No abstract

It is well established that the energy cost per unit distance traveled is minimal at an intermediate walking speed in humans, defining an energetically optimal walking speed. However, little is known about the optimal walking speed while carrying a load. In this work, we studied the effect of speed and load on the energy expenditure of walking. The O(2) consumption and CO(2) production were measured in ten subjects while standing or walking at different speeds from 0.5 to 1.7 m s(-1) with loads from 0 to 75% of their body mass (M(b)). The loads were carried in typical trekker's backpacks with hip support. Our results show that the mass-specific gross metabolic power increases curvilinearly with speed and is directly proportional to the load at any speed. For all loading conditions, the gross metabolic energy cost (J kg(-1) m(-1)) presents a U-shaped curve with a minimum at around 1.3 m s(-1). At that optimal speed, a load up to 1/4 M(b) seems appropriate for long-distance walks. In addition, the optimal speed for net cost minimization is around 1.06 m s(-1) and is independent of load.

We have addressed the nature of the postural control signals contained within the discharge activity of neurons in the pontomedullary reticular formation, including reticulospinal neurons, during a reaching task in the cat. We recorded the activity of 142 neurons during ipsilateral reaching movements that required anticipatory postural adjustments (APAs) in the supporting limbs to maintain equilibrium. Discharge activity in 82/142 (58%) neurons was significantly increased before the onset of the reach. Most of these neurons discharged either in a phasic (22/82), tonic (10/82), or phasic/tonic (41/82) pattern. In each of these 3 groups, the onset of the discharge activity in some neurons was temporally related either to the go signal or to the onset of the movement. In many neurons, one component of the discharge sequence was better related to the go signal and another to the onset of the movement. Based on our previous behavioral study during the same task, we suggest that reticular neurons in which the discharge activity is better related to the go signal contribute to the initiation of the APAs that precede the movement. Neurons in which the discharge activity is better related to the movement signal might contribute to the initiation of the movement and to the production of the postural responses that accompany that movement. Together our results suggest the existence of neurons that signal posture and movement independently and others that encode a convergent signal that contributes to the control of both posture and movement.

Children consume more energy per unit body mass to walk at a given speed than do adults (DeJaeger et al., 2001). The difference in the net mass-specific metabolic energy cost per unit distance (i.e. the cost of transport, the energy required to operate the locomotory machinery) between adults and children is greater the higher the speed and the younger the subject. For example, at a speed of 1·m·s -1 , a 3-4-year-old has a net oxygen consumption 33% greater than adults. This difference disappears by the age of 11-12·years.In order to take into account the difference in size between children and adults, the speed of progression can be normalised using the dimensionless Froude number, V -f 2 /(gl), where V -f is mean walking speed, g is acceleration of gravity and l is leg length (Alexander, 1989). In this case, the difference in the cost of transport between children and adults for the most part disappears. This indicates that, after the age of 3-4·years, the difference in the cost of transport may be explained mostly on the basis of body size (DeJaeger et al., 2001).As previously observed in running (Schepens et al., 2001), body size can also affect the positive muscle-tendon work (W tot) performed during walking. Wtot naturally falls into two categories: the external work (Wext), which is the work necessary to sustain the displacement of the centre of mass of the body (COM) relative to the surroundings, and the internal work (W int), which is the work that does not directly lead to a displacement of the COM. Only some of Wint can be measured: (1) the internal work done to accelerate the body segments relative to the COM (Wint,k) and (2) the internal work done during the double contact phase of walking by the back leg, which generates energy that will be absorbed by the front leg (Wint,dc). On the contrary, the internal mechanical work done for stretching the series elastic components of the muscles during isometric contractions, to overcome antagonistic cocontractions, to overcome viscosity and friction cannot be directly measured (although this unmeasured internal work will affect the efficiency of positive work production; Willems et al., 1995).Walking is characterised by a pendulum-like exchange between the kinetic and potential energy of the COM. In children, the 'optimal speed' at which these pendulum-like transfers are maximal increases progressively with age from 0.8·m·s -1 in 2-year-olds up to 1.4·m·s -1 in 12-year-olds and adults (Cavagna et al., 1983). At all ages, the optimal speed is close to the speed at which the mass-specific work to move the COM a given distance, Wext, is at a minimum. Above the optimal speed, the energy transfers decrease. This decrease is greater the younger the subject. The decreased transfers result in a greater power required to move the COM: at 1.25·m·s -1 , The effect of age and body size on the total mechanical work done during walking is studied in children of 3-12·years of age and in adults. The total mechanical work per stride (W tot ) is measured as the sum of the ex...

Walking is characterized by a pendular exchange between gravitational potential energy and kinetic energy of the centre of mass of the body, whereas running is characterized by an elastic rebound of the body within each step. The recovery of mechanical energy through the pendular mechanism of walking has been found to change with age and body size in children, attaining a maximum at lower speeds in the younger subjects (Cavagna, Franzetti & Fuchimoto, 1983). To our knowledge, the changes in the bouncing mechanism of running during growth have not been investigated. In a previous study (Cavagna, Franzetti, Heglund & Willems, 1988), the mechanism of running was analysed in adult humans and other vertebrates. The step period and the vertical oscillation of the centre of mass of the body were divided into two parts: one during which the vertical ground reaction force is greater than body weight (the lower part of the oscillation, taking place during the contact of the foot on the ground) and the other during which the vertical ground reaction force is less than body weight (the upper part of the oscillation, taking place both during ground contact and aerial phase). The duration of the lower part of the oscillation was considered to be one-half of the period of the elastic bounce of the body. The upper part of the oscillation may be compared with the second half of the oscillation only if the aerial phase is nil. If the body leaves the ground, the elastic model is not valid during the upper part of the oscillation because the restoring force is no longer proportional to the displacement (since the vertical acceleration is constant). However, even when an aerial phase exists, the elastic mechanism functions during the lower part of the oscillation. In adults running at low and intermediate speeds, the duration and the amplitude of the lower part of the oscillation of the centre of mass of the body are about equal to those of the upper part (symmetric rebound). In this case, the freely chosen step frequency equals the natural frequency of the body bouncing system responsible for the rebound. At high speeds of running, the duration and the amplitude of the upper part of the oscillation are greater than those of the lower part due to the relative increase in the aerial phase (asymmetric rebound). In this case, the freely chosen step frequency is lower than the natural frequency of the bouncing system. 1. The effect of age and body size on the bouncing mechanism of running was studied in children aged 2-16 years. 2. The natural frequency of the bouncing system (fs) and the external work required to move the centre of mass of the body were measured using a force platform. 3. At all ages, during running below •11 km h¢, the freely chosen step frequency (f) is about equal to fs (symmetric rebound), independent of speed, although it decreases with age from 4 Hz at 2 years to 2·5 Hz above 12 years. 4. The decrease of step frequency with age is associated with a decrease in the mass-specific vertical stiffness of the bouncing ...