Adaptations in response to repeated bouts of eccentric exercise

Following a single, acute bout of eccentric exercise, previous studies have shown evidence of EEIMD. However, following a single bout of eccentric exercise, skeletal muscle can be protected against damage caused by subsequent bouts of eccentric muscle action. This effect is commonly referred to as the repeated bout effect (RBE) (McHugh, 2003, Nosaka and Clarkson, 1995).

2.3.1

Evidence of adaptations

Section 2.1 examined the markers of change in response to acute bouts of eccentric exercise, which included structural and biochemical (enzyme and protein leakage, loss of Ca2+ homeostasis, inflammatory response and DOMS) alterations. In muscle experiencing strenuous eccentric exercise following prior bouts of eccentric exercise there is attenuation of the appearance of damage markers, a near absence of weakness and reduction of perceptions of soreness (Brown et al., 1997a, Howatson et al., 2007, Howatson and van Someren, 2007, McHugh et al., 2001, McHugh et al., 1999, Nosaka and Clarkson, 1995, Nosaka and Newton, 2002b, Nosaka et al., 2001).

The RBE has been evidenced to appear as early as 2 days after the initial exercise bout (Nosaka and Newton, 2002b, Paddon-Jones et al., 2000) and is diminished between 9 and 12 months later (Nosaka et al., 2001). Further, the effects of 8 weeks of consecutive eccentric training found that pronounced soreness and muscle fibre disruption was evident only following the first 2 to 3 eccentric training sessions, with marked decreases in these variables during the remainder of the study (Fridén and Lieber, 2001). However, minimal amounts of prior exercise can also confer the effect. As few as 2 maximal eccentric contractions have been shown to offer protection against symptoms of EEIMD when the same muscle group performed a larger amount of maximal eccentric work 2 weeks later (Nosaka and Sakamoto, 2001). It has also been shown that light eccentric exercise which does not induce symptoms of muscle damage would confer a protective effect. Exercise equating to just 10% maximal voluntary

Chapter 2 │ Literature review │ Responses to repeat bouts of eccentric exercise

contraction (MVC) is as effective in attenuating the markers of EEIMD from a subsequent bout of eccentric exercise that was greater in magnitude to the initial bout (40% MVC) performed 48 hours later (Lavender and Nosaka, 2008).

Care must be taken when comparing studies that utilise differing protocols because these have provided contrasting timeframes of the RBE. For example, eccentric elbow flexion studies resulted in significantly greater and a more focal degree of muscle injury than downhill treadmill running. Additionally, the fact that most individuals spend more time using their legs for purposes of walking may result in an already present protective effect, which is lacking in elbow extension studies in the less frequently used arms.

The RBE has provided evidence that an initial bout of eccentric exercise offers a protective effect on subsequent bouts of similar exercise type for as long as 6 to 9 months. In most of the studies, the RBE is evidenced by a more rapid return to full force generating capacity, decreased rating of perceived soreness and reduced serum CK activity following repeated versus unaccustomed eccentric muscular activity. Furthermore, the initial bout does not have to be particularly severe to bring about the adaptation. An underlying mechanism responsible for the adaptive process has not been fully revealed, although several have been offered as an explanation.

2.3.2

Theories on mechanisms for adaptive process

There are three theories used to account for the adaptation to an initial bout of eccentric exercise, which can be categorised into neural, connective tissue and cellular theories (McHugh, 2003, McHugh et al., 1999).

The neural theory proposes that the initial damage to skeletal muscle stimulates an increased motor unit activity (Komi and Buskirk, 1972), slow-twitch fibre recruitment and motor unit synchronization following damage (Golden and Dudley, 1992). Subsequently, forces encountered during a repeated bout of eccentric contractions are spread across more fibres (Nosaka and Clarkson, 1995).

The connective tissue theory suggests that adaptive increase in the amount of intramuscular connective tissue (Lapier et al., 1995) and remodelling of the intermediate

Chapter 2 │ Literature review │ Responses to repeat bouts of eccentric exercise

filaments (Fridén et al., 1983a) in response to eccentric contractions account for the attenuation of damage and DOMS in response to repeated eccentric work.

Cellular adaptation theories suggest a strengthening of cell membranes (Clarkson and Tremblay, 1988), removal of weak fibres (Armstrong et al., 1983) and longitudinal addition of sarcomeres (Lynn and Morgan, 1994) as likely candidates for the adaptive response to eccentric muscle work. A significant reduction in the levels of serum CK activity measured after a repeated bout of eccentric exercise have been put forward as an indication that the cell membrane has been strengthened and is less likely to allow a loss of Ca2+ homeostasis and subsequent necrosis of cellular components (Clarkson and Tremblay, 1988).

In summary, the effect of RBE research shows that adaptation has been produced even when the initial bout is less demanding than the second bout, whether that is by number of contractions performed or the length the muscle is contracting through. The ability for the conferral of the protective effect is not necessarily reliant on damage being evident following the first bout of eccentric exercise. A number of adaptive responses are likely to be involved in the mechanism of protecting the muscle from further damage although there is no unified theory that can explain why this mechanism occurs.

Chapter 2 │ Literature review │ Aims and objectives