Dominic Stone
The human body is made up of over 600 muscles, which are responsible for everything from helping us move our bodies to keeping us alive. Muscle actions can be voluntary, which are actions we choose to perform, or involuntary, which happen automatically. Muscle actions can be further classified into concentric, eccentric, and isometric actions. Concentric actions refer to the shortening of a muscle, eccentric actions refer to lengthening, and isometric actions refer to a muscle staying the same length.
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What You'll Learn
Muscle contraction
Isotonic action is most likely to occur under static conditions, and there is a rise time of tension buildup, an intermediate phase of maximal tension, and a final decay time of tension decrease. An example of an isotonic contraction is when the tension in the muscle remains constant despite a change in muscle length, such as when the force of contraction matches the total load on the muscle.
If the muscle length changes while muscle tension remains the same, then the muscle contraction is concentric or eccentric. A concentric contraction occurs when muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. An eccentric contraction occurs when the muscle lengthens as it contracts. An example of these two contractions is lifting a dumbbell while working out. When you pick the dumbbell up, your bicep muscle tightens and contracts to lift the weight. When you lower the weight down, your bicep muscle remains contracted, but it lengthens.
The complex process leading to muscle contraction is called excitation-contraction coupling, and it begins when an action potential causes depolarization in the myocyte membrane. The depolarization is spread via the transverse (T) tubules, which help spread depolarization signals to the entire muscle fibre. This causes a conformational change, allowing the myosin heads to attach to the actin filaments, creating what is known as a cross-bridge. Cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head.
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Muscle relaxation
> Concentric – Action in which the proximal and distal muscle attachments move towards one another.
> Eccentric – Action in which the proximal and distal muscle attachments move away from one another.
> Isometric – Action in which the proximal and distal muscle attachments do not move relative to one another.
Isometric means 'the same length', which only occurs when a muscle is relaxed. However, it is not muscle length but joint angle that remains constant. Contraction means 'shortening', so isometric contraction involves internal movement processes that shorten muscle fibres. This occurs when there is no external movement or change in joint angle or distance between attachments.
Progressive muscle relaxation (PMR) is a technique that can be used to reduce stress and promote relaxation. Developed by Dr Edmund Jacobson in the 1920s, it involves tensing and relaxing different muscle groups in a certain order. This helps to build awareness of the sensations of tension and deep relaxation. The technique can be performed sitting or lying down, and it is recommended to wear comfortable clothing and be in a quiet, distraction-free space.
PMR is a two-step process: first, tension is created in a specific muscle group, and the individual focuses on what that tension feels like. Then, the tension is released, and the individual notices the difference in sensation as the muscle group relaxes. This process is repeated for different muscle groups, working up the body. It is important not to strain or overly tense the muscles, and to breathe normally while performing PMR. The rhythmic pattern of breathing and movement can enhance the feeling of relaxation.
PMR has been used to treat a variety of conditions, including insomnia, anxiety, chronic pain, headaches, high blood pressure, and digestive issues. It is a simple and effective technique that can be learned by almost anyone.
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Muscle attachments
Direct attachments occur when a muscle connects directly to a bone. Tendons, which are cord-like connective tissues, play a vital role in indirect attachments, allowing muscles to connect to bones indirectly. Tendons serve as a means to concentrate the pull of the muscle on a small area of the bone. This mechanism enables the transmission of tension generated by the muscle's contractile component to the associated bones, facilitating joint movement.
The terms "origin" and "insertion" are used to describe muscle attachments in relation to their function during movement. An origin is a muscle attachment site on a bone that remains stationary during a particular action, while an insertion refers to the attachment on a bone that moves during that action. For example, the triceps brachii has three points of origin (two on the humerus and one on the scapula) and one insertion point on the ulna.
The line of pull of a muscle is a useful concept for predicting its action. By drawing an imaginary double-headed arrow pointing toward the centre of the muscle, with its base at each attachment, one can visualise the movement of the interposed joint. This visualisation technique helps in understanding muscle actions and promotes better retention of knowledge compared to rote memorisation.
Additionally, muscles can be categorised into prime movers (agonists) and antagonists, depending on their role in a specific movement. Prime movers are the muscles that generate the primary force to drive an action, while antagonists oppose the prime movers by providing resistance or reversing the movement. Synergists assist prime movers, and stabilisers maintain bone immobility when necessary.
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Muscle activation
Muscles can only "contract" or "relax", but scientists prefer to use the term "muscle action" as muscles do not actually contract. Instead, the active component of a muscle, the actin-myosin system, behaves as a sliding filament model where the adjacent actin and myosin muscle filaments slide past one another. This shortens the distance between the two attachments at the ends of a muscle group when a joint angle changes, giving the appearance of contraction.
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that work with bones, tendons, and ligaments to support the body's weight and move it. They attach to bones at two or more places, with the attachment on an immobile bone called an "origin" and the attachment on a bone that moves during an action called an "insertion". Tendons attach skeletal muscles to bones, and some muscle fibres contract quickly using short bursts of energy (fast-twitch muscles), while others move slowly, like back muscles that aid posture.
Cardiac muscle, or myocardium, makes up the middle layers of the heart. Smooth muscle lines the body's hollow organs, like the stomach and intestines, and is controlled by the autonomic nervous system.
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Muscle movement
The human body is home to over 600 muscles, which help us do everything from moving our bodies to breathing and keeping us alive. Skeletal muscles, which are part of the musculoskeletal system, work with bones, tendons, and ligaments to support our weight and move us. Tendons attach skeletal muscles to bones, and these muscles are voluntary, moving when we think about moving.
Muscles can only "contract" or "relax", and scientists prefer to refer to muscle "action" rather than "contraction". This is because the active component of a muscle, the actin-myosin system, behaves as a sliding filament model, with adjacent actin and myosin muscle filaments sliding between one another. This shortens the distance between the two attachments of the ends of a muscle group when a joint angle changes.
There are three types of muscle action: concentric, eccentric, and isometric. In concentric action, the proximal and distal muscle attachments move towards one another. In eccentric action, the attachments move away from one another. In isometric action, the attachments do not move relative to one another, and the muscle length remains constant. Isometric contraction occurs when there is no external movement or change in the joint angle or distance between attachments.
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