Characteristics and adaptation of skeletal muscle to endurance exercise

[摘要] ENGLISH ABSTRACT: Skeletal muscle adapts to stimuli by modifying structural and metabolic protein expression.Furthermore, a muscle group may vary within itself to accommodate specialisation in regions.Structural and metabolic characteristics of an individual are regulated partly by genotype, butcontraction duration and intensity may play a greater role in muscle phenotype. The aims of thisdissertation were to investigate: structural and metabolic regionalisation in a muscle group, possiblerelationships between training volume and intensity and hybrid fibres, muscle characteristics ofathletes from two different ethnic groups, and muscle adaptation in already well-trained athletessubjected to high intensity interval training.Myosin heavy chain (MHC) isoform content and citrate synthase (CS) activities were measured inthe Quadriceps femoris (QF) muscle of 18 female rats. Muscle was divided into superficial, middleand deep, distal, central and proximal parts. MHC IIb and IIx were more abundant in superficialregions (P < 0.05) with low CS activities compared to deeper parts. Isoform content varied along thelength of deep regions. This study showed that the QF has regional specialisation. Therefore,standardisation of sampling site is important.Hybrid fibre proportions in muscle biopsies of 12 middle distance runners and 12 non-runners wereinvestigated. MHC IIa/IIx correlated with training volume/week in runners (r = -0.66, P < 0.05) andMHC IIa/IIx correlated with exercise hours/week in non-runners (r = -0.72, P < 0.01). Averagepreferred racing distance (PRDA) correlated better with MHC IIa/IIx in runners (r = -0.85, P <0.001). MHC IIa/IIx may therefore be more closely related to exercise intensity than previouslythought.Fibre type characteristics and performance markers were investigated in 13 Xhosa and 13 Caucasiandistance runners, matched for performance, training volume and PRDA. Xhosa runners had lessMHC I and more MHC IIa fibres in muscle biopsies than Caucasian runners (P < 0.05). Xhosarunners had lower plasma lactate at 80% peak treadmill speed (PTS) (P < 0.05), but higher lactatedehydrogenase (LDH) (P < 0.01) and phosphofructokinase (P = 0.07) activities in homogenatemuscle samples. LDH activities in MHC I (P = 0.05) and IIa (P < 0.05) fibre pools were higher inXhosa runners. Xhosa athletes may thus have a genetic advantage or they may have adapted torunning at a higher intensity.Six weeks of individually standardised high intensity interval treadmill training (HIIT) wereinvestigated in 15 well-trained runners. PTS increased after HIIT (P < 0.01), while maximumoxygen consumption (VO2max) only showed a tendency to have increased as a result of HIIT (P = 0.06). Sub-maximal tests showed lower plasma lactate at 64% PTS (P = 0.06), with lower heartrates at workloads from 64% to 80% PTS (P < 0.01) after HIIT. No changes were observed forcross-sectional area, capillary supply and enzyme activities in homogenates muscle samples. LDHactivity showed a trend (P = 0.06) to have increased in MHC IIa pools after HIIT. Higher HIITspeed was related to decreases in MHC I fibres, but increases in MHC IIa/IIx fibres (r = -0.70 and r= 0.68, respectively, P < 0.05). Therefore, HIIT may alter muscle fibre composition in well-trainedrunners, with a concomitant improvement in performance markers.

[发布机构] Stellenbosch University