Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Types of muscle contraction
The term ‘muscle contraction’ refers to the development of tension within the muscle.
The term is a little confusing, as contraction means becoming smaller in much English
usage. Some sport and exercise scientists would prefer the term ‘action’ to be used
instead, but as this has yet to be widely adopted, I will use muscle contraction, of which
there are three main types:
In isometric, or static, contraction, the muscle develops tension with no change in
overall muscle length, as when holding a dumbbell stationary in a biceps curl.
In concentric contraction, the muscle shortens as tension is developed, as when a
dumbbell is raised in a biceps curl.
In eccentric contraction, the muscle develops tension while it lengthens, as in the
lowering movement in a biceps curl.
Both concentric and eccentric contractions can, theoretically, be at constant tension
(isotonic) or constant speed (isokinetic). However, most contractions normally involve
neither constant tension nor constant speed.
Group action of muscles
When a fibre or muscle develops tension both ends tend to move; whether these
movements actually occur depends on the resistance to movement and on the activity
of other muscles. Furthermore, when a muscle develops tension, it tends to perform all
of its possible actions at all joints it crosses. Because of these axioms, muscles act
together rather than individually to bring about the movements of the human body,
with each muscle playing a specific role – this is one important feature of coordinated
movement. In such group actions, muscles are classified according to their role, as
follows. Please note that the muscle group terminology used here is that normally used
in sports biomechanics; however, many anatomists now prefer different names for these
muscle group roles.
The muscles that directly bring about a movement by contracting concentrically
are known as the agonists, which means ‘movers’. This group is sometimes divided
into prime movers, which always contract to cause the movement, and assistant
movers, which only contract against resistance or at high speed. However, electromyography
(see below) does not usually support such a simple distinction. If we
accept such a distinction, then brachialis and biceps brachii would be prime movers
for elbow flexion while brachioradialis would be generally considered to be an
assistant mover. This distinction is closely related to the idea of spurt and shunt
muscles touched on later in this section.
Antagonists are muscles that cause the opposite movement from that of specified
agonists. Their normal role in group action is to relax when the agonists contract,