Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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The series elastic element lies in series with the contractile component and transmits the tension
produced by the contractile component to the attachment points of the muscle. The tendons
account for by far the major part of this series elasticity, with elastic structures within the muscle
cells contributing the remainder. The parallel elastic element comprises the epimysium, perimysium,
endomysium and sarcolemma. The elastic elements store elastic energy when they are
stretched and release this energy when the muscle recoils. The series elastic element is more
important than the parallel elastic element in this respect. The elastic elements are important as
they keep the muscle ready for contraction and ensure the smooth production and transmission
of tension during contraction. They also ensure the return of the contractile component to its
resting position after contraction. They may also help to prevent the passive overstretching of the
contractile component when relaxed, reducing the risk of injury. In practice, the series and
parallel elastic elements are viscoelastic rather than simply elastic, enabling them to absorb
energy at a rate proportional to that at which force is applied and to dissipate energy at a rate
that is time-dependent; this would require the addition of a damping element to each elastic
element in Figure 6.10. In toe-touching, the initial stretch is elastic followed by a further elongation
of the muscle–tendon unit owing to its viscosity.
The mechanics of muscular contraction
This section will consider the gross mechanical response of a muscle to various neural
stimuli. Much of this information is derived from in vitro, electrical stimulation of the
frog gastrocnemius. However, we will assume that similar responses occur in vivo for
the stimulation of human muscle by motor nerves. Although each muscle fibre can only
respond in an all-or-none way, a muscle contains many fibres and can contract with
various force and time characteristics.
The muscle twitch
The muscle twitch is the mechanical response of a muscle to a single, brief, lowintensity
stimulus. The muscle contracts and then relaxes, as represented in Figure
6.11(a). After stimulation, there is a short period of a few milliseconds when excitation–
contraction coupling occurs and no tension is developed. This can be considered as the
time to take up the slack in the elastic elements and is known as the latency (or latent)
period. The contraction time is the time from onset of tension development to peak
tension (Figure 6.11(a)) and lasts from 10 to 100 ms, depending on the make-up of the
muscle fibres. If the tension developed exceeds the resisting load, the muscle will
shorten. During the following relaxation time the tension drops to zero. If the muscle
had shortened, it now returns to its initial length. The muscle twitch is normally a
laboratory rather than an in vivo event. In most human movement, contractions are
long and smooth and variations of the response are referred to as graded responses.
These are regulated by two neural control mechanisms. The first – increasing the