Environmental temperature exhibits profound effects on the activity and ecology of ectotherms through its impact on muscle contractile physiology. While the performance of locomotor behaviors powered by muscle contraction directly decreases by at least 33% over a 10°C drop in body temperature, chameleons are known to feed, presumably with high performance, at body temperatures where sympatric lizard species remain inactive. I propose that ballistic movements that are powered by the recoil of preloaded elastic and collagenous tissues are less thermally dependent than movements that rely on direct muscular power. Despite the reduced thermal sensitivity of the elastic-recoil powered movement, I propose that the muscles associated with preloading these elastic tissues are themselves thermally sensitive and at low temperature, will take longer to load the elastic tissues. Finally, I expect that because of the different effect of temperature on elastic-recoil-powered and muscle-powered movements, performance declines for elastic-recoil-powered tongue projection at low temperature will not vary between species along an environmental temperature gradient (i.e., thermal effects will be the same for all species). Conversely, performance declines for muscle powered tongue retraction at low temperature will be lower in species from colder environments along an environmental temperature gradient. To test these predictions, I used high-speed videography, electromyography and in vitro muscle contractile experiment techniques in conjunction with temperature manipulations to test the mechanistic principles in Chamaeleo calyptratus. I then used high-speed videography at different temperatures in three Bradypodion species from different habitats in South Africa to compare thermal effects on elastic-recoil and muscle-powered movements in different species. I found that the elastic-recoil mechanism of tongue projection in chameleons circumvents the constraints that low temperature imposes on muscle rate properties, thereby reducing the thermal dependence of tongue projection. In all species examined, tongue projection was relatively thermally robust, maintaining a high degree of maximal performance at temperatures as low as 15°C. In contrast, the associated muscle-powered tongue retraction was strongly effected by temperature and experienced substantial performance declines over the same temperature range. While tongue projection performance was itself thermally robust, muscle contractile dynamics of the tongue projector muscle, which preloads the elastic elements responsible for powering projection, was strongly affected by temperature. Similarly, at cooler temperatures the tongue projector muscle became active earlier relative to the onset of tongue projection, due to the reduced rate of tension buildup and the resulting increase in time required to load the elastic elements of the tongue with the required force to subsequently power tongue projection. Further, the effect of temperature on both tongue projection performance and tongue retraction performance was found to vary between species living in different thermal environments. This suggests that despite differences in how temperature affects the performance of these different movement types, both elastic-recoil-powered movements and muscle-powered movements may experience selective pressure to optimize their performance to their environments. Based on these findings, I suggest that the relative thermal independence of tongue projection in chameleons is a more general characteristic of elastic-recoil-powered mechanisms and organisms that use elastic recoil mechanisms for ecologically important movements, such as feeding and locomotion, may benefit from an expanded thermal niche. Further, given the prevalence of elastic power-amplification mechanisms in ectotherms, the benefit of reduced thermal sensitivity may promote the evolution of these mechanisms in other ectothermic animals. Finally, I propose that temperature manipulations may be a useful methodological approach to testing for the presence or prevalence of elastic recoil in powering other biomechanical systems. While these studies examined thermal effects on ballistic tongue projection and tongue retraction in chameleons at difference mechanistic levels and within the framework of how these thermal relationships may be affected by their local environment, many of the results apply more broadly to similar systems in other ectotherms. Comparison of these findings to similar elastically powered systems may help solidify the generality of these findings among other taxa.

Angiotensin Converting Enzyme (ACE) IID, and a actinin 3 (AC1N3) R577X polymorphisms have been linked to the strength and power performance of elite athletes and suggested to influence skeletal muscle function in the general popUlation. This research investigated the association of these two candidate gene polymorphisms with the muscle function of young and older men, and the distribution of these genotypes in frequent fallers compared to controls. Muscle function measurements of young and older men included isometric strength, absolute and relative isokinetic strength at high velocity (ratio of torque at 2400 ·s"; torque at 30°·s") and the time course of an evoked twitch. Additionally body composition was measured by skinfold thickness (young men) and DXA scanning (old men) to estimate fat-free mass, an index of muscularity, and fat mass. ACE and AC1N3 genotypes were determined from whole blood samples using polymerase chain reaction, and serum ACE activity using spectrophotometry. The gemtypes of frequent fallers referred to a Falls Clinic were compared to a control group of healthy men. ACE genotype was not associated with any measure of muscle function, including the time course of an evoked twitch or absolute and relative high velocity torque, or body composition in these populations (ANOVA, 0.12<P<0.97). Serum ACE activity appeared to be weakly associated with knee extensor (R = 0.19, P = 0.07) and elbow flexor (R = 0.20, P = 0.06) isometric strength in older men, and was negatively correlated with the relative torque at high velocity (R = -0.23, P = 0.03). AC1N3 genotype was associated with fat mass in older men (P = 0.04), but was not associated with any measure of muscle function or muscularity (KruskalWaIIis, 0.26<P<0.95). Finally there was no apparent difference in the distribution of ACE IID (r: = 0.54, P = 0.77) and AC1N3 RIX (r: = 0.76, P = 0.68) genotypes between frequent fallers and controls. Any influence of these individual polymorphisms seems unlikely to be of sufficient magnitude to produce genotype related differences in muscle function in young or older free living UK Caucasian men. Serum ACE activity may have a small association with the isometric and dynamic strength of older men. However, AC1N3 genotype was associated with increased fat mass in XX individuals, that suggests this polymorphism may have an association with the accumulation of body fat over the life span of older men.

The primary purpose of this thesis was to assess the effect of knee-joint angle on the neuromechanics of maximal and explosive contractions, specifically torque and neuromuscular activation, as well as the influence of isometric resistance training (RT) on these variables and thus joint angle specificity of training adaptations. It was found that electrode location had a pronounced effect on surface electromyography (sEMG) amplitude during maximum isometric voluntary contractions (MVCs) and moderate relationship between subcutaneous tissue thickness and sEMG amplitude (R2=0.31 up to 0.38) was reduced but not consistently removed by maximal M-Wave (MMAX) normalization [up to R2= 0.16 (peak-to-peak) and R2= 0.23 (Area)]. Thus, MMAX peak-to-peak was the better normalization parameter that removed the influence of electrode location and substantially reduced the influence of subcutaneous tissue thickness. Maximal torque-angle relationship presented an inverted U shape with both, agonist (measure by two different techniques) and antagonist neuromuscular activation both differing with knee-joint angle and thus, both likely contributing to the torque-angle relationship. Absolute explosive torque-angle relationship exhibited higher torques at mid-range knee joint angles in a similar manner to maximal strength, whilst the ability to explosively express the available torque (i.e. relative to maximal strength) revealed only subtle differences between joint angles. Agonist neuromuscular activation showed increases from extended to flexed positions during both maximum and explosive contractions (at all time points; ~6% to ~34%) and evoked contractile properties presented opposite patterns with twitch torque increasing (~5% to ~30%) and octet torque decreasing (~2% to ~14%) with knee flexion. Finally, after 4 weeks of RT at a 65° knee-joint angle evidence of joint angle specificity was provided from both within-group (greater gains at 3 angles than others) and between-group evidence (greater gains at 2 angles than others) for maximal strength but not for explosive strength and neuromuscular activation. In summary, this thesis demonstrated: (1) higher strength values at middle knee-joint positions than more flexed and/or extended positions during maximal and explosive contractions; (2) how agonist neuromuscular activation contributes to the beforementioned changes in strength; (3) how muscle contractile properties contribute to the explosive strength across knee-joint angles; and finally (4) that joint angle specificity has a neural basis.

Neural activation is thought to be essential for the expression of maximal muscle performance, but the exact contribution of neural mechanisms such as the level of agonist, antagonist and stabiliser muscle activation to muscle strength is not fully understood. Explosive neuromuscular performance, including the ability to initiate (the electromechanical delay, EMD) and develop force rapidly (termed, rate of force development, RFD) are considered essential for the performance of explosive sporting tasks and joint stabilisation and thus injury avoidance. The thesis aimed to improve our understanding of the contribution of neural factors to muscle performance, with a specific focus on explosive neuromuscular performance. The work in this thesis utilised a range of approaches to achieve this aim. Initially, the association between muscle activation and rate of force development and EMD was established. Comparison of unilateral and bilateral actions was then undertaken. Finally interventions with the aim to both negatively affect and improve muscle strength, which included fatigue and resistance training (RT), respectively was undertaken and the neural contributions to changes in performance established. Agonist activation during the early phase of voluntary force production was shown to be an important determinant of voluntary EMD, explaining 41% of its inter-individual variability. Agonist activation was an important determinant of early, but not late phase RFD. Use of bilateral actions resulted in a reduction in explosive strength, which was thought to be due to differences in postural stability between unilateral and bilateral strength tasks. The level of stabiliser activation was strongly related to the level of agonist activation during the early phase of explosive force development and had a high association with explosive force production. Task-specific adaptations following isoinertial RT, specifically, the greater increase in isoinertial lifting strength than maximal isometric strength were due to training-specific changes in the level of agonist activation. High-intensity fatigue achieved a more substantial decline in explosive than maximal isometric strength, and this was postulated to be due to neural mechanisms, specifically decreased agonist activation. This work provides an in depth analysis of the neural contributions to maximal muscle performance.

Neuromuscular explosive strength (defined as rate of force development; RFD) is considered important during explosive functional human movements; however this association has been poorly documented. It is also unclear how different variants of strength training may influence RFD and its neuromuscular determinants. Furthermore, RFD has typically been measured in isometric situations, but how it is influenced by the types of contraction (isometric, concentric, eccentric) is unknown. This thesis compared neuromuscular function in explosive power athletes (athletes) and untrained controls, and assessed the relationship between RFD in isometric squats with sprint and jump performance. The athletes achieved a greater RFD normalised to maximum strength (+74%) during the initial phase of explosive contractions, due to greater agonist activation (+71%) in this time. Furthermore, there were strong correlations (r2 = 0.39) between normalised RFD in the initial phase of explosive squats and sprint performance, and between later phase absolute explosive force and jump height (r2 = 0.37), confirming an association between explosive athletic performance and RFD. This thesis also assessed the differential effects of short-term (4 weeks) training for maximum vs. explosive strength, and whilst the former increased maximum strength (+20%) it had no effect on RFD. In contrast explosive strength training improved explosive force production over short (first 50 ms; +70%) and long (>50 ms; +15%) time periods, due to improved agonist activation (+65%) and maximum strength (+11%), respectively. Explosive strength training therefore appears to have greater functional benefits than maximum strength training. Finally, the influence of contraction type on RFD was assessed, and the results provided unique evidence that explosive concentric contractions are 60% more effective at utilising the available force capacity of the muscle, that was explained by superior agonist activation. This work provides a comprehensive analysis of the association between athletic performance and RFD, the differential effects of maximum vs. explosive strength training, and the influence of contraction type on the capacity for RFD.

Dans un contexte de performance sportive, de réhabilitation ou de santé, les étirements sont couramment utilisés dans le but de préparer le système neuromusculaire à l’exercice subséquent. Si la littérature montre majoritairement que les étirements altèrent de façon aigue la performance musculaire et/ou fonctionnelle, il existe toutefois des études dont les résultats diffèrent, suggérant l’influence d’un certain nombre de facteurs sur la variabilité des réponses du système neuromusculaire. Pour une meilleure compréhension de cette variabilité et des mécanismes impliqués, cette thèse avait pour objectif d’examiner l’influence de différents paramètres sur le système neuromusculaire : la durée d’étirement, le groupe musculaire et la modalité d’étirement. Au travers de quatre études, il a été montré que les étirements statiques pouvaient altérer la production de force maximale et les propriétés neuromusculaires indépendamment de la durée d’étirement. Cependant, il est apparu que ces modulations étaient spécifiques au muscle, et plus précisément à la raideur relative du système musculo-tendineux. Indépendamment de la durée d’étirement, les étirements statiques ne seraient pas efficaces pour préparer le système neuromusculaire à une activité musculaire subséquente, et notamment pour des muscles raides. Plus particulièrement, les étirements dynamiques ne paraissent pas plus efficaces. Même si les effets néfastes ont été diminués comparativement aux étirements statiques, ils ne favoriseraient pas davantage la production de force ni la commande nerveuse. Toutefois, il est intéressant de noter qu’une durée courte d’étirements dynamiques peut réduire la résistance passive à l’étirement, et donc augmenter potentiellement l’amplitude articulaire maximale, sans affecter les capacités de production de force. Finalement, les étirements dynamiques pourraient être envisagés comme une activité musculaire dynamique, qui compense partiellement les effets néfastes de l’étirement sur la performance musculaire. D’un point de vue pratique, ceci suggère que ce type d’étirements peut être réalisé avant une performance, mais néanmoins accompagnés d’une activité musculaire de plus haute intensité, afin d’optimiser les effets de l’échauffement musculaire sur le système neuromusculaire. En définitive, la variabilité des réponses du système neuromusculaire serait dépendante de facteurs spécifiques, tels que la modalité d’étirement et la raideur relative du système musculo-tendineux, soulignant l’importance de les prendre en considération dans la pratique. / Stretching is traditionally incorporated into pre-exercise routines in health, rehabilitation and sporting environments to condition the neuromuscular system for exercise. While a large body of evidence reported that stretching may acutely impair the subsequent muscular performance, some conflicting results highlight possible mitigating factors of neuromuscular responses variability. Because the limited data available do not present a clear consensus, the aim of this thesis was to investigate the effects of different factors on neuromuscular responses to stretch: the muscle group, the stretch duration and modality. By means of four studies, it has been shown that static stretching may alter maximal voluntary torque and neuromuscular properties, irrespective of the stretch duration. However, these modulations were dependent on the muscle group, and more specifically on the intrinsic stiffness of the muscle-tendon system. Regardless of the stretch duration, static stretching appears to be not effective, or even detrimental, to prepare the musculotendinous system for subsequent exercises, and especially for stiff muscles. It is also relevant that dynamic stretching was not better than static stretching. Even if stretch-induced impairments were mitigated compared to static stretching, dynamic stretching would not optimize muscle strength capacities and central nervous system. However, a short duration of dynamic stretching may be sufficient to reduce passive resistive torque, and therefore potentially increase maximal range of motion, without affecting muscle strength capacities. Finally, dynamic stretching could be considered as dynamic muscle activity, which would partly counteract deleterious muscle-tendon stretching effects. From a practical point of view, dynamic stretching could be a part of warm-up procedure, but should be associated to stronger contractions to optimize the improvement in muscle strength capacities. In summary, the variability of neuromuscular responses to stretch would be dependent on specific factors, such as stretching modality and relative stiffness of the musculotendinous system, emphasizing the importance of taking it into consideration in practice.

Résumé en français :<p>L’objectif de nos investigations a été d’étudier, chez le sujet humain, les mécanismes responsables de l'altération des performances motrices au cours du vieillissement. La première étude était destinée d’une part, à comparer la diminution maximale de force observée dans les différents régimes de contraction et, d’autre part, à contribuer à la compréhension des mécanismes à l’origine de la meilleure préservation de force pour le régime excentrique. Etant donné que jusqu’à présent, seuls des facteurs musculaires avaient été proposés afin d’expliquer les différences existant en fonction du régime de contraction, nous nous sommes plus particulièrement intéressés à une éventuelle participation de facteurs nerveux. <p>Dans la seconde étude, nous avons testé de manière indirecte (via l’enregistrement d’activités réflexes) l’efficacité des afférences périphériques au cours du vieillissement. En effet, sous le contrôle de la commande centrale, celles-ci contribuent également à réguler le niveau d’excitabilité des motoneurones. Une moindre efficacité de celles-ci pourrait donc être un facteur supplémentaire responsable de l’altération de la fonction motrice. <p>Nous avons ensuite analysé (études III et IV) les propriétés contractiles et le comportement des unités motrices lors de contractions lentes et très rapides (contractions balistiques). Cela nous a permis de vérifier s’il existait chez les seniors une relation entre la diminution de force, de sa vitesse d’installation, et la stratégie d’activation des unités motrices.<p>Enfin, dans l’étude V, nous avons analysé la fatigabilité neuromusculaire chez les seniors, comparativement aux sujets jeunes. Plus spécifiquement, nous avons étudié les contractions concentriques et excentriques maximales fatigantes. La fatigue n’a en effet pas encore été étudiée lors de ces deux types de contraction chez les adultes jeunes et âgés. L’objectif de cette dernière étude était surtout d’analyser la part relative des mécanismes nerveux et musculaires à l’origine de la fatigue ainsi que leur décours dans le temps en fonction de l’âge des sujets.<p>L’approche méthodologique utilisée nous a permis d’analyser les modifications fonctionnelles du muscle et de ses unités motrices avec l’âge. Un intérêt tout particulier a été porté aux ajustements de la commande nerveuse et à la compréhension des mécanismes sous-jacents. L’ensemble de nos résultats expérimentaux documentent de manière originale l’interaction étroite entre les adaptations nerveuses et musculaires au cours du vieillissement. Plus largement, notre travail doctoral contribue à une meilleure connaissance de la remarquable plasticité du système neuromusculaire chez l’homme. <p>Résumé en anglais :\ / Doctorat en Sciences de la motricité / info:eu-repo/semantics/nonPublished