48 Control of Muscle Tension

Force of Muscle Contraction

The force a muscle generates is dependent on its length and shortening velocity.

Muscle Force Generation

The force a muscle generates is dependent on the length of the muscle and its shortening velocity. These two fundamental properties limit many key biomechanical properties, including running speed, strength, and jumping distance.

Force-Length Relationship

Due to the presence of titin, muscles are innately elastic. Skeletal muscles are attached to bones via tendons that maintain the muscle under a constant level of stretch called the resting length. If this attachment was removed, for example if the bicep was detached from the scapula or radius, the muscle would shorten in length.

Muscles exist in this state to optimize the force produced during contraction, which is modulated by the interlaced myofilaments of the sarcomere. When a sarcomere contracts, myosin heads attach to actin to form cross-bridges. Then, the thin filaments slide over the thick filaments as the heads pull the actin. This results in sarcomere shortening, creating the tension of the muscle contraction. If a sarcomere is stretched too far, there will be insufficient overlap of the myofilaments and the less force will be produced. If the muscle is over-contracted, the potential for further contraction is reduced, which in turn reduces the amount of force produced.

Simply put,  the tension generated in skeletal muscle is a function of the magnitude of overlap between actin and myosin myofilaments.

In mammals, there is a strong overlap between the optimum and actual resting length of sarcomeres.

Force-Velocity Relationship

The force-velocity relationship in muscle relates the speed at which a muscle changes length with the force of this contraction and the resultant power output (force x velocity = power). The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force. However, cross-bridge formation is not immediate, so if myofilaments slide over each other at a faster rate the ability to form cross bridges and resultant force are both reduced.

At maximum velocity no cross-bridges can form, so no force is generated, resulting in the production of zero power (right edge of graph). The reverse is true for stretching of muscle. Although the force of the muscle is increased, there is no velocity of contraction and zero power is generated (left edge of graph). Maximum power is generated at approximately one-third of maximum shortening velocity.

Velocity and Duration of Muscle Contraction

The shortening velocity affects the amount of force generated by a muscle.

Muscle Contraction Velocity

Skeletal muscle contractions can be broadly separated into twitch and tetanic contractions. In a twitch contraction, a short burst of stimulation causes the muscle to contract, but the duration is so brief that the muscle begins relaxing before reaching peak force. If another contraction occurs before complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous twitch, a phenomenon called summation. If the stimulation is long enough, the muscle reaches peak force and plateaus at this level, resulting in a tetanic contraction.

Force-Velocity Relationship

The force-velocity relationship in muscle relates the speed at which a muscle changes length to the force of this contraction and the resultant power output (force x velocity = power). The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force. However, cross-bridge formation is not immediate and if myofilaments slide over each other at a faster rate, their ability to form cross-bridges and subsequent force are both reduced.

At a maximum velocity no cross-bridges can form so no force is generated, resulting in the production of zero power (right edge of graph). The reverse is true for stretching of muscle; although the force of the muscle is increased, there is no velocity of contraction and zero power is generated (left edge of graph). Maximum power is generated at approximately one-third of maximum shortening velocity.

Twitch contractions are short in duration. Though they have high velocity, they begin resting before reaching peak force. Tetanic contractions, which are long in duration, reach peak force and plateau.

Motor Units

The motor unit is the functional unit of muscle contraction and includes the motor nerve fiber and the muscle fibers it innervates.

Motor Units

A motor unit consists of the motor neuron and the grouping of muscle fibers innervated by the neuron. The number of muscle fibers within a motor unit varies, and is a function of the muscle’s ability for accurate and refined motion. Precision is inversely proportional to the size of the motor unit. Thus, small motor units can exercise greater precision of movement compared to larger motor units. For instance, thigh muscles, responsible for large powerful movements, can have a thousand fibers in each unit, while eye muscles, requiring small precise movements, might only have ten. Groups of motor units are innervated to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are considered a motor pool.

Motor Pools and Force

There are often multiple sizes of motor unit within a motor pool as a means of modulating the precision and force produced by a single muscle. For example, a small motor unit in the biceps can be activated for small precise movements, while a larger motor unit can be activated to facilitate more forceful actions.

These multiple motor units of different sizes within a motor pool allow for very fine control of force either spatially or temporally.

• Spatial: The recruitment of more or larger motor units to increase force.
• Temporal: The repeated stimulation of a motor unit so that the force of contraction generated is summed.

Muscle Tone

Muscle tone is a measure of a muscle’s resistance to stretching while in a passive resting state.

Even when at rest, muscle fibers are at least partially contracted, possessing a small degree of tension which is termed muscle tone or tonus. Muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons.

This influence leads to the generation of reflexes in the spinal cord, such as the immediately obvious knee jerk reaction but also including key functions such as the posture maintenance and proper digestive system function..

Control of Muscle Tone

The main regulator of muscle tone is the muscle spindle, a small sensory unit that is closely associated with and lies parallel to a muscle. Connecting to the endomysium of a muscle fiber, muscle spindles are composed of nuclear bag fibers and nuclear chain fibers. Both are similar to muscle fibers in that they contain actin and myosin myofilaments that allow them to stretch with the muscle. However, unlike skeletal muscle fibers where the nuclei are spread out and located at the periphery of the cell, in nuclear bag and nuclear chain fibers the nuclei are located in a central region which is enlarged in nuclear bag fibers.

Both cells of the muscle spindle contain sensory neurons. When stretched, muscle spindles become activated, triggering impulses to the spinal cord that can generate an immediate reflex. Spindles can also trigger impulses to the cerebral cortex providing information about the degree of stretch within the muscle.

To maintain tone, spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles. If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction. With this contraction, the spindle is no longer stretched.

A similar system is found in the tendons attaching muscle to bone. Distinct stretch receptors called golgi tendon organs assess the level of stretch within the tendon. The sensitivity of the golgi tendon organ is significantly less than that of the spindle, so it is thought they exist to prevent damage rather than control muscle tone.

Smooth and Cardiac Muscles

Smooth and cardiac muscles do not have specialized muscle spindles. Tone is maintained through autonomous feedback from the muscle fibers, neurons, and associated tissues.

Types of Muscle Contractions: Isotonic and Isometric

Muscle contractions are defined by changes in the length of the muscle during contraction.

A muscle fiber generates tension through actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same. Although the term contraction implies shortening, when referring to the muscular system, it means the generation of tension within a muscle fiber. Several types of muscle contractions occur and are defined by the changes in the length of the muscle during contraction.

Isotonic Contractions

Isotonic contractions maintain constant tension in the muscle as the muscle changes length. Isotonic muscle contractions can be either concentric or eccentric.

Concentric Contractions

A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance. For example, when lifting a heavy weight, a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder. Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.

Eccentric Contractions

An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated. Eccentric contractions can be both voluntary and involuntary. For example, a voluntary eccentric contraction would be the controlled lowering of the heavy weight raised during the above concentric contraction. An involuntary eccentric contraction may occur when a weight is too great for a muscle to bear and so it is slowly lowered while under tension. Cross-bridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.

Isometric Contractions

In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip. Using the above example, the muscle contraction required to grip but not move a heavy object prior to lifting would be isometric. Isometric contractions are frequently used to maintain posture.

Isometric contractions are sometimes described as yielding or overcoming.

Yielding

A yielding contraction occurs when a muscle contraction is opposed by resistance. For example, when holding a heavy weight steady, neither raising nor lowering it.

Overcoming

An overcoming contraction occurs when a muscle contraction is opposed by an immovable object, such as the contraction generated in the muscles when pushing against a wall.

In both instances, cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.