (Trachemys) scripta elegans. J Morphol 225:193-211
Bevan, L; Laouris, Y; Garland, S J et al. (1993) Prolonged depression of force developed by single motor units after their intermittent activation in adult cats. Brain Res Bull 30:127-31
Spielmann, J M; Laouris, Y; Nordstrom, M A et al. (1993) Adaptation of cat motoneurons to sustained and intermittent extracellular activation. J Physiol 464:75-120
Enoka, R M; Stuart, D G (1992) Neurobiology of muscle fatigue. J Appl Physiol 72:1631-48
Bevan, L; Laouris, Y; Reinking, R M et al. (1992) The effect of the stimulation pattern on the fatigue of single motor units in adult cats. J Physiol 449:85-108
Reinking, R M; Laouris, Y (1991) Triggering module for waveform digitization. Brain Res Bull 27:145-9

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