Literature review
1. Introduction
The contractile response of skeletal muscle to a volitional command or electrically induced stimuli is somewhat determined by its contractile history (Kilduff et al., 2007; Requena et al., 2011). Repetitive contractile stimulation can result in attenuation in performance due to the onset and development of neuromuscular fatigue (Robbins, 2005). Neuromuscular fatigue can be briefly described as an acute decrease in maximal peak force and considerable shifts in the force time curve of exerted muscles after repeated muscle contractions (Häkkinen, 1993).
Contrastingly, unlike fatigue, which acts to impair force production, there is evidence that suggests the contractile history of skeletal muscle may also facilitate the volitional force that the muscle can produce (Sale, 2002) during subsequent twitch contractions. This phenomenon is known as Postactivation Potentiation (PAP). Fatigue and PAP coexist and are both produced from the contractile history of the muscle(s) involved, however, it is the net balance of these two mechanisms that will determine if the force generated in subsequent contractions will be superior, inferior or unchanged (Macintosh and Rassier, 2002). The relationship between PAP and fatigue is influenced by numerous variables such as the conditioning contraction (CC) volume and intensity, recovery period following the CC, the type of CC, type of subsequent activity and the subject characteristics (Tillin and Bishop, 2009). Although these variables have shown to affect force production, optimal conditions for these variables have not been determined and therefore their effect on subsequent activities is unclear.
A significant feature of PAP is that it is greater in fast Type II muscle fibres because fast fibres undergo greater phosphorylation of myosin regulatory light chains (MRLC) (See 2.1) in response to a conditioning activity (Sweeney, Bowman and Stull, 1993). Consequently, muscles with a higher proportion of Type II fibres, and individuals with a higher proportion of Type II fibres within a muscle, will exhibit a greater PAP (Sale, 2002). Therefore, theoretically, if effectively utilised, PAP could be implemented into a power training routine to enhance the training stimulus of fast twitch or explosive exercises, such as plyometrics (Robbins, 2005). Furthermore, if PAP was to be induced prior to a competition, it could prove to be superior to current conventional warmups at improving performance in explosive sports such as those involving jumping, throwing and sprinting (Gullich and Schmidtbleicher, 1996). However, the current literature is conflicted on the effects that PAP has and may have on sports performance (Tillin and Bishop, 2009). These inconsistencies are likely due to the variables that affect PAP and the manner in which they were implemented across each study independently.
2. Mechanisms of PAP
It has been proposed that there are two principle mechanisms responsible for PAP. The first of which is considered to be the phosphorylation of MRLC (Gossen and Sale, 2000; Sale, 2002; Hodgson, Docherty and Robbins, 2005), and the second is an increase in the recruitment of higher order motor units (Gullich and Schmidtbleicher, 1996; Chiu et al., 2003; Hodgson, Docherty and Robbins, 2005). It has also been suggested that a change in the pennation angle of the muscle(s), after the conditioning stimulus may also contribute to PAP (Tillin and Bishop, 2009).
2.1 Phosphorylation of Myosin Regulatory Light Chains
Phosphorylation of MRLC, is catalysed by the enzyme myosin light chain kinase (MRLCK), which in theory renders actin-myosin interaction more sensitive to Ca2+ released from the sarcoplasmic reticulum (Sweeney, Bowman and Stull, 1993) during muscular contraction. The activated MRLCK is thought to phosphorylate a specific region of the S-1 myosin head, near it’s hinge region with the S-2 component (Hodgson, Docherty and Robbins, 2005) and it has been suggested that this binding of phosphate provokes a structural alteration in this portion of the myosin molecule during subsequent contractions, which leads to an increase in the rate by which myosin cross-bridges move from a non-force producing state to a force producing state (Grange, Vandenboom and Houston, 1993; Sweeney, Bowman and Stull, 1993). Evidence has also shown that phosphorylation of MRLC renders actin-myosin interactions more sensitive to myoplasmic Ca2+ (Szczesna et al., 2002), thus its greatest effect is at relatively low concentrations of Ca2+, which is the case during twitch or low-frequency tetanic contractions (Sale, 2002; Hodgson, Docherty and Robbins, 2005).
Skinned animal models have shown an acute increase in phosphorylation of MRLC and a corresponding potentiation of twitch tension, following tetanic stimulation of specific efferent neural fibres
2.2 Recruitment of Higher order Motor Units
The H-reflex is a measurement tool utilised by researchers to study the effects of contractile history on neuromuscular response (Hodgson, Docherty and Robbins, 2005). It is traditionally defined as an excitation potential generated as a segmental spinal reflex following maximal impulses to activate the muscle contractile apparatus (Trimble and Harp, 1998). In theory the link between H-reflex and corresponding enhancement of volitional force production can be explained as: H-reflex amplitude is a function of the number and size of recruited motor units (Hugon, 1973). Therefore, in respect it has on PAP, H-reflex may superimpose on the subsequent motor signals, thus increasing the signal to the active muscle(s) (Chiu et al., 2003). – IS THIS EVEN RELEVANT TO MOTOR UNITS, IF SO HOW TO LINK IT TOGETHERE?
2.3 Pennation Angle