Muscle-tendon unit morphology, architecture and stiffness in relation to strength and responses to strength training

This thesis examined the change in skeletal muscle architecture with contractile force production, the relationship of architecture with muscle strength parameters and if muscle tendinous tissue stiffness determines in vivo explosive strength (i.e. rate of torque development, RTD). Muscle and tendinous tissue adaptations to contrasting strength training regimes, and the potential capacity of these tissues to adapt following chronic strength training were also explored. Quadriceps femoris fascicle length (FL) decreased, while the pennation angle (PA) increased in a curvi-linearly manner from rest to maximal voluntary contraction (MVC) torque. Consequently, effective physiological cross-sectional area (effPCSA) during MVC was 27% greater than at rest, although effPCSA measured at rest and during MVC had similar correlations to maximal strength. In the earliest phase of contraction, FL, but not PA, was negatively related (R2=0.187) to voluntary RTD. Neither FL nor PA was related to maximal isometric or dynamic strength. Muscle-tendon unit (MTU) and patellar tendon (PT) stiffness were unrelated to voluntary and evoked RTD. Relative PT stiffness was also unrelated to relative RTD, although relative MTU stiffness was related to voluntary RTD (25-55%MVT, R2≤0.188) and evoked RTD (5-50%MVT, R2≤0.194). MTU stiffness increased after sustained-contraction (SCT, +21%), though not explosive-contraction strength training (ECT). PT stiffness increased similarly after ECT (+20%) and SCT (+16%), yet neither induced tendon hypertrophy. SCT produced modest muscle (+8%) and aponeurosis (+7%) hypertrophy. Chronic strength trained (CST: >3 years) males had substantially greater muscle and aponeurosis size, but similar tendon size as untrained controls (UNT) and short-term (12 weeks) strength trained (STT) individuals. Between these groups, at the highest common force, MTU stiffness was indifferent, while PT stiffness was similarly greater in STT and CST than UNT. These results suggest FL and PA have little influence on muscle strength and tendon stiffness has no influence on RTD. Maximum strength negated any qualitative influence of MTU stiffness on in vivo RTD. Component MTU tissues (muscle-aponeurosis vs. external tendon) adapt differentially depending on the strength training regime. Specifically, free tendon appeared to adapt to high magnitude loading, while loading duration is also an important stimulus for the muscle-aponeurosis. However, chronic strength training was not concordant with greater higher force MTU stiffness, and does not further increase higher force PT stiffness beyond the adaptations that occur after 12 weeks of strength training. Finally, no evidence was found for tendon hypertrophy in response to strength training

The molecular athlete: exercise physiology from mechanisms to medals

Human skeletal muscle demonstrates remarkable plasticity, adapting to numerous external stimuli including the habitual level of contractile loading. Accordingly, muscle function and exercise capacity encompass a broad spectrum, from inactive individuals with low levels of endurance and strength, to elite athletes who produce prodigious performances underpinned by pleiotropic training-induced muscular adaptations. Our current understanding of the signal integration, interpretation and output coordination of the cellular and molecular mechanisms that govern muscle plasticity across this continuum is incomplete. As such, training methods and their application to elite athletes largely rely on a "trial and error" approach with the experience and practices of successful coaches and athletes often providing the bases for "post hoc" scientific enquiry and research. This review provides a synopsis of the morphological and functional changes along with the molecular mechanisms underlying exercise adaptation to endurance- and resistance-based training. These traits are placed in the context of innate genetic and inter-individual differences in exercise capacity and performance, with special considerations given to the ageing athletes. Collectively, we provide a comprehensive overview of skeletal muscle plasticity in response to different modes of exercise, and how such adaptations translate from "molecules to medals"

Oxygen uptake in the frequency domain as a test for cardiorespiratory fitness.

Oxygen uptake kinetics describe the characteristics of the rate of change of VO[2] in response to the onset of exercise or a change in work rate. There is a lack of knowledge concerning the use of VO[2] kinetics in the frequency domain as a test for cardiorespiratory fitness. The PRBS exercise test has been developed to study the dynamic responses of the cardiorespiratory system to random changes in submaximal work rate. This exercise test technique provides a multi-frequent assessment of VO[2] kinetics that can be expressed in terms of amplitude (ml-min[-1]W[-1]) or phase shift (degrees) over a frequency range of 0.0022 to 0.0089 Hz. The VO[2] kinetics of young women were investigated using this submaximal test during which the work rate was alternated between two levels. The upper work rate level was chosen to be below the ventilatory threshold. In the first experiment, the variability of replicate tests was investigated in a cohort of eight moderately active women (age = 22.6 +/- 0.8 years). Although there were wide limits of agreement between the two tests there was no significant difference between test 1 and test 2.In a second experiment to test the discriminant ability, oxygen uptake kinetics were compared to VO[2peak] in twenty-eight sedentary or moderately active young women (age = 22.9 +/-3.1 years). The PRBS exercise test technique was able to discriminate between a group of subjects with lower VO[2peak] (VO[2peak] = 32.3 +/- 3.3 ml-kg-1min-1) and a group of subjects with higher VO[2peak] (VO[2peak] = 41.1 +/- 3.2 ml-kg-1min-1). Differences in VO[2] kinetics occurred at frequencies of 0.0022 Hz for amplitude, and at frequencies of 0.0022 Hz to 0.0067 Hz for phase shift. Significant relationships were found to exist between VO[2peak] and VO[2] kinetics at frequencies of 0.0022Hz, 0.0044 Hz and 0.0067 Hz. The following model explained the highest proportion of the variation between VO[2peak] and VO[2] kinetics (r = - 0.72, P0.001): VO[2peak] (in ml-kg[-1]min[-1]) = 0.503(phase shift at 0.0067 Hz) (in degrees) + 72.24In a third experiment to test the sensitivity to detect change, both VO[2] kinetics and VO[2peak] were measured before, during and after an eight week endurance-type training programme completed by fifteen young women (age = 21.6 +/- 1.9 years). Thirteen young women (age = 24.3 +/-3.5 years) acted as a non-training control group. Faster VO[2] kinetics were measured at a frequency of 0.0044 Hz for amplitude and at frequencies of 0.0022Hz to 0.0067 Hz for phase shift following the training programme. Increases in VO[2peak] also occurred as a result of the exercise regimen. No changes in either VO[2] kinetics or VO[2peak] were observed in the non-training group. This study showed that the PRBS exercise test technique was sensitive to short-term endurance-type training adaptations. In conclusion, the parameters measured during the PRBS exercise test provide valuable information that can not be gained from a standard assessment of VO[2] kinetics in the time domain. It is proposed that this exercise test technique has potential as a means of assessing cardiorespiratory fitness within the area of sports science and within the clinical environment