Introduction to Sports Biomechanics: Analysing Human Movement ...

QUALITATIVE ANALYSIS OF SPORTS MOVEMENTS
This is also known as the principle of minimum task complexity. The chain of
body segments proceeds from the most proximal to the most distal segment.
Coordination of that chain becomes more complex as the number of degrees of
freedom – the possible axes of rotation plus directions of linear motion at each joint
– increases. A simple segment chain from shoulder girdle to the fingers contains
at least 17 degrees of freedom. Obviously many of these need to be ‘controlled’ to
permit movement replication. For example, in a basketball set shot players may
keep their elbow well into the body to reduce the redundant degrees of freedom.
The forces need to be applied in the required direction of motion. This principle
explains why skilled movements look so simple.
Partially general principles – these apply to groups of sports tasks, such as
those dominated by speed generation
Sequential action of muscles.
This principle – also referred to as the summation of internal forces, serial organisation,
or the transfer of angular momentum along the segment chain – is most
important in activities requiring speed or force, such as discus throwing. It involves
the recruitment of body segments into the movement at the correct time. Movements
are generally initiated by the large muscle groups, which produce force to
overcome the inertia of the whole body plus any sports implement. The sequence is
continued by the faster muscles of the extremities, which not only have a larger
range of movement and speed but also improved accuracy owing to the smaller
number of muscle fibres innervated by each motor neuron. In correct sequencing,
proximal segments move ahead of distal ones, which ensures that muscles are
stretched to develop tension when they contract; the principle appears to break
down for long axis rotations, such as medial–lateral rotation of the upper arm and
pronation–supination of the forearm, which occur out of sequence in, for example,
the tennis serve.
Minimisation of inertia (increasing acceleration of movement).
This is most important in endurance and speed activities. Movements at any joint
should be initiated with the distal joints in a position that minimises the moment
of inertia, to maximise rotational acceleration. For example, in the recovery phase
of sprinting, the hip is flexed with the knee also flexed; this configuration has a far
lower moment of inertia than an extended or semi-flexed knee. This principle relates
to the generation and transfer of angular momentum (see Chapter 5), which are
affected by changes in the moment of inertia.
Impulse generation or absorption.
This principle is mainly important in force and speed activities. It relates to the
impulse–momentum relationship (see also Chapter 5): impulse = change of
momentum = average force multiplied by the time the force acts. This shows that a
large impulse is needed to produce a large change of momentum; this requires
a large average force or a long time of action. In impulse generation, the former
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