Mechanical energy in toddler gait A trade-off between economy and stability?

Abstract

During walking metabolic energy is consumed, even if the average walking speed is constant and there is no net change in height of the body. Energy is lost at each step and has to be put into the system again. The total amount of positive muscle-tendon work that has to be performed can be divided into two categories: external work and internal work.External work (Wext) is the amount of work performed to lift and accelerate the centre of mass. To minimize Wext adults make use of an imperfect inverted pendulum (IP) mechanism of energy exchange (Fig.·1), which was first formulated by Cavagna and others (Cavagna et al., 1963(Cavagna et al., , 1966(Cavagna et al., , 1976) over 40 years ago. The IP is characterized by an out-of-phase oscillation of potential (Ep) and kinetic energy (Ek) allowing energy exchange to occur. At preferred walking speed, as much as 70% of the required external mechanical energy can be recovered due to this energy saving mechanism (Cavagna et al., 1977). The other 30% of external mechanical energy is lost from the system and must be supplied by the muscles.Internal work comprises all the work performed by the muscles and tendons that does not directly lead to a displacement of the centre of mass. In the past only the work necessary to accelerate the body segments relative to the centre Mechanical energy expenditure was investigated in children who are just learning to walk and compared with adult mechanical energy expenditure during walking. First, we determined whether the inverted pendulum (IP) mechanism of energy exchange was present in toddlers. It seems that new walkers partially make use of this energy saving mechanism, but it is less efficient than in adults. The reduced recovery values (R=40% at optimal speeds in toddlers compared to 70% in adults) can be explained by their low self-selected walking speed in combination with their tossing gait (large vertical oscillations of the body) and by the observation that during as much as 25-50% of the gait cycle kinetic and potential energy are oscillating in-phase.The second step was to calculate positive external mechanical work (Wext). Since the IP mechanism is less efficient in toddlers, more mass-specific positive work has to be performed to lift and accelerate the centre of mass than in adults walking at the same speed, even when differences in body size are taken into account.The amount of positive internal work (Wint,k) necessary to move the body segments relative to the centre of mass was the third parameter we calculated. In toddlers W int,k is largely determined by the kinetic energy of the lower limb. Compared to adults, toddlers have to perform less mass-specific work per unit distance to accelerate the body segments since the upper body is kept relatively stiff during walking and there is no arm swing.Apart from work performed on the centre of mass and work performed to move the body segments relative to the centre of mass, when walking some work is also performed during double contact as both legs are pushing against each othe...

Development of pendulum mechanism and kinematic coordination from the first unsupported steps in toddlers

“…Both the normalised speed (Froude number), the correlation coefficient between E k and E p and energy recovery increased with age, but 1-5 months after the onset of independent walking they were still lower than these values in adults or in older children (Fig.·5). The recovery of mechanical energy for this age group (1-5 months after the onset of independent walking) was similar to that reported by Hallemans et al (2004).…”

“…Or is it acquired with walking experience in the developing child? Previous studies have demonstrated conclusively that the mechanics of walking of children 2-12 years old, in particular the pendular recovery of energy at each step, is very similar to that of the adults when the walking speed is normalised with the Froude number (Bastien et al, 2003;Cavagna et al, 1983;Schepens et al, 2004), though in younger children (from 2 weeks to 6 months after the onset of independent walking) the mechanical energy exchange occurs to a lesser degree (Hallemans et al, 2004). However, to the best of our knowledge, the pendulum mechanism has not been investigated in toddlers who are just beginning independent, unsupported locomotion, roughly around 1 year of age.…”

Changes in Step Variability of New Walkers With Typical Development and With Down Syndrome

“…During that time, toddlers experience a new task and discover how to control many components through practice (Bril & Brenière;Thelen & Ulrich, 1991). As a result, many parameters stabilize, including intralimb coordination, interlimb coordination, and speed (Bril & Brenière;Clark & Phillips, 1993;Hallemans et al, 2004;Ivanenko et al, 2004). So, why might step width become less stable as step length stabilizes?…”

“…By 5 months of walking experience, toddlers have combined the postural requirements for remaining upright with forward progression, leading to a decrease in step width and in lateral acceleration of the center of mass (Bril & Brenière, 1992). By 2−4 months after the onset of independent walking, toddlers begin to use their legs effectively as inverted pendulums (Hallemans, Aerts, Otten, De Deyn, & De Clercq, 2004;Ivanenko et al, 2004). That observation implies that the rhythmic pendular oscillations seen in adult walking are not innate and that before that pattern can emerge, active neural control and intersegmental coordination are required (Ivanenko et al).…”

The dynamics of hylobatid bipedalism: evidence for an energy-saving mechanism?

“…The ground reaction forces are used to calculate the fluctuations of the centre of mass via the procedure first described in 1975 (Cavagna, 1975), and which has been applied in a large number of recent publications (Farley and Ko, 1997;Roberts and Scales, 2002;Ahn et al, 2004;Griffin et al, 2004;Hallemans et al, 2004;Parchman et al, 2003). In this method, body mass is obtained by integration of the total vertical force (of both hind limbs) over stride duration.…”