Variables y operativización de variables

The aim was to compare the metabolic consequences of equivalent concentric and eccentric work. Changes to mitochondria ultrastructure following eccentric exercise (Su et al., 2010) may be conducive to reduced lipid oxidation and IMCL accumulation. Favourable changes to the lipid and lipoprotein profile following eccentric exercise might increase the uptake and storage of fatty acids from circulating FFAs and lipoproteins within muscle (Nikolaidis et al., 2008). A higher [IMCL] in v.lateralis was

observed 24 hours post eccentric exercise. The advantage of the one-legged model employed over a cross-over design where the same leg is measured on two different occasions (such as walking up, then separately down a hill) is that the same circulating substrates, hormones and cytokines were available to both legs concurrently in the recovery period. This is especially important because the availability of circulating substrates has a primary influence on IMCL repletion after exercise (Johnson et al., 2006, Kiens and Richter, 1998) and any necessary walking during recovery similarly incurs the same metabolic cost in each leg. Therefore, any observed differences between legs in the dependent variables measured must result from changes within the muscle itself.

Also observed was a metabolic alteration within the m. quadriceps femoris that had

undertaken the eccentric protocol, which was reflected in the higher [Pi] observed 24

and 48 hours post exercise. Elevated [Pi], a product of PCr hydrolysis, is associated

with fatigue in healthy muscle and dysfunction in diseased muscle. The ratio of Pi /PCr

has been utilised to reflect the degree of metabolic activity that is occurring within skeletal muscle (McCully et al., 1988). Alterations in this ratio can occur in a number of circumstances, such as muscular myopathies, metabolic disorders and post exercise fatigue from both short and long duration exercise protocols (Baker et al., 1993, Widrick, 2002). The data from this study showed the Pi / PCr ratio measured in the

control condition as 0.15 ± 0.04, which is in agreement with values reported in the literature (Vandenborne et al., 1995). The data from this study revealed both an increase in resting [Pi] and an increase in the Pi / PCr (0.21 ± 0.03) in the eccentrically exercised

leg. Both these non-specific markers of muscle damage can be considered to be evident in the eccentrically exercised leg and absent from the concentrically exercised leg.

Chapter 4 │ IMCL concentration following EEIMD

This is the first study to show that skeletal muscle exposed to very strenuous eccentric exercise and therefore EEIMD, responds differently in terms of IMCL accumulation in first 48 hours of recovery. Previous reports have observed a reduced ability to accrue muscle glycogen following EEIMD (Widrick et al., 1992) and that IMCL accumulates when glycogen is unable to accrue (Stannard and Johnson, 2004). It was therefore expected that a relative increase in IMCL in the treatment (damaged) leg would occur. Instead, a significant interaction between conditions and time was observed: the damaged leg showing higher IMCL at 24 hours but did not increase in content after that. In contrast, the IMCL content of the previously concentrically contracted leg was lower at 24 hours and increased in content over the subsequent 24 hours period.

This observation could be interpreted in a number of ways. Firstly, whilst the external work was balanced between legs, the increased negative work borne by the passive (non-contractile) elements in the muscle during eccentric work (Fridén et al., 1983b), means that the metabolic cost of contraction was higher in the control (concentric) leg. This, along with the fact that greater number contractions were performed in the control leg, means that more IMCL may have been used as a fuel during the concentric contractions. The lower concentrations of IMCL seen at the 24 hour period may therefore reflect partial recovery of that substrate store. In support of this hypothesis,

eccentric exercise appears not to follow the ‘Size Principle’ (Nardone et al., 1989), such that there may be relatively greater reliance upon the larger, less oxidative motor units during strenuous negative work. The treatment (eccentric) leg may therefore have used proportionally more glycogen to support its 300 contractions, with less reliance upon the more oxidative (smaller) motor units. A limitation of this study is that measures of [IMCL] were not collected prior to the exercise interventions.

A second explanation for the observed changes in IMCL may involve changes in resting creatine resulting from EEIMD. As stated in the methodology, the Cr signal from the

1_{H-MRS spectra was utilised as the internal standard and hence the IMCL signal is}

referenced to Cr. The IMCL peak was referenced to the creatine peak rather than the water peak because EEIMD is known to result in oedema. However it cannot be excluded based on the data gathered that the water retention may have affected the T1 and T2 (transverse) relaxation times differently. Significantly larger changes in muscle volume in the treatment leg (Table 1) were observed, which is consistent with this.

Chapter 4 │ IMCL concentration following EEIMD

Thus, if the intracellular creatine concentration is altered by EEIMD, then this may affect the measure of apparent [IMCL]. However, Hesselink et al. (1998) showed that

the PCr content of rat tibialis anterior subjected to 240 eccentric contractions and

resulting in the structural changes associated with sustained eccentric work, was not different after 24 hours recovery in comparison to the contralateral control muscle (Hesselink et al., 1998). The proportion of PCr that constitutes the total creatine (TCr) pool is assumed to be stable in resting skeletal muscle and has routinely been measured using 31P-MRS to indicate [TCr] (Kemp et al., 2007). The data from this study similarly reveal that there was no difference in [PCr] between the two conditions and it can therefore be assumed that the usage of the Cr signal as an internal reference to quantify IMCL is valid, though this may warrant further investigation.

Nikolaidis et al (2008) observed decreases in the blood lipid profile post eccentric exercise. They speculated that these may have been brought about by the increase in activity of lipoprotein lipase (LPL) that promotes lipolysis and FFA release, which in turn, could be taken up by the skeletal muscle and then either oxidised or esterified as IMCL. However, the current study can go some way to disproving this speculation. As both the control and damaged muscle were exposed to the same circulation (and thus lipoprotein/FFA delivery), any differences observed between legs in IMCL content must be due to intramuscular factors. However, it has been established that unaccustomed eccentric exercise can lead to substantial microvascular dysfunction (Kano et al., 2004). There is contradictory evidence as to whether this microvascular dysfunction affects muscle oxygenation, seeing as measures of muscle oxygenation are concerned with the transition from rest to intense exercise. It is therefore hard to establish whether the changes exist in a rested state and would therefore impact on the results herein.

The present study has highlighted that strenuous eccentric exercise resulting in EEIMD produces changes in IMCL content and that this is associated with altered [Pi] in the

muscle. Since EEIMD is also associated with impaired insulin-mediated glucose uptake, our observation may help to shed further light upon the role of skeletal muscle lipid metabolism in the development of EEIMD induced insulin resistance. Further, it is evident that high energy phosphate compounds are not changed at rest in muscle exposed to prior eccentric contractions and therefore considered to be damaged.

5.

Repeat bout effect on eccentric