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Mechanical determinants of the minimum energy cost of gradient running in humans
No full textThe metabolic cost and the mechanical work of running at different speeds and gradients were measured on five human subjects. The mechanical work was partitioned into the internal work (Wint) due to the speed changes of body segments with respect to the body centre of mass and the external work (Wext) due to the position and speed changes of the body centre of mass in the environment. Wext was further divided into a positive part (W+ext) and a negative part (W-ext), associated with the energy increases and decreases, respectively, over the stride period. For all constant speeds, the most economical gradient was -10.6 +/-0.5% (S.D., N = 5) with a metabolic cost of 146.8 +/- 3.8 ml O2 kg-1 km-1. At each gradient, there was a unique W+ext/W-ext ratio (which was 1 in level running), irrespective of speed, with a tendency for W-ext and W+ext to disappear above a gradient of +30% and below a gradient of -30%, respectively. Wint was constant within each speed from a gradient of -15% to level running. This was the result of a nearly constant stride frequency at all negative gradients. The constancy of Wint within this gradient range implies that Wint has no role in determining the optimum gradient. The metabolic cost C was predicted from the mechanical experimental data according to the following equation: [formula: see text] where eff- (0.80), eff+ (0.18) and effi (0.30) are the efficiencies of W-ext, W+ext and Wint, respectively, and el- and el+ represent the amounts of stored and released elastic energy, which are assumed to be 55J step-1. The predicted C versus gradient curve coincides with the curve obtained from metabolic measurements. We conclude that W+ext/W-ext partitioning and the eff+/eff- ratio, i.e. the different efficiency of the muscles during acceleration and braking, explain the metabolic optimum gradient for running of about -10%
Simultaneous registration in 3 spatial directions of acceleration to which one is subjected during marching
No full textVentilatory work during exercise at high altitude
No full textOxygen consumption, ventilation, and dynamic respiratory work were measured in three male subjects during cycling at 122 and 3500 mm above sea level (ASL). At a given ventilation the dynamic respiratory work was 20% less at 3500 m ASL; this change was due to a decrease of airway resistance. At a given submaximal exercise intensity, the respiratory work was significantly higher at 3500 m ASL (+ 140%-180%); hence, the increase of ventilation was not compensated for by the decrease of airway resistance. At V̇(O2)max the respiratory work was predicted to reach its maximal value at 5800 m ASL where it was 30% higher than at sea level
Biomechanical and physiological aspects of legged locomotion in humans
No full textWalking and running, the two basic gaits used by man, are very complex movements. They can, however, be described using two simple models: an inverted pendulum and a spring. Muscles must contract at each step to move the body segments in the proper sequence but the work done is, in part, relieved by the interplay of mechanical energies, potential and kinetic in walking, and elastic in running. This explains why there is an optimal speed of walking (minimal metabolic cost of about 2 J.kg(-1).m(-1) at about 1.11 m.s(-1)) and why the cost of running is constant and independent of speed (about 4 J.kg(-1).m(-1)). Historically, the mechanical work of locomotion has been divided into external and internal work. The former is the work done to raise and accelerate the body centre of mass (m) within the environment, the latter is the work done to accelerate the body segments with respect to the centre of m. The total work has been calculated, somewhat arbitrarily, as the sum of the two. While the changes of potential and kinetic energies can be accurately measured, the contribution of the elastic energy cannot easily be assessed, nor can the true work performed by the muscles. Many factors can affect the work of locomotion--the gradient of the terrain, body size (height and body m), and gravity. The partitioning of positive and negative work and their different efficiencies explain why the most economical gradient is about -10% (1.1 J.kg(-1).m(-1) at 1.3 m.s(-1) for walking, and 3.1 J.kg(-1).m(-1) at between 3 and 4 m.s(-1) for running). The mechanics of walking of children, pigmies and dwarfs, in particular the recovery of energy at each step, is not different from that of taller (normal sized) individuals when the speed is expressed in dynamically equivalent terms (Froude number). An extra load, external or internal (obesity) affects internal and external work according to the distribution of the added m. Different gravitational environments determine the optimal speed of walking and the speed of transition from walking to running: at more than 1 g it is easier to walk than to run, and it is the opposite at less than 1 g. Passive aids, such as skis or skates, allow an increase in the speed of progression, but the mechanics of the locomotion cannot be simply described using the models for walking and running because step frequency, the proportion of step duration during which the foot is in contact with the ground, the position of the limbs, the force exerted on the ground and the time of its application are all different
The transition between walking and running in humans : metabolic and mechanical aspects at different gradients
No full textFive subjects walked and ran at overlapping speeds and different gradients on a motorized treadmill. At each gradient the speed was obtained at which walking and running have the same metabolic cost (Sm) and the speed of spontaneous (Ss) transition between the two gaits was measured. Ss was found to be statistically lower than Sm at all gradients, the difference being in the range of 0.5-0.9 km h-1. The motion analysis of walking reveals that at all gradients and at increasing speed: (1) the percentage of recovery, an index of mechanical energy saving related to the pendulum-like characteristic of walking, decreases; (2) the lower limb spread reaches a limit in walking; and consequently (3) both the stride frequency and the internal mechanical work, due to limb acceleration in relation to the body centre of mass, increase much more in walking than in running. Switching to a run, although implying a higher frequency, makes the internal work decrease as a result of the lower limb spread. In this paper several influences, such as the 'ratings of perceived exertion' (RPE), on the choice of beginning to run when it is more economical to walk, are discussed. A tentative hypothesis on the determinants of Ss, which is emphasized to be a speed which has to be studied in detail but is generally avoided in locomotion, is based on a comfort criterion from peripheric afferences and is reflected by the fact that at Ss a running stride costs as much as a walking stride.(ABSTRACT TRUNCATED AT 250 WORDS
The regulation of respiratory resistance in exercising horses
No full textHorses display remarkable aerobic capabilities, attaining during muscular exercise a maximal rate of oxygen consumption about 30-fold higher than the resting value, and 2.5-fold higher than that of other mammals of similar body mass. Under these circumstances an enormous mechanical burden is expected to impinge on the equine respiratory pump and regulatory mechanisms aiming to minimize this load may play an important role in determining the adequacy of the respiratory system to the metabolic requirements. The behaviour of the respiratory system has been investigated in horses at rest and during treadmill locomotion at different velocities and gaits. During exercise hyperpnoea, horses exhibit a significant reduction in the lung viscous resistance not observed in other mammals, such as dogs and humans. Therefore, the exercise-dependent increase in the rate of mechanical work of breathing is lower in the horse than in other mammals. This increase in the equine airway patency during exercise appeared to be mainly determined by the pattern of laryngeal movements. In fact, during exercise, the laryngeal cross-sectional area, determined with a video-endoscopic imaging technique at the level of rima glottidis (CSArg), undergoes during inspiration an increase averaging up to over 4 times the resting expiratory values. Although a significant linear correlation was found between CSArg and minute ventilation (VE), the laryngeal activation contributes to increase lung conductance only when CSArg is narrower than the tracheal section. It appears therefore that in exercising horses pulmonary resistive features are finely controlled to reduce the mechanical load supported by the respiratory muscles and to counterbalance the increase in the ventilatory energetic requirements inherent in the remarkably enhanced aerobic performance observed in this species
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