To get the most from clients it’s imperative that we complete periods of stress and rest – this balance is fundamental in the development of almost all physiological structures and processes. Fatigue is a rather vague term that encompases a variety of definitions from impairment of muscular function, to feelings of tiredness. When we consider what information this provides us, and more importantly our clients, it becomes clear that more precise mechanics and elements need to be defined. Enoka (2002) in an excellent text on Neuromechanics gathered and listed mechanisms of fatigue. The following blog will run through a brief overview of some of the physiological processes that can be effected by sustained activity.
Motor Command: The major mechanism effecting motor output appears to be a disturbance in the neurotransmitter levels. Serotonin, dopamine and norepinephrine are the most researched neurotransmitters due to their influence on emotion, mood and sleep. Despite these being the most researched it is likely that several others also effect exercise capacity including glutamate, gamma-aminobutyric acid and acetylcholine. Aside from neurotransmitters, hyperthermia and hypoglycaemia have been shown to reduce motor output.
Spinal Activation: When force exerted by a muscle fibre is insufficient to sustain a muscle contraction, the central nervous system can adjust the output from the spinal cord to compensate (Enoka 2002). However when the defect cannot be met, the CNS has been shown to reduce its drive.
Neuromuscular Propagation: The process of converting axonal action potential to sarcolemma action potential is reffered to as neuromuscular propagation. Potential impairments may include branch point failure and a decreased sensitivity of the post synaptic membrane.
Excitation Contraction Coupling: Several physiological processes are involved in the conversion of the muscle fibre action potential to fibre force, which can be impaired resulting in fatigue. Such processes as decreases in sensitivity to calcium of the myofibrils caused by acidosis have been observed, or the accumulation of ADP which has been shown to reduce shortening velocities.
While it is critical that we push clients session intensity progressively, we must consider the immediate impacts this may have on subsequent elements of the session or subsequent sessions in the week.