Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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to brief summations or longer subtetanic or tetanic contractions of the whole muscle.
For precise but weak movements only a few motor units will be recruited while far more
will be recruited for forceful contractions. The smallest motor units with the fewest
muscle fibres, normally type I (see below), are recruited first. The larger motor units,
normally type IIA, then type IIB (see below) are activated only if needed. Both wave
summation and multiple motor unit summation are factors in producing the smooth
movements of skeletal muscle. Multiple motor unit summation is primarily responsible
for the control of the force of contraction.
Treppe
The initial contractions in the muscle are relatively weak, only about half as strong as
those that occur later. The tension development then has a staircase pattern called
treppe, which is related to the suddenly increased availability of calcium ions. This
effect, along with the increased enzymatic activity, the increase in conduction velocity
across the sarcolemma, and the increased elasticity of the elastic elements, which are all
consequent on the rise in muscle temperature, leads to the pattern of increasingly strong
contractions with successive stimuli. The effect could be postulated as a reason for
warming up before an event, but this view is not universally accepted.
Development of tension in a muscle
The tension developed in a muscle depends upon:
The number of fibres recruited and their firing (or stimulation) rate and synchrony.
The relative size of the muscle – the tension is proportional to the physiological
cross-sectional area of the muscle; about 0.3 N force can be exerted per square
millimetre of cross-sectional area.
The temperature of the muscle and muscle fatigue.
The pre-stretch of the muscle – a muscle that develops tension after being stretched
(the stretch–shortening cycle, see below) performs more work because of elastic
energy storage and other mechanisms; the energy is stored mostly in the series elastic
elements but also in the parallel elastic ones.
The mechanical properties of the muscle, as expressed by the length–tension, force–
velocity and tension–time relationships (see below).
It should be noted that there are distinct differences in the rates of contraction, tension
development and susceptibility to fatigue of individual muscle fibres. The main
factor here is the muscle fibre type. Slow-twitch, oxidative type I fibres are suited for
prolonged, low-intensity effort as they are fatigue-resistant because of their aerobic
metabolism. However, they produce little tension as they are small and contract only
slowly. Fast-twitch, glycolytic type IIB fibres have a larger diameter and contract
quickly. They produce high tension but for only a short time, as they fatigue quickly