Effects of load carriage, load position, and walking speed on energy cost of walking

Abstract

Purpose: The purpose of this study was to examine the effects of load, load position, and walking speed on the energy cost of walking per unit distance (C_w: ml/kg/m).

Methods: Eight young male subjects walked on a treadmill at various speeds with and without load in the hands, on the back, and on the legs. The C_w values were determined from the ratio of 2-min steady-state oxygen consumption (V_O2) above resting value () to the walking speed (v): .

Results: An energy-saving phenomenon was observed when the load was carried on the back at slower speeds. This phenomenon diminished at faster speeds, particularly when walking faster than 90 m/min. It was also observed when the load was carried in the hands at slower speeds.

Conclusions: These findings partly supported our hypothesis that an energy-saving phenomenon would be observed due to an interaction between rotative torque around the center of body mass and excessive burden on the lower muscles as a function of speed.

Introduction

Energy expenditure during walking with and without loads has been studied previously to examine physical and psychological tolerance as well as physiological responses in military research (Datta and Ramanathan, 1971; Soule et al., 1978; Jones et al. (1984), Jones et al. (1986); Duggan and Haisman, 1992; Legg et al., 1992). It has been suggested that metabolic demand during walking with load carriage increases linearly with the carrying weight (Soule and Goldman, 1969; Soule et al., 1978; Keren et al., 1981; Gordon et al., 1983; Francis and Hoobler, 1986). However, some researchers revealed that African women can carry an extra-heavy load that amounts to 40% of the body mass on their head (Maloiy et al., 1986; Charteris et al., 1989b). It was reported that the metabolic cost while walking with loads did not necessarily increase if the load was less than 20% of the subject's body mass (Charteris et al., 1989a). These authors termed the phenomenon ‘free-ride’ (Charteris et al., 1989b), and referred to free-ride as carrying loads on the head. They demonstrated that free-ride diminished with an increase of carrying weight.

Charteris et al. (1989a) suggested that changes in both step frequency and stride length are possible biomechanical mechanisms to explain free-ride. However, it is well known that humans naturally select their own optimum step frequency and stride length during walking and/or running (Maloiy et al., 1986). These biomechanical factors did not change significantly in men carrying loads of up to 40 kg (Martin and Nelson, 1986), suggesting that the minimum energy cost of walking must be automatically selected to minimize energy expenditure. This means that changes in step frequency and stride length due to load carriage might be an incidental phenomenon to minimize energy expenditure. Furthermore, the effects of varying loads and load position on an energy-saving phenomenon when walking with loads on the back as a function of speed have not yet been fully studied.

It was hypothesized that the main factors affecting this phenomenon when walking with loads on the back would be due to an interaction between the following two conflicting effects (Fig. 1):

• Load weight on the back would induce rotative torque functioning around the center of the body mass as a positive effect, similar to the free-ride phenomenon.

• Load weight on the back would also put an excessive burden on the lower body muscles as a negative effect.

The rotative torque functioning around the center of the body mass can be defined as follows:

$$\text{F}\text{=AB}\text{×}\text{load}\text{weight,}$$

where F is the rotative torque and AB is a radius of rotation, represented as C in Fig. 1. A possible mechanism to save the energy cost of walking by the rotative torque is to save production of the propulsive force done by leg muscles, resulting in a decrease in the energy cost of walking. On the other hand, load carriage may put an extra burden on the postural trunk muscles and leg muscles, resulting in an increase in the energy cost of walking.

The purpose of this study was to examine the effects of load, load position, and walking speed on the energy cost of walking. For the purpose of the subjects’ safety during laboratory testing, we did not attach the loads to the subjects’ heads, instead, the load was carried on the subjects’ backs. If our hypothesis is correct, then a phenomenon similar to free-ride, which was previously defined, will be observed when loads are carried on the back when walking. It was also hypothesized that a phenomenon similar to free-ride would not be observed when the loads were carried in the hands and on the legs because rotative torque functioning around the center of the body mass cannot be counted on as an energy-saving mechanism when walking with loads in the hands and on the legs.