The maximum force developed in each motor unit of a muscle is related to
the number of fibres recruited, their firing (or stimulation) rate and synchrony,
and the physiological cross-sectional area of the motor unit. The maximum
force depends on the number of cross-bridges attached; the maximum
contraction velocity reflects the maximum rate of cross-bridge turnover, but
is independent of the number of cross-bridges operating.

The factors affecting a muscle’s ability to produce force include its length, velocity, fibre type,
physiological cross-sectional area and activation (see also Bartlett, 1997).
The force per unit physiological cross-sectional area is often known as the
‘specific tension’ of the muscle. A range of values for specific tension have
been reported (e.g. Pierrynowski, 1995); a maximum value of 350 kPa is
often used to estimate the maximum muscle force from its physiological cross-
sectional area (pcsa). It should be noted that pcsa=(m cosa)/(rf/ρ), where m
and ρ are the mass and density of the muscle, rf is the muscle fibre length and
α is the fibre pennation angle.

 The last two of these are defined
when the muscle’s sarcomeres are at the optimal length (2.8 µm) for tension
generation (Pierrynowski, 1995). The different values of specific tension cited
in the literature may be caused by different fibre composition, determination
of pcsa or neural factors (Fukunaga et al., 1992). The effects of training may
also be important (see below).

Muscle activation is regulated through motor unit recruitment and the
motor unit stimulation rate (or rate-coding). The former is an orderly sequence
based on the size of the a-motoneuron. The smaller ones are recruited first,
these are typically slow twitch with a low maximum tension and a long
contraction time. The extent of rate-coding is muscle-dependent. If more
motor units can be recruited, then this dominates. Smaller muscles have fewer
motor units and depend more on increasing their stimulation rate.

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