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How do humans conserve energy during gait?

Frank Plasschaert

Where do humans save energy during gait?

Human gait is a very complex movement that requires energy tot sustain it. To make our locomotion as efficient as possible, in order to keep balanced with the energy delivery by the cardiovascular system, several mechanisms have been adopted. Muscles must contract at each step to move the body segments in the proper sequence. The work done however is in part relieved by an interplay of mechanical energies (potential-kinetic-elastic energy). Historically, the mechanical work of locomotion has been divided into internal and external work. The latter is described as the work done to raise and accelerate the body centre of mass within the environment. The internal work is the work done to accelerate the different body segments with respect to the body centre of mass.

In order to decrease the energy expenditure for walking, the excursion of the centre of gravity is minimised. Walking with stiff lower limbs and without pelvic rotation results in a need to lift our centre of mass with approximately 9.5 cm with each step. This requires a significant investment of energy to perform external work. By combining pelvic rotations in three planes (rotation, tilt and obliquity) and using a coordinated hip, knee and ankle motion humans do manage to reduce the vertical excursion with on average 5.1 cm. (Inman et al. 1981). Although vertical excursion can be minimised, our centre of mass needs to be raised with 4.4 cm to his highest point during single support in midstance. Each time the centre of mass drops about 50% of the potential energy of the body is recovered as kinetic energy (Cochran 1982).

The moments exerted by the muscles at different joints can be adjusted and synchronised to keep the ground reaction force in line with the lower limb, so that no further muscle action is required to remain stable and work requirements are minimised. In the second half of stance phase in physiological gait, the ground reaction force (GRF) is kept in front of the knee and behind the hip, in order to maintain knee and hip extension without active use of the quadriceps and hip extensors. This control of momentum is well used in certain patients groups, e.g. Duchennes’ Muscular Dystrophy where toe walking is used to keep the GRF balanced in front of the knee joint centre to substitute for a weak quadriceps.

A third mechanism to save energy whilst walking has been shown by Yack et al. (1988). Biarticular muscles have the capacity to absorb energy at one level whilst generating power at another level. A well known example is the function of the rectus femoris muscle during rapid walking. At the hip the rectus femoris works concentrically as a hip flexor and assists hip flexion in pre- and initial swing. At the level of the knee the rectus femoris puts trough the patellar tendon a brake on flexion of the shank as an eccentric decelerator. By doing so the rectus transfers (a part of the) energy at the level of the knee towards the hip.

Closely related to the previous mechanism tendon and muscle can be used as any other spring to store elastic strain energy and to return it by elastic recoil. The mechanism(s) is (are) not well known and even the size of the effect is still being debated (Ingen Schenau et al. 1997 J. Applied Biomechanics). It is generally agreed that an eccentric activity of the muscle is needed to store/release elastic energy, otherwise the actine and myosin would not be “attached” and the sarcomers just “plastically” lengthened without loading the spring. This mechanism is considered to assist in the 3rd ankle rocker portion of gait. Though called “push-off” and widely held to be one of the most important components of power generation of human gait, EMG studies of normal gait clearly demonstrate an absence of electrical activity in the plantarflexors during all but the earliest fraction of this plantarflexion motion. This has led some to postulate that elastic energy is being stored in this muscle group during its elongation in second rocker which must then be released during the 3rd rocker to create the propulsion previously assumed to be caused by an active, concentric contraction of the triceps surae. This has yet to be clearly shown, and this theory is not without its critics.