Muscle

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. 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.

Muscle Length-tension Relationship Muscle Fatigue & Blood Supply       Greatest tension is developed at point B (slightly stretched) with less tension developed at points A (contracted) and C (stretched).   Muscle Fatigue (a) muscle with intact circulation (b) isolate muscle. Note sustained muscular contraction may occlude local vasculature momentarily impeding blood flow to activated muscle. Also see Pump and Burn. Fast-Twitch versus Slow Twitch Fibers   Slow oxidative (SO) Fast Oxidative Glycolytic (FOG) Fast Glycolytic (FG)   Type I (red) Type IIa (white) Type IIb (white) Speed of contraction Slow Fast Fast Force of contraction Low High High Anaerobic capacity Low Medium High Aerobic capacity High Medium Low Capillary density High Medium Low Mitochondrial density High High Low Motor neuron siz Small Medium Large Major substrate Triglycerides CP, Glycogen CP, Glycogen Activity Prolonged low intense Prolonged high intense Short high intense Average fiber percentage 50% 35% 15% Ratio of both types of muscle fiber varies in each individual and each muscle. Ratio is thought to be genetically determined. Both types of fibers can improve their metabolic capabilities through specific strength and endurance training. Sprinters and weight lifters have a large percentage of fast-twitch fibers. Marathon runners generally have a higher percentage of slow twitch fibers. Muscles that primarily maintain posture against gravity require more endurance and generally have a higher percentage of slow-twitch fibers. Muscles that produce powerful, rapid, explosive strength movements tend to have a greater percentage of fast-twitch fibers.       An artery branching into capillaries between three muscle fibers Record of successive conctractions of elbow flexor muscles         One end of muscle fiber showing attachment of tendon to sarcolemma Part of a muscle fiber specially prepared to bring out the numerous nuclei