Muscles... what do they actually do?!

Skeletal muscle attaches to bone via tendons to facilitate movement of the skeleton. It is voluntary muscle as we have conscious control over it through our somatic nervous system. A muscle consists of a large number of muscle fibres. The outer sheath is a muscle is called the epimysium. Within this the cells are grouped into fasciculi which is wrapped in its own connective tissue called perimysium. Within the fascicles are individual muscle fibres that are wrapped in endomysium. Each connective tissue runs the length of the muscle and binds together at the ends to form tendons, or aponeuroses. Muscle cells or fibres are cylindrical in shape with several nuclei and are made up of many contractile protein myofibrils which are arranged in repeating units known as sarcomeres. The protein myofilaments of the sarcomere are of two types, thin filaments made up of actin, and thicker filaments of protein myosin. The overlapping arrangement of these filaments gives a distinctive striated appearance.

The primary function of skeletal muscle is contraction facilitating movement. Typically, a muscle spans a joint and is attached to bone either end by tendons. One of the bones remains fixed or stable whilst the other moves as a result of muscle contraction. Many muscle are arranged in antagonist pairs so that their actions oppose each other. As one group contracts, the opposing muscle or group relaxes to prevent injury. The point where a muscle attaches to a bone that moves is known as insertion, and where the muscle attaches to a stable or fixed bone is called the origin.

Only a small number of muscle fibres are needed to contract to bend a limb, but if we put more stress on a joint, for example, lifting a weight, then more muscle fibres are recruited to perform the action and give the same outcome. This is called muscle fibre recruitment. When stimulated, muscles fibres contract on an all or nothing basis. It means that muscle fibres contract to their full level along their entire length, or not at all. The force therefore generated by a muscle is not regulated by the strength or level of contraction within a fibre, but due to the number of fibres recruited to contract to produce the movement.

Skeletal muscle cells contract in response to stimulation from a nerve fibre usually half way down its length. The muscle contracts when actin filaments slide over the myosin filaments, resulting in a shortening of the length of the sarcomeres. The actin and myosin filaments do not change length themselves, they slide past each other and is so known as sliding filament mechanism. The shortening of the sarcomeres makes the muscle itself contract and shorten resulting in an action at the joint.

The muscles used predominantly in the establishment and maintenance of body posture can be described as postural muscles, and are all extensor muscles. They help to oppose us to gravity, and are also sometimes referred to as core stability muscles. They are constantly at work making small adjustments, so need endurance and therefore are mainly made up of slow twitch muscle fibres. They are prone to overuse and have a tendency to shorten and tighten. From the foot up, examples are soleus, gastrocnemius, tensor fasciae latae, piriformis, adductor group, rectus femoris, iliopsoas, cervical and lumbar erector spinae, quadratus lumborum, pectoralis minor and major, levator scapulae, upper trapezius, scalenes, subscapularis, biceps brachii, sternocleidomastoid, suboccipitals, and temporalis.

Postural muscles can be further classified into groups. Local stability muscles group the muscles located deep and more central to the body. They provide segmental stability to allow movement rather than producing the movement themselves. They have a broad spectrum of attachment points within the trunk, on or near the vertebra, making them ideal for increasing stabilisation. They are monoarticular, crossing only one joint, and primarily function eccentrically to control movement and maintain static stabilisation. Continuously active, they are primarily made up of slow twitch, high density muscle fibres, and with overuse and dysfunction can become weak and inhibited. They undergo only a small change in length thereby not having a large impact on actual movement of a joint. They have static holding capacities and contract isometrically to increase joint stiffness. Local stability muscles work in anticipation of movement. They can become inhibited which can lead to over dominance of more global muscles leading to complex biomechanical injuries.

Examples that fit within this classification of muscle are multifidus, posterior psoas, transverse abdominis, the central portion of erector spinae, the medial fibres of quadratus lumborum, and the posterior segment on the internal obliques.

The monoarticular muscles that connect the trunk to the extremities can be classified as global stabilisers. The function eccentrically to control range of movement and power, and maintain posture by contracting isometrically. Because of the constant active role in postural maintenance, they are prone to weakness and inhibition. These postural muscles of the core include the superficial multifidi, lateral fibres of QL, oblique abdominis, anterior psoas, the gluteals and soleus. Global muscles are predominantly larger and more superficial than local muscles and attach from the pelvis to the ribcage and/or the upper and lower extremities. They are vital in the role of force absorption of movement and contribute towards movement through concentric contraction.

Global mobility muscles cross more than one joint and generally work concentrically to produce power and large ranges of movement, and eccentrically to decelerate high loads, and have a role in shock absorption of load. They tend to be more superficial, can work aerobically and anaerobically containing high levels of fast twitch muscle fibres allowing for non continuous, sharp bursts of activity. As they tend to take on the failed stability role as well as their own, it is common overtime for them to become overused, stressed and tight. A tight mobiliser muscle can cause imbalance and dysfunctional when pulling against an inhibited stabiliser muscle. This can lead to an overuse injury and negatively affect joint alignment, posture, and movement patterns. They are direction specific and so their anatomical attachment point.

Examples of global mobilisers are the gastrocnemius, hamstrings, tensor fascia latae, piriformis, rectus abdominis, rhomboids, iliocostalis, latissimus dorsi, scalenes and levator scapulae.

Prime Mover

The prime mover muscle is the primary muscle that acts directly to produce a movement by contracting. It is often referred to as the agonist, working as the opposing half of an antagonist in an antagonistic pair. In a bicep curl, the prime mover is the biceps brachii.

Synergist Muscles

Synergist muscles help to create movement. The synergist in a movement is the muscle or muscles which stabilise a joint around which movement is occurring, helping the agonist to function effectively. For example, in a bicep curl, the agonist or prime mover is the biceps brachii, and its antagonist is the triceps brachii. The synergist is the deltoid which assists flexion at the elbow joint by helping to stabilise the upper arm. They are usually smaller muscles which act at the associated joint to the prime mover.


As well as agonists, antagonists and synergists, fixators are a type of muscle grouping term. Fixators are the muscle or muscles that fix or hold a bone so that the prime mover or agonist can contract and carry out the intended movement. These stabilisers provide the necessary support to assist holding the body in place whilst movement occurs, acting to eliminate unwanted movement at the prime movers origin and insertion points. Many muscles attach to more than one bone capable of movement and are said to be multiarticulate or multijoint muscles. When they contract, they are able to move both bones attached. Fixators are essential to restricting and performing only intended movements. They are usually larger postural muscles.

Using the bicep curl as an example again, the biceps brachii flexes the elbow joint. The biceps brachii contracts concentrically to move the radius, where its distal attachment point is. One of the origin points of the biceps brachii is on the scapula. To prevent the scapula from drawing closer to the radius when this movement occurs, the rhomboids and upper trapezius work isometrically keeping the scapula from moving on the torso.

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