Muscle

ExRx.net > Exercise Information > Info

Translate

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. Stretch-shortening Cycle Muscular forces can be enhanced when the muscle is stretched (causing eccentric tension) immediately before concentric contraction. This stretch-shorten cycle (SSC) occurs naturally in running, jumping, and all other activities in which muscles are suddenly stretched by impact or other external forces. The stretch-shortening cycle saves energy by temporarily storing potential energy through elastic recoil resulting from an external quick stretching force. Plyometric vertical jumps folowing quick landings from a 0.4 m height have been found to be higher than made from a semisquat, with or without a dip or countermovement (Asmussen & Bonde-Peterson 1976). Also see Depth Jump Heights and Speed of Contraction. Recoils or windups immediately before throwing, stiking, or kicking can augment the muscle contraction forces to come. In running, the shock abosorption occuring at foot plant contributes to greater force production at push-off (Cavagna, Dusman & Margaria 1968). In sprint running, the quick stretch (0.15 to 0.1 seconds) applied at ground contact during push-off increased force production (Kreighbaum 1996). In weight training, even a one second delay at the bottom of a bench press would result in a 55% loss of force. Slow Oxidative (SO) fibers have the greater elastic properties than Fast Glycolytic (FG) (Kimi & Bosco 1978, Milner 1988) Musculotendionous Receptors Muscle Spindles Muscle spindles are a type of receptor located thoughout the muscle and situated between and parallel to individual muscle fibers. In this position, they are stretched along with adjacent muscle fibers causing a reflex contraction of their associated host muscle, also known as myotatic or stretch reflex. The knee-jerk reflex is an example. Both the stretch reflex and recoil of stored elastice energy used in striking or throwing movements. Muscle spindles also are involved in motor control, providing constant monitoring and regulation of sensorimotor function, enabling appropriate body movement, both reflective and voluntary (Schmidt 1988, Gregor 1989). Golgi Tendon Organs Golgi Tendon Organs (GTO) are receptors located in tendon merging with its muscle (myotendinous junction) and positioned in line with the direction of muscle contraction. These receptors are sensitive to both the passive and active pull of muscle tendon (Gregor, 1989). The GTO inhibits the contraction of the associated muscle (autogenic inhibition) and excites the antagonistic muscle group. Also see Golgi Tendon Organs Question/Answer. Fibers Types   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 size 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 contractions 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 Also see motor unit recruitment.