(a) A muscle that crosses on the
anterior side
of a joint produces
flexion*
(b) A muscle that crosses on the
posterior side
of a joint produces
extension*
(c) A muscle that crosses on the
lateral side
of a joint produces
abduction
(d) A muscle that crosses on the
medial side
of a joint produces
adduction
These generalities do not apply to the knee and ankle because the lower limb is rotated during development. The
muscles that cross these joints posteriorly produce flexion, and those that cross anteriorly produce extension.
*
Example:
Pectoralis major
(anterior view)
Example: Latissimus
dorsi (posterior view)
The latissimus dorsi
is the antagonist of
the pectoralis major.
Example: Deltoid
middle fibers
(anterolateral view).
The teres major
is the antagonist
of the deltoid.
Example:
Teres major
(posterolateral view)
Figure 10.1
The action of a muscle can be inferred by the position
of the muscle relative to the joint it crosses. (Examples given relate
to the shoulder joint.)
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Muscle Action
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