Introduction to Sports Biomechanics: Analysing Human Movement ...

ecause of their anaerobic, lactic metabolism. Fast-twitch, oxidative-glycolytic type IIA
fibres are intermediate between the other two, being moderately resistant to fatigue
because of their mainly aerobic metabolism. These fibres are able to develop high
tension but are susceptible to fatigue at high rates of activity.
The length–tension relationship
For a single muscle fibre, the tension developed when it is stimulated to contract
depends on its length. Maximum tension occurs at about the resting length of the fibre,
because the actin and myosin filaments overlap along their entire length, maximising
the number of cross-bridges attached. If the fibre is stretched, the sarcomeres lengthen
and the number of cross-bridges attached to the thin actin filaments decreases.
Conversely, the shortening of the sarcomeres to below resting length results in the
overlapping of actin filaments from opposite ends of the sarcomere. In both cases the
active tension is reduced. In a whole muscle contraction, the passive tension caused by
the stretching of the elastic elements must also be considered, as shown in Figure 6.12,
as well as the active tension developed by the contractile component, which is similar to
the active tension of an isolated fibre.
The total tension (Figure 6.12) is the sum of the active and passive tensions and
depends upon the amount of connective tissue – the elastic elements – that the muscle
contains. For single joint muscles, such as brachialis, the amount of stretch is not
usually sufficient for the passive tension to be important. In two-joint muscles, such as
three of the four heads of the hamstrings – semitendinosus, semimembranosus and
the long head of biceps femoris – the extremes of the length–tension relationship may
be reached, with maximal total tension being developed in the stretched muscle, as in
Figure 6.12.
Figure 6.12 Length–tension relationship for whole muscle contraction.
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