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.

Ce travail de thèse s'est intéressé à l'étude des facteurs permettant d'expliquer les performances fonctionnelles des membres inférieurs des personnes âgées. Dans une première étude, nous avons montré que la puissance maximale (Pmax) ainsi que sa composante appelée la vitesse optimale (Vopt) obtenues lors d'un test de force-vitesse des extenseurs du genou, sont des facteurs déterminants de la performance fonctionnelle des personnes âgées évaluées sur 3 tests : marche sur 6m, se relever et s'assoir 5 fois de suite à partir d'une chaise et monter 6 marches d'escalier. Ainsi, plus une personne possède une Pmax et une Vopt élevées, plus ses performances lors de tests fonctionnels seront élevées. De plus, la Vopt étant considérée comme une évaluation indirecte du pourcentage de fibres rapides, les sujets ayant les plus grandes valeurs de Vopt sont les plus performantes aux tests fonctionnels. La deuxième étude a mesuré et comparé l'activité électromyographique (EMG) du quadriceps lors des tests de contraction isométrique volontaire maximal (IMVC), de détermination de la puissance maximale et des tests fonctionnels cités cidessus. La mesure du signal EMG au cours de ces différents tests a permis de déterminer dans quelle mesure le type de tâche induit une activation maximale ou non. Les personnes âgées montrent une capacité à davantage activer leurs quadriceps lors des tests de de contraction concentrique plutôt qu'isométrique ou excentrique. Les tests utilisant l'IMVC comparé aux tests de détermination de la puissance et de monté de marche, et de lever de chaise ne seraient pas les tests les plus appropriés pour mesurer les capacités maximales d'activation neuromusculaires chez la personne âgée. Il ressort de ces résultats une amélioration du testing de la personne âgée et des orientations pour la réhabilitation / This work has focused on the study of factors that explain the functionals performances of the lower limbs of the elderly. In a first study we showed that the maximum power (Pmax) and its component called the optimal speed (Vopt) obtained during a test of force-velocity of the knee extensors, are important determinants of functional performance elderly evaluated on three tests: walking on 6m, sit to stand 5 times from a chair and climbing 5 stairs. The more a person has a high Pmax and Vopt, the more its performance during functional tests will be. In addition, Vopt is considered an indirect assessment of the percentage of fast fibers; subjects with the highest values of Vopt are best performers during functional tests. The second study measured and compared the electromyographic (EMG) activity of the quadriceps when tested for isometric maximum voluntary contraction (IMVC), determination of maximum power and functional tests mentioned above. Measurement of EMG signal during these tests was used to determine to what extent the type of task induced a maximum activation or not. Older people show a greater ability to activate their quadriceps muscles when tested for concentric contraction rather than isometric or eccentric contractions. Tests using the IMVC compared to tests for determining the power, sit-tostand and stairs climbing would not be the most appropriate tests to measure the maximum neuromuscular activation capacity in the elderly. From these results improved testing of the elderly and guidelines for rehabilitation are proposed