Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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the racket arm may still be in their ‘preparation’ phase. The extension of the three-phase
ballistic model to other activities is unhelpful, although common. The biomechanical
structure of other movements may require more than three phases, as in the following
examples.
Phase analysis of running
In Chapter 1, we divided the running cycle into two rather obvious phases: stance and
swing, for which the key events are clearly touchdown and toe-off. The functions of these
two phases can then be defined as follows. The stance phase absorbs impact, allows an
active landing, maintains forward momentum, and accelerates to toe-off. The swing
phase allows recovery of the non-support leg and prepares for foot descent to hit
the ground with an active landing. This two-phase schema results in overly complex
biomechanics of the phases. An alternative, more attractive schema biomechanically,
subdivides the movement into three phases each within the above stance and swing
phases, as follows. Although this phase analysis is far more biomechanically sound, it is
unlikely that the phase boundaries of the short duration foot-strike phase will be
discernible from 50 or 60 Hz digital video recordings, an important practical drawback.
Foot strike – starts at first foot contact (touchdown). This is a very short phase
in which the foot should, ideally, hit the ground moving backwards. The main
function of this phase is impact absorption.
Mid-support – starts after the impact peak as limb length starts to shorten. This
phase is characterised by relative shortening of the overall length of the support limb
towards the centre of mass. The hips and knees flex (see also Figure 3.13(a), region
a–b), the ankle dorsiflexes and the foot pronates. Its functions are maintenance of
forward momentum, support of body weight and storage of elastic energy.
Take-off – starts as limb length starts to increase, ends at toe-off. This phase involves
relative lengthening of the overall limb length. The hips and knees extend (see also
Figure 3.13(a), region c–d), the ankle plantar flexes and the foot supinates. Its
functions are to accelerate the body forwards and upwards and to transfer energy
from the powerful thigh muscles to the faster muscles of the calf.
Follow-through – starts at toe-off. This phase is characterised by slowing of hip
extension then start of hip flexion, both accompanied by, and the latter assisting,
knee flexion (see also Figure 3.13(a), region d–e). It functions as a decelerating phase
for the recovery leg.
Forward swing – starts as the foot begins to move forward relative to the body. The
hip flexes with the knee flexing or flexed (see also Figure 3.13(a), at f). Its main
function is to coordinate the recovery leg movement to enhance the forwards–
upwards reaction force from the ground, as this phase coincides with the take-off
phase of the other leg (similar to the coordination of arm and leg movements in the
vertical jump examples in Chapter 1). The phase also prepares for the next, foot
descent, phase.