Dominic Stone
Muscle contraction is the activation of tension-generating sites within muscle cells. Muscle contractions can be categorised into skeletal, smooth, and cardiac muscles. Skeletal muscle contractions are neurogenic, requiring input from motor neurons, while smooth and cardiac muscles are myogenic, meaning they are initiated by the muscle cells themselves. Muscle contractions can be further classified into isometric, eccentric, and isotonic contractions, with the latter having two subtypes: concentric and eccentric. The fixator muscle plays a crucial role in stabilising the origin of the prime mover or agonist during a given body movement. For example, during a bicep curl, the shoulder muscles act as fixators, maintaining a fixed posture and preventing unwanted movements, thus stabilising the shoulder joint for the biceps to contract and lift weights.
| Characteristics | Values |
|---|---|
| Type | Muscle contraction |
| Definition | The activation of tension-generating sites within muscle cells |
| Variables | Length and tension |
| Muscle subtypes | Striated muscle fibers and smooth muscle fibers |
| Striated muscle fibers | Contain actin and myosin filaments that power contraction |
| Skeletal muscle contractions | Neurogenic, requiring synaptic input from motor neurons |
| Smooth and cardiac muscles | Myogenic, initiated by the smooth or heart muscle cells themselves |
| Muscle contraction types | Isometric, eccentric, isotonic, passive stretch |
| Fixator muscle | Stabilizes the origin of the primary mover, or agonist, during a given body movement |
| Muscle co-contraction | Highest number of studies providing supporting evidence for a stabilizing role |
| Muscle mechanics characteristics | Joint stiffness, moment arm, local and global muscles |
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What You'll Learn
Fixator muscles stabilise the origin point of the agonist
The fixator in a movement is the muscle that stabilises the origin of the agonist and the joint that the origin spans, in order to help the agonist function most effectively. The prime mover, or agonist, is the principal muscle involved in a movement. The fixator muscle stabilises the bone that is the attachment for the prime mover's origin.
For example, when extending the elbow against gravity, such as in a push-up, the tricep is the agonist and the bicep is the antagonist. The brachioradialis and brachialis are synergist muscles that assist the biceps and stabilise the elbow joint. The fixator muscle in this case stabilises the origin of the tricep and the joint that the origin spans.
In the case of knee flexion, the hamstrings are the agonists and the quadriceps femoris are the antagonists. In the squat exercise, the quadriceps contract concentrically in the upward phase and eccentrically in the downward phase. The fixator muscle in this case stabilises the origin of the hamstrings and the joint that the origin spans.
The biceps brachii is the prime mover in forearm flexion, such as when lifting a cup. The brachialis is a synergist in this action. A synergist can also be a fixator that stabilises the origin of the agonist.
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Muscle co-contraction increases joint stiffness and stability
Muscle co-contraction can be quantified using an EMG-based Co-Contraction Index (CCI), which provides an estimate of joint stiffness. This is particularly useful for understanding the impact of joint stiffness on dynamic movements such as walking or running.
The process of muscle contraction is complex and involves the interaction of thin and thick filaments within the muscle fibers. These filaments are composed of proteins, such as actin and myosin, which form cross-bridges when bound together. This cross-bridge cycling is fueled by ATP, which provides the energy for the contraction process.
There are different types of muscle contractions, including concentric and eccentric contractions. Concentric contractions occur when muscle myofilaments of myosin and actin slide past each other, pulling the Z-lines together and resulting in a positive work output. Eccentric contractions, on the other hand, occur when the myofilaments slide past each other in the opposite direction, resulting in negative work. An example of an eccentric contraction is lowering a heavy object; the muscle lengthens as it actively manages the weight.
Muscle co-contraction can increase joint stiffness, which may be beneficial for individuals with impaired movement. However, it is important to consider the trade-off between increased stability and the higher energetic cost associated with muscle co-contraction.
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Isometric contractions occur when muscles are held at a set length
Muscle contractions can be described based on two variables: force and length. Isometric contractions occur when there is a change in muscle tension without a change in muscle length. In other words, isometric contractions happen when muscles are actively held at a set length. This means that instead of lengthening or shortening, the muscle is held in a position that requires a specific length once activated. An example of this type of contraction is carrying something in your arms in front of you. You are not trying to raise or lower the object but keep it at a steady position.
Isometric contractions are performed without joint motion, and the muscle length remains constant. They are often used when there is joint damage, and joint motion is likely to increase pain. They are also used during early strengthening when the limb is supported or to promote circulation through alternating contractions. Isometric contractions are also required for some functional activities.
In contrast, isotonic contractions are performed with joint motion, and the muscle length changes. During isotonic contractions, the muscle tension remains constant despite a change in muscle length. This occurs when a muscle's force of contraction matches the total load on the muscle. For example, isotonic contractions are seen during activities such as walking, running, or squatting.
Concentric contractions occur when there is sufficient muscle tension to overcome the load, and the muscle contracts and shortens. This occurs when the force generated by the muscle exceeds the load opposing its contraction. During a concentric contraction, a muscle is stimulated to contract according to the sliding filament theory. This occurs throughout the length of the muscle, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint. An example of a concentric muscle contraction is picking up a heavy box.
Eccentric contractions occur when the muscle controls movement against resistance (including gravity) by lengthening or slowing the movement. Eccentric contractions happen when you lower something heavy. Your muscle has to remain tight to manage the weight, but it lengthens to shift the weight into a different position.
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Skeletal muscle contractions are neurogenic
The process begins with an action potential in the motor neuron, which causes depolarization in the myocyte membrane. This depolarization spreads through the transverse (T) tubules, causing a conformational change in the dihydropyridine receptors. As a result, nearby ryanodine receptors on the sarcoplasmic reticulum (SR) open, releasing calcium ions. The calcium ions bind to troponin C, leading to a conformational change that shifts tropomyosin and allows the myosin heads to attach to the actin filaments, forming cross-bridges.
The cross-bridge cycling is fuelled by adenosine triphosphate (ATP), which binds to an ATP-binding domain on the myosin head. ATP is then hydrolyzed into adenosine diphosphate (ADP) and phosphate (P), causing the myosin heads to change conformation and move towards the positive end of the actin filament. The phosphate is released, and the ADP-bound myosin binds to a new site on the actin filament. As ADP is released, the myosin returns to its original position, pulling on the actin filament and causing the sarcomere and muscle fibre to contract.
These cycles continue until calcium levels in the myocyte decrease, leading to tropomyosin covering the actin filament's myosin-binding sites. At this point, myosin dissociates from actin, breaking the cross-bridge and allowing the muscle to relax.
Skeletal muscle contractions differ from those of smooth and cardiac muscles, which are myogenic and initiated by the muscle cells themselves rather than external nerve stimulation. Skeletal muscle contractions are under voluntary control and are responsible for locomotor activity, breathing, swallowing, and eye movements.
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Smooth and cardiac muscles are myogenic
Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and are responsible for voluntary movements such as walking, talking, and lifting objects.
Unlike skeletal muscles, the contractions of smooth and cardiac muscles are myogenic. This means that they are initiated by the smooth or heart muscle cells themselves, instead of being stimulated by an external event such as nerve stimulation.
Smooth muscles are generally involuntary and control many of our internal processes that we do not consciously think about. For example, when we eat, it is the smooth muscles in our gut that help push the food along to be digested. Smooth muscles are also responsible for regulating blood flow by contracting blood vessels. Smooth muscle fibres do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body.
Cardiac muscle, also called myocardium, is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. The rhythmic contraction of cardiac muscle occurs automatically and in a highly regulated manner, ensuring the continuous and efficient circulation of blood.
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