Chapter 11: The Muscular System The Motors of the Body

Muscle The distinguishing characteristic of muscle is its ability to actively shorten and produce tension.

Behavioral Properties of the Musculotendinous Unit The characteristic behavioral properties of muscle are: extensibility, elasticity, irritability, and the ability to develop tension.

Behavioral Properties of the Musculotendinous Unit Extensibility - the ability to be stretched or to increase in length. Elasticity - the ability to return to normal length after extension or contraction.

Behavioral Properties of the Musculotendinous Unit Parallel elastic component - muscle membranes that provide resistive tension when a muscle is passively stretched. Series elastic component - tendons that act as a spring to store elastic energy when an active muscle is stretched.

Series and parallel elastic elements in muscle. Resting muscle contains elastic elements in series with the contractile elements (sarcomeres) and in parallel with them. During an isometric contraction, the muscle does not change length, but sarcomeres shorten, stretching the series elastic elements. During isotonic contraction, the contractile elements shorten, stretching the series elastic elements, before they develop tension to lift the load. Muscle begins to shorten when contractile elements shorten further.

Behavioral Properties of the Musculotendinous Unit Both the PEC and SEC have a viscous property that enables muscle to stretch and recoil in a time-dependent fashion.

Behavioral Properties of the Musculotendinous Unit When a static stretch of a muscle group such as the hamstrings is maintained over a period of time, the muscle progressively lengthens, increasing joint range of motion.

Behavioral Properties of the Musculotendinous Unit Likewise, after a muscle group has been stretched, it does not recoil to resting length immediately, but shortens gradually over a period of time. This viscoelastic response of muscle is independent of gender.

Behavioral Properties of the Musculotendinous Unit Irritability - the ability to respond to a stimulus. Stimuli affecting muscles are either electrochemical, such as an action potential from the attaching nerve, or mechanical, such as an external blow to a portion of a muscle.

Behavioral Properties of the Musculotendinous Unit If the stimulus is of sufficient magnitude, muscle responds by developing tension. Contractility - the ability of a muscle to shorten in length.

Muscle Contraction When a muscle contracts, it pulls with equal force on each attachment.

Muscle Contraction A muscle’s line of pull refers to the direction of the resultant force produced at an attachment.

Skeletal Muscle Function The magnitude of the torque generated is the product of the force developed by the muscle and the perpendicular distance of the line of action of that force from the center of rotation at the joint.

Skeletal Muscle Function In keeping with the laws of vector addition, the net torque present at that joint determines the direction of any resulting movement.

Skeletal Muscle Function The weight of the attached body segment, external forces acting on the body, and tension in any muscle crossing a joint can all generate torques at that joint.

Skeletal Muscle Function Concentric muscle action - when a muscle shortens under tension. Eccentric muscle action - when a muscle lengthens under tension. Isometric muscle action - when a muscle produces tension, but there is not movement.

Skeletal Muscle Function Agonist - a muscle that causes movement. The prime mover. Antagonist - a muscle that resists movement. Synergist - a muscle that assists the agonist in performing a movement.

Skeletal Muscle Function Stabilizer, Neutralizer, Fixator - role played by a muscle acting to stabilize a body part against some other force or eliminate an unwanted action produced by an agonist. Two joint muscles - muscles which cross two joints.

Skeletal Muscle Function They can fail to produce force when slack (active insufficiency) and can restrict range of motion when fully stretched (passive insufficiency).

Functional Organization of Skeletal Muscle: Muscle fibers - skeletal muscle fibers grow in length and diameter from birth to adulthood, with a fivefold increase in fiber diameter during this period. Fiber diameter can also be increased by resistance training.

Functional Organization of Skeletal Muscle: The number of muscle fibers present is genetically determined and varies from person to person.

Functional Organization of Skeletal Muscle: Summation - progressively additive effect of repeated stimuli. Tetanus - state of muscle producing sustained maximal tension resulting from repetitive stimulation.

Functional Organization of Skeletal Muscle: Motor units - a single motor neuron and all fibers it innervates. Fiber types. Recruitment of motor units - slow twitch motor units always produce tension first, whether the final movement is slow or fast.

Fiber architecture: Parallel fiber arrangement - fibers are alongside each other. Pennate fiber arrangement - arrow. The tibialis posterior, rectus femoris, and deltoid muscles are pennate.

Parallel Muscle Fibers

Pennate Fibers

Fiber architecture: When tension is developed in a parallel-fiber muscle, any shortening of the muscle is primarily the result of the shortening of the fibers.

Fiber architecture: When the fibers of a pennate muscle shorten, the rotate about their tendon attachments, progressively increasing the angle of pennation.

Fiber architecture: Pennate fiber arrangements promote muscle force production and parallel fiber arrangements facilitate muscle shortening.

Fiber architecture: Although pennation reduces the effective force generated at a given level of fiber tension, this arrangement allows the packing of more fibers than the amount that can be packed into a longitudinal muscle occupying equal space.

Fiber architecture: Because pennate muscles contain more fibers per unit of muscle volume, they can generate more force than parallel fibered muscles of the same size.

Mechanical Factors Affecting Muscular Force The magnitude of the force generated by muscle is also related to: the velocity of muscle shortening, the length of the muscle when it is stimulated, and the period of time since the muscle received a stimulus.

Mechanical Factors Affecting Muscular Force The relationship between the concentric force exerted by a muscle and the velocity at which the muscle is capable of shortening is inverse.

Mechanical Factors Affecting Muscular Force When a muscle develops concentric tension against a high load, the velocity of muscle shortening must be relatively slow.

Mechanical Factors Affecting Muscular Force When resistance is low, the velocity of shortening can be relatively fast.

Mechanical Factors Affecting Muscular Force The stronger a muscle, the greater the magnitude of its isometric maximum on the force-velocity curve.

Mechanical Factors Affecting Muscular Force Eccentric strength training involves the use of resistance that is greater than the athlete's maximum isometric force generating capabilities.

Mechanical Factors Affecting Muscular Force Eccentric training is also associated with increased muscle soreness.

Force-Length Relationship: In single muscle fibers and isolated muscle preparations, force generation is at its peak when the muscle is at normal resting length (neither stretched nor contracted).

Force-Length relationship: When the length of the muscle increases or decreases beyond resting length, the maximum force the muscle can produce decreases following the form of a bell-shaped curve.

Length-Tension Relationship (sarcomere only)

Length-Tension Relationship (sarcomere & elastic component) Rapid stretch of muscle increases force during ensuing concentric phase SSC Elastic component Stretch reflex

Force-Length relationship: Within the human body, however, force generation capability increases when the muscle is slightly stretched.

Force-Length relationship: Parallel-fiber muscles produce maximum tensions at just over resting length, and pennate fiber muscles generate maximum tensions at between 120% and 130% of resting length.

Force-Length relationship: This phenomenon is due to the contribution of the elastic components of muscle (primarily the SEC), which add to the tension present in the muscle when the muscle is stretched.

Force-Length relationship: When a muscle is actively stretched, the SEC causes an elastic recoil effect, and the stretch reflex simultaneously initiates the development of tension in the muscle. Thus, a stretch promotes subsequent forceful shortening of the muscle.

Force-Length relationship: This pattern of eccentric contraction, followed immediately by concentric contraction, is known as the stretch-shortening cycle.

Force-Time relationship: When a muscle is stimulated, a brief period of time elapses before the muscle begins to develop tension. Referred to as electromechanical delay.

Force-Time relationship: Muscular strength is most commonly measured as the amount of torque a muscle group can generate at a joint.

Force-Time relationship: The tension-generating capability of a muscle is related to its cross-sectional area and training state.

Force-Time relationship: With both concentric and eccentric strength training, gains in strength over at least the first 12 weeks appear to be related to factors such as improved innervation of the trained muscle rather than to the increase in its cross-sectional area.

Force-Time relationship: Muscular power - the product of force and velocity. Maximum power occurs at approximately one-third of maximum velocity and at approximately one-third of maximum concentric force.

Force-Time relationship: Muscular endurance - the ability of the muscle to exert tension over a period of time.

Muscle Force Effect of muscle temperature - as body temperature elevates, the speeds of nerve and muscle functions increase.