Biomechanics of the skeletal muscles

Objectives  Identify the basic behavioral properties of the musculotendinous unit.  Explain the relationships of fiber types and fiber architecture to muscle function.  Explain how skeletal muscles function to produce coordinated movement of the human body.  Discuss the effects of the force-velocity and length- tension relationships and electromechanical delay on muscle function.  Discuss the concepts of strength, power, and endurance from a biomechanical perspective.

Structural Organization of Skeletal Muscle Human body has approx. 434 muscles –40-45% of total body weight in adults –75 muscle pairs responsible for bodily movements and posture  Muscle Fibers  Motor Units  Fiber Types

Muscle Fibers  During contraction, cross-bridges form Sarcoplasmic Reticulum –Transverse Tubules Endomysium Perimysium –Fascicles Epimysium  Variation of length and diameter within muscles seen in adults.

Muscle Fibers Contain:  sarcolemma  sarcoplasm  nuclei  mitochondria  myofibrils  myofilaments Sarcomere Z lines M line A band myosin filaments I band actin filaments H zone

Motor Units  single motor neuron and all fibers it innervates  considered the functional unit of the neuromuscular system

Behavioral Properties of the Musculotendinous Unit Behavioral properties of muscle tissue: –Extensibility –Elasticity –Irritability –Ability to develop tension Behavioral properties common to all muscle: –Cardiac, smooth, skeletal

Extensibility and Elasticity Extensibility:ability to be stretched or to increase in length Elasticity:ability to return to normal resting length following a stretch Two components: –Parallel elastic component (PEC) –Series elastic component (SEC) Contractile component Visoelastic

Irritability and the Ability to Develop Tension Irritability –The ability to respond to electrical or mechanical stimulus. –Response is the development of tension. Not necessarily a contraction

Fiber Types Fast Twitch (FT) –Type IIA –Type IIB Slow Twitch (ST) –Type I Peak tension reached in FT in 1/7 time of ST

Fiber Types Effects of training: –Endurance training can increase ST contraction velocity by 20% –Resistance training can convert FT fibers from Type IIB to Type IIA Affected by: –Age and Obesity

Skeletal Muscle Function Recruitment of motor units Change in length with tension development Roles assumed by muscles Two-joint and multijoint muscles

Recruitment of Motor Units CNS enables matching of speed and magnitude of muscle contraction to requirement of movement. Threshold activation –ST activated first (low threshold) –With an increase in speed, force, and/or duration requirement, higher threshold motor units are activated (FT fibers )

Change in Muscle Length with Tension Development Concentric: Shortening of muscle with tension development Isometric: No change in muscle length with tension development Eccentric: Lengthening of muscle with tension development

Roles Assumed by Muscles  Agonist : Acts to cause movement  Antagonist: Acts to slow/stop a movement  Stabilizer: Acts to stabilize a body part against some other force  Neutralizer: Acts to eliminate an unwanted action produce by an agonist  Agonists and Antagonists are typically positioned on opposite sides of a joint.

Two-joint and Multijoint Muscles Movement effectiveness depends on: –Location and orientation of muscle’s attachment relative to the joint –Tightness or laxity of musculotendinous unit –Actions of other muscles crossing the joint Disadvantages: active insufficiency: failure to produce force when slack passive insufficiency: restriction of joint range of motion when fully stretched

Factors Affecting Muscular Force Generation Force-Velocity Relationship Length-Tension Relationship Electromechanical Delay Stretch-Shortening Cycle

Force-Velocity Relationship The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity. Velocity Force (Low resistance, high contraction velocity)

Force-Velocity Relationship The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum. Velocity Force isometric maximum

Length-Tension Relationship The length-tension relationship: Tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers and the passive tension provided by the tendons and membranes.

Electromechanical Delay

Stretch-Shortening Cycle

Muscular Strength, Power, and Endurance Muscular Strength Muscular Power Muscular Endurance Muscular Fatigue Effect of Muscle Temperature

Muscular Fatigue

Velocit y Force Normal body temperature Elevated body temperature

Although muscle injuries pose no real emergency,first aiders have a lot of opportunities to take care of them.

What to look for?  A sharp pain.  Severe weakness and loss of function of the injured part.  A “snap” sound heard.  Stiffness and pain while moving muscle.

Muscle is the only biological tissue capable of developing tension. Resulting actions can be concentric, eccentric, isometric for muscle shortening, lengthening or remaining unchanged in length Force production is the combination of many relationships (ex: force-velocity) Specific activity performance is related power, endurance, and strength muscle injuries