Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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A component of magnitude F g, which tends to spin the bone about its longitudinal
axis.
A rotating component of magnitude F t. In Figure 6.17 this is shown in one plane of
movement only; in reality, this muscle force component may be capable of causing
movement in both the sagittal and frontal planes as, for example, flexor carpi ulnaris
flexing and adducting the wrist.
A component of magnitude F s along the longitudinal axis of the bone, which
normally stabilises the joint, as in Figures 6.17(c) and (d). This component may
sometimes tend to dislocate the joint, as in Figure 6.17(f). Joint stabilisation is an
important function of muscle force, particularly for shunt muscles (see below).
The relative importance of the last two components of muscle force is determined
by the angle of pull (θ); this is illustrated for the brachialis acting at the elbow in
Figure 6.17(a), assuming F s to be zero. The optimum angle of pull is 90° when F t = F
(Figure 6.17(e)) and all the muscle force contributes to rotating the bone. The angle
of pull is defined as the angle between the muscle’s line of pull along the tendon and
the mechanical axis of the bone. It is usually small at the start of the movement, as in
Figure 6.17(b), and increases as the bone moves away from its reference position, as in
Figures 6.17(e) and (f).
For collinear muscles, the ratio of the distance of the muscle’s origin from a joint
to the distance of its insertion from that same joint (a/b in Figure 6.17(b)) is known as
the partition ratio, p. It can be used to define two kinematically different types of
muscle. Spurt muscles are muscles for which p > 1; the origin is further from the joint
than is the insertion. The muscle force mainly acts to rotate the moving bone.
Examples are the biceps brachii and brachialis for forearm flexion. Such muscles are
often prime movers. Shunt muscles, by contrast, have p < 1 because the origin is
nearer the joint than is the insertion. Even for rotations well away from the reference
position, the angle of pull is always small. The force is, therefore, directed mostly
along the bone so that these muscles act mainly to provide a stabilising rather than a
rotating force. An example is brachioradialis in forearm flexion. Such muscles may
also provide the centripetal force, which is largely directed along the longitudinal axis
of the bone towards the joint, for fast movements. Two-joint muscles are usually spurt
muscles at one joint, as for the long head of biceps brachii acting at the elbow, and
shunt muscles for the other joint, as for the long head of biceps brachii acting at the
shoulder.
Within the human musculoskeletal system, anatomical pulleys serve to change the
direction in which a force acts by applying it at a different angle and, sometimes,
achieving an altered line of movement. The insertion tendon of peroneus longus is one
such example. This muscle runs down the lateral aspect of the calf and passes around
the lateral malleolus of the fibula to a notch in the cuboid bone of the foot. It then turns
under the foot to insert into the medial cuneiform bone and the first metatarsal bone.
The pulley action of the lateral malleolus and the cuboid accomplishes two changes of
direction. The result is that contraction of this muscle plantar flexes the foot about the
ankle joint (among other actions). Without the pulleys, the muscle would insert