Positive correlations between increases in RLC phosphorylation and potentiation have been reported in many animal studies (29, 30), but very few studies have been conducted on human subjects (10). Of the research conducted on human subjects, numerous have found no significant RLC phosphorylation (31, 32). Whilst a correlation between RLC phosphorylation and potentiation is evident in animal studies, there appears to be an inconsistency in the research on humans. This suggests that other factors may be responsible for inducing PAP in humans.
An interesting finding regarding the RLC phosphorylation theory, is type II muscle fibres appear to experience greater phosphorylation than type I muscle fibres (33). This suggests that individuals with a higher percentage of type II fibres, such as strength and power athletes, may experience a greater potentiation (10). However, this would potentially only apply if the phosphorylation theory is correct.
Potentiated H-reflex response
The second theory thought to be responsible for PAP is an excitation of several neurological mechanisms following a conditioning exercise. There are several neural responses suggested to experience a state of excitation following a conditioning exercise, such as H-reflex potentiation, an increase in motor unit synchronisation, desensitisation of alpha motor neuron input, and a decrease in the reciprocal inhibition of the antagonistic muscles (34). However, H-reflex potentiation appears to be the most dominant neural mechanism found within the research (35, 36, 10). The H-reflex is an electromyographic (EMG) measurement of the level of excitability of a muscle. Quite simply, higher H-reflexes are associated with higher excitability.
The H-reflex is the result of an afferent neural volley in response to single-pulse sub-maximal stimulation of the relevant nerve bundle. With sufficient recovery PAP increases H-reflex amplitude, this is thought to be due to increased recruitment of high-order motor neurons at the spinal cord (10). An increased recruitment of high-order motor neurons would, therefore, lead to a faster and more forceful muscle contraction, resulting in an improved performance.
Pennation angle of the muscle fibres
In recent years, a third theory appears to be arising within the literature (37). This theory suggests there is a decrease in the pennation angle of the muscle fibres following a conditioning exercise. A decrease in the pennation angle of the muscle means more force can be transmitted through the tendon and ultimately to the bone upon contraction – a more forceful contraction means a better performance (36). However, there is little research supporting this theory, therefore more research is warranted before more accurate assumptions can be made (10).