Dominic Stone
Muscles are organs that contain cells that can contract, allowing them to generate force and movement. The force generated by a muscle is influenced by various factors, including the length of the muscle, its shortening velocity, and the number of muscle fibres contracting. When a muscle contracts, myosin heads attach to actin to form cross-bridges, resulting in the thin filaments sliding over the thick filaments and creating tension. This tension can be increased through multiple motor unit summation and multiple-wave summation, leading to a greater force of contraction. The force-velocity relationship in muscle describes how the speed of muscle length changes influences the force of contraction and the resulting power output. Additionally, the length-tension relationship illustrates how changes in muscle fibre length affect the tensions or forces produced. Understanding these mechanisms is crucial for optimising performance and adapting to different activity patterns.
| Characteristics | Values |
|---|---|
| Nature of muscle force | Force is generated by myosin crossbridges interacting with actin |
| Muscle contraction | The activation of tension-generating sites within muscle cells |
| Muscle tension | The force exerted by a muscle on an object |
| Load | The force exerted by an object on the muscle |
| Isometric contraction | Muscle tension changes without any corresponding changes in muscle length |
| Isotonic contraction | Muscle length changes while muscle tension remains the same |
| Excitation-contraction coupling | The muscle fiber action potential always precedes the increase in intracellular Ca2+, which always precedes muscle contraction |
| Muscle fiber action potential | The increase in intracellular Ca2+ from sarcoplasmic reticulum release produces a single muscle contraction known as a twitch |
| Length-tension relationship | The strength of an isometric contraction is related to the length of the muscle at which the contraction occurs |
| Active tension | The tension developed from the cross-bridge cycle and is proportional to the actual number of cross-bridges |
| Passive tension | The tension that results from increasing the muscle length |
| Force-velocity relationship | The velocity of muscle shortening as a function of afterload, which is the force against which the muscle contracts |
| Motor units | A motor unit consists of a single motor neuron and all the muscle fibers it innervates |
| Force of contraction | Increased by multiple motor unit summation and multiple-wave summation |
| Skeletal muscle force | Dependent on the number of cross-bridges in the strongly bound, high-force state |
| Peak power | Obtained at loads considerably below 50% of Po and is correlated with the percentage of fast-twitch fibers |
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What You'll Learn
Muscle contraction
The physiological concept of muscle contraction is based on two variables: length and tension. The length-tension relationship illustrates the forces produced from the cross-bridge cycle as a result of changes in muscle fibre length. The tension is determined by altering the resting length of a muscle that has already undergone isometric contraction. This resting length, also known as preload, is the passive precontraction from isometric contraction. Passive tension results from increasing the muscle length and can be thought of as the tension produced in an elastic rubber band as it stretches. Active tension, on the other hand, is the tension developed from the cross-bridge cycle and is proportional to the actual number of cross-bridges. This tension is highest when there is an optimum overlap between myosin and actin, resulting in a maximal number of cross-bridges.
The complex process leading to muscle contraction is called excitation-contraction coupling, which involves a sequence of events that always exhibit a temporal relationship. The muscle fibre action potential always precedes the increase in intracellular Ca2+, which always precedes muscle contraction. A single action potential leading to an increased intracellular Ca2+ from the sarcoplasmic reticulum produces a single muscle contraction known as a twitch. If an already active muscle fibre becomes stimulated again, there is insufficient time for the sarcoplasmic reticulum to reaccumulate Ca2+. Consequently, intracellular Ca2+ remains high, and the force of the second stimulus becomes an additive effect to the remainder of the first stimulus, resulting in additional force.
The sliding filament theory describes a process used by muscles to contract. It involves the thin and thick filaments that form myofibrils, the basic functional organelles in the skeletal muscle system. The thin filaments are predominantly composed of actin, while the thick filaments consist of myosin. The theory explains that muscle contraction occurs when the protein filaments within each skeletal muscle fibre slide past each other.
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Force-velocity relationship
The force-velocity relationship is a fundamental property of muscle, describing the fact that a muscle's force-generating capacity depends on the contraction velocity and on whether it is an eccentric or concentric contraction. In other words, the force-velocity relationship refers to the velocity of muscle shortening as a function of afterload, or the force against which the muscle contracts.
The force-velocity relationship was first measured by A.V. Hill in 1938, who found that it could be described by a hyperbolic equation. The hyperbolic relationship between muscle force (F) and velocity (v) for concentric (shortening) contractions can be expressed as:
P + a) V = b(Po − P)
Where vmax > 0 is the maximum shortening velocity, Fim is the maximum isometric force, and F/Fim is the normalised muscle force.
The force-velocity relationship has important implications for muscle performance, as it can be used to inform the optimisation of human performance. For example, the maximum shortening velocity depends on temperature and is determined primarily by the fibre type. FT fibres are able to shorten three to four times faster than ST fibres. For human muscles, maximum shortening velocities are two and six fibre lengths per second for slow and fast-twitch muscles, respectively.
The force-velocity relationship also has implications for the energy output rate in a contracting muscle, which consists of work output (PV) and heat production (aV), depending on the amount of external load (P). The force-velocity curve becomes more linear with increasing temperature, and the maximum stress (Fim divided by muscle cross-sectional area) is almost independent of animal size and fibre type.
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Muscle fibre action potential
The process of muscle fibre action potential involves the transmission of electrical signals along the sarcolemma, which is the muscle cell membrane. This transmission is facilitated by the presence of T-tubules, which are deep invaginations within the muscle fibre. These T-tubules carry the action potential into the interior of the cell, triggering the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). The SR is a specialised organelle that stores high concentrations of calcium ions in skeletal muscle.
The release of Ca++ ions from the SR initiates the contraction of the muscle fibre by its contractile units, called sarcomeres. The Ca++ ions interact with shielding proteins, forcing them to move aside and exposing the actin-binding sites for attachment by myosin heads. This process is known as excitation-contraction coupling, where the excitation of the muscle fibre through the action potential is coupled with the release of calcium ions for contraction.
The sequence of events in muscle fibre action potential is as follows: firstly, a signal in the form of the neurotransmitter acetylcholine (ACh) is released from the motor neuron innervating the muscle fibre. This ACh diffuses across the synaptic cleft and binds to ACh receptors on the motor end-plate of the sarcolemma. As a result, positively charged sodium ions (Na+) enter the muscle fibre, causing it to depolarize and triggering an action potential. This action potential then spreads along the entire membrane, including the T-tubules, leading to the release of Ca++ ions and subsequent muscle contraction.
The duration of the action potential is shorter than the twitch duration, allowing the muscle fibre to be activated again before muscle relaxation occurs. If an already active muscle fibre is stimulated again, the additive effect of the stimuli results in additional force. This process is important for generating force and movement in skeletal muscle, which works in conjunction with the bones of the skeleton.
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Muscle relaxation
PMR is a deep relaxation technique that has been used to control stress and anxiety, relieve insomnia, and reduce symptoms of certain types of chronic pain. It involves tightening and relaxing specific muscle groups one at a time, in a sequential manner, to release tension and promote mental calmness. The technique can be performed in a seated or lying position and typically takes 10 to 20 minutes per day. It is recommended to start with the lower extremities and end with the face, abdomen, and chest.
During PMR, individuals are instructed to inhale and contract a specific muscle group for 5 to 10 seconds, and then exhale and release the tension in that group. The individual then focuses on the changes in feeling as the muscle group relaxes for 10 to 20 seconds before moving on to the next group. This process is repeated for various muscle groups, including the upper thighs, buttocks, calf muscles, hands, arms, abdominal muscles, and chest.
The technique helps individuals recognise and manage the physical effects of stress and tension. It has been found to reduce symptoms of anxiety, depression, and stress, while improving overall well-being and quality of life. PMR can be easily learned and practised by almost anyone, even when feeling calm, to familiarise themselves with the method.
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Motor units
A motor unit is a fundamental structure in the nervous system that generates skeletal muscle contraction, coordinates motor behaviours, and forms the basis of motor control. It is defined as a single alpha motor neuron and all of the muscle fibres it innervates. The number of muscle fibres that a single motor unit innervates (innervation ratio) can vary greatly depending on the function of the muscle.
The activation of a motor unit requires the generation of an action potential at the initial segment region of its innervating motor neuron. This action potential is then propagated down its efferent axon to neuromuscular junctions, resulting in the activation and contraction of all the individual muscle fibres that make up the motor unit.
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Frequently asked questions
Muscle contraction is the activation of tension-generating sites within muscle cells. It does not necessarily mean muscle shortening, as muscle tension can be produced without changes in muscle length.
The force-velocity relationship in muscles refers to the velocity of muscle shortening as a function of afterload, or the force against which the muscle contracts. As the afterload increases, the shortening velocity decreases.
Muscles operate with the greatest active tension when close to their ideal length, often their resting length. When stretched or shortened beyond this, the maximum active tension generated decreases.
The peak force and power output of a muscle depend on several factors, including muscle and fiber size and length, architecture, fiber type, number of cross-bridges in parallel, force per cross-bridge, and the force-frequency relationship.
