PHYSIOLOGY OF HUMAN RUNNING: FROM MOTORS TO FUEL PUMPS

PHYSIOLOGY OF HUMAN RUNNING: FROM MOTORS TO FUEL PUMPS

Taken from:

http://havemeyerfoundation.org/monograph.htm

A. J. Blazevich and N. C. C. Sharp
Sport Sciences, Brunel University, Uxbridge, UB8 3PH, UK

THE ELASTIC HUMAN
In order for a human to run quickly and efficiently for a given
period of time a number of physiological, biochemical and
biomechanical processes must be optimised. Human running is
accomplished by performing a series of bounces as gravitational
energy is stored in our `leg springs' during the leg shortening, or
impact, phase and is released during the leg lengthening, or
propulsion, phase (Fig 1). Approximately 0.6 J of energy are stored
and released per kilogram per bounce in the foot and calf (Ker et al.
1987), compared with about 1.1 J per kg in a 0.5 ton horse (Minetti
et al. 1999). The total energy stored and released in the whole leg
represents a substantial portion (about half in humans) of the energy
required to propel the body into the next step. Because the highly
elastic (ie high energy return) tendons are most responsible for this
spring-like behaviour, and their properties change in response to
loading, it is reasonable that some portion of training should target
the tendon. The tendon stiffness that is optimum for performance
depends on the force transmitted through the tendon and on the tendon
lengthening velocity during the run (eg it differs for different
tendons and between sprint and long-distance running). However, it is
known that chronic endurance or strength training increases tendon
stiffness, while flexibility training reduces it. The effects of
plyometrics training have not been measured directly in humans,
although in rats there is evidence that it reduces stiffness and
hysteresis (ie reduces the energy lost from the tendon). Thus,
training a human runner may require some portion of these training
modalities in order to optimise tendon.....

OPTIMUM MUSCLE CONTROL
The remaining energy required for running must come from muscle
contraction. It has been held traditionally that muscles lengthen, or
work eccentrically, during the impact phase of running and shorten,
or work concentrically, during the propulsive phase. Recent evidence
from human research, and experiments on animals, shows however that
muscles contract quasi-isometrically during the propulsive phase of
many stretchshorten- type movements (eg Kurokawa et al. 2003), or
during high-speed movements performed without a counter-movement
(Kurokawa et al. 2001). This makes sense when one considers the work
of Hill (1938), who showed that concentrically-contracting muscle
uses more energy than isometrically-contracting muscle, with the
disparity increasing as muscle force or length change (or velocity)
increased. As muscle power increases, the relative cost of performing
work by concentric muscle action increases, and the benefit of using
stored energy becomes greater.

During running, the leg shortens during the impact phase as a result
of the ground reaction force (GRF; A), and then lengthens during the
propulsion phase as a result of the work performed by tendons (reuse
of stored elastic energy) and muscles. Thus, the leg essentially
functions as a spring (B) with about half of the work required to
continue the spring bouncing being done by the muscles.

CONSIDERATIONAS IN PHYSIOLOGICAL TRAINING
The efficient performance of running involves a complex interaction
of physiological, biochemical and biomechanical processes. Each
process can be targeted by specific training, with resistance,
plyometrics, flexibility, anaerobic, aerobic and interval training
all required. The difficulty for the athlete, coach or scientist is
to programme the appropriate training with the appropriate volume,
intensity and timing; so called periodisation. It is well known that
adaptations of specific processes are compromised when training is
performed concurrently, ie, training 2 processes on the same day or
in the same session (Docherty and Sporer 2000), so informed planning
is required.

Ultimately, the task is to determine what the appropriate
requirements are for a particular runner, examine what his/her
current fitness parameters are, then target training to optimise
these parameters. Such process should result in performance
improvements, and reduce the risk of muscle-tendon, skeletal,
cardiovascular, and heatand fatigue-related injury