Chapter 14 Neuromuscular Fatigue

What is neuromuscular fatigue? A temporary decrease in muscular performance seen as a failure to maintain or develop an expected force or power output. –Central fatigue occurs mainly in the brain. –Peripheral fatigue occurs in the motor neurons, the peripheral nerves, the motor endplates, and the muscle fibers.

Effects of fatigue Causes loss of muscle strength Causes reflexes to fade with repeated stimulation Degrades coordination of complex movements

The relationships between force production and time to exhaustion for continuous isometric, intermittent isometric, and dynamic muscle actions

Central versus peripheral causes of fatigue Central factors are not involved to any great extent in fatigue during high- intensity exercise. Fatigue in low-intensity effort over one or two hours is likely a failure in nervous system drive.

Factors contributing to central fatigue Sensory feedback from working muscles Accumulation of ammonia Increase in serotonin in the brain

Setchenov phenomena More work can be produced after a pause with diverting activity than after a passive rest pause. Sensory feedback of nerve impulses from fatigued muscles impinges on the reticular formation in the brain and inhibits voluntary effort. Diverting activities, such as exercise with non- fatigued muscle group, produce an increased inflow of impulse from non-fatigued body parts to the reticular formation, shifting balance between inhibition toward facilitation.

Accumulation hypothesis –Build up of fatigue-causing metabolites within muscle fibers –Build-up of lactate interferes with muscle contraction by affecting Calcium release from the saracoplasmic reticulum Actin-myosin binding ATP breakdown ATP production

Depletion hypothesis –Depletion of fuels used to produce ATP Phosphagen depletion—during very high intensity activity Glycogen depletion—over a long period of time during moderate-intensity exercise

Temperature and muscle fatigue Muscle fatigue occurs at both low and high muscle temperatures. At high temperatures, metabolites accumulate in the muscle. Low muscle temperatures may affect neuromuscular electrical transmission of the contractile properties of the muscle.

The relationship between time to exhaustion and muscle temperature during submaximal isometric muscle actions

Electomyography Records muscle action potentials (MAPs) Qualitative electromyography-- Developed for use of physicians in diagnosing abnormal neuromuscular function Quantitative electromyography—Study of amount of electrical activity present in a given muscle under varying conditions

Differences between recordings of single motor-unit potentials with needle electrodes (left) and summated potentials from many motor units with surface electrodes (right)

Integration of muscle action potentials to get true mean value of their amplitude.

Electromyography and fatigue Electrical activity in muscle tissue increases over time during submaximal fatiguing tasks. Increase results from recruitment of additional motor units and increases in the firing rate of already activated motor units to make up for the force that is lost as motor units fatigue.

The relationship between EMG voltage and time to exhaustion during fatiguing tasks with increasing force outputs

The relationship between EMG voltage and time at non-fatiguing and fatiguing power outputs during cycle ergometry