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Biceps: when the muscle shortens, points (a) insertion
and (b) origin are brought closer together and the arm
is bent, or flexed at the elbow. |
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Antique apparatus for recording successive muscular contractions |
Sliding Filaments
Myofibrils comprised of actin and myosin myofilaments
Tension Potential
- A muscle's ability to contract dependent upon its length,
or degree of contraction. A muscle can contract more forcefully
when it is slightly stretched. Muscle generates maximal concentric
tension at a length 1.2 times its resting length. Beyond this
length, active tension decreases due to insufficient sarcomere
overlap (Norkin & Levangie 1992). A muscle tension becomes
weaker as it nears complete contraction. See muscle
length-tension relation graph below. Also see active
insufficiency and passive insufficiency
below.
Active Insufficiency
The
inability for a biarticulate
muscle to exert enough tension to shorten sufficiently to
complete full range of motion in both joints simultaneously.
Active insufficiency explains the quantity of cross
bridges active from the myosin to the actin dependent upon
the muscle's length. Also see tension
potential above and muscle length-tension
relation graph below These diagrams illustrate how hamstring
involvement is influenced by the position of the knee. In the
first figure, the hamstrings are in a mechanical advantage during
hip extension when the knee is straight or nearly straight. In
the second figure, the hamstring muscles are relaxed when the
knee is bent, particularly as the hip is further extended; the
gluteus maximus are thereby more exclusively involved in hip
extension.
Passive Insufficiency
- The inability for a biarticulate
muscle to stretch enough to complete full range of motion
in both joints simultaneously. Also see tension
potential above and muscle length-tension
relation graph below The above diagrams also illustrate how
flexion at the hip joint is influenced by the position of the
knee. In the first figure, the hamstrings are tightly stretched
when the knee is straight. In the second figure, the muscles
are relaxed when the knee is bent; a greater amount of hip flexion
is thereby permitted.
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