During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way), and energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on König's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e.g., the pendulum-like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the past 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g., low gravity, unsteady speed). This narrative review aims to summarize current knowledge of the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in unsteady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g., frictional internal work and elastic contribution) deserves future research that would improve our knowledge of the

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The aim of this article was to understand how active power is used in squat and countermovement jumps. A simple empirical model comprising a mass, a spring, an active element and a damper, together with an optimisation principle, was used to identify the mechanical factors that maximise performance of jumps without countermovement (squat jumps, SJ) and with countermovement (CMJ). Twelve amateur volleyball players performed SJ from two initial positions and CMJ with two degrees of counterbalancing, while kinematic data were collected (jump height, push-off duration and position of the centre of mass). The model adjusted well to real data of SJ through all the impulse phase, and slightly less adequately at the end of this phase for CMJ. Nevertheless, it provides a satisfactory explanation for the generation and utilisation of active power for both type of jumps. On average, the estimated power of the active elements, the spring, and the damper were greater in the SJ. Based upon the result obtained with this model, we suggest that active power is best evaluated with SJ. The reason for this is that, during this kind of jump, the elements associated with the damper consume much of the energy produced by the active elements. The participation of the elements that consume the energy generated by the active elements is less in CMJ than in SJ, allowing for a better utilisation of this energy. In this way it is possible to achieve a better performance in CMJ with less active power.

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This work analysed the changes that occur in kinetic-temporal parameters and the electromyographic activity of lower limb muscles during the performance of maximum Counter Movement Jumps (CMJ) done with and without muscular fatigue, to explain the changes in performance in both situations. The vertical strength of fifty jumps performed by ten male sportsmen before and after a fatigue protocol was registered. Records of surface electromyography were obtained for six lower limb muscles; in addition, activation level and intermuscular coordination were analysed. For analysis purposes, negative and positive mechanic working times were considered separately. In both conditions, the temporal course of strength distribution is more important for the performance than any other instantaneous parameter. The negative external working time was significantly lower in fatigue conditions. The electromyographic analysis showed an increase in the activation level of all muscles studied and significant changes in the activation sequence. The specific kinetic-temporal variables were not good predictors of jump height. When CMJ is performed without muscle fatigue, a high value of active state favours the positive work. During fatigue, there could be a partial compensation of the performance due to the increasing activity of the contractile elements, although the activation sequence undergoes significant changes. Thus, the changes in performance would be mainly associated with the decrease in the capacity to transmit power in the proximal-distal direction.

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