The Muscular System

Muscle tissue is body tissue made from specialised cells called muscle fibres. These fibres can contract and relax, allowing the body to produce movement, maintain posture, stabilise joints and generate heat.

There are three types:

  • Skeletal muscle: Moves the bones and is controlled voluntarily.

  • Cardiac muscle: Forms the heart and pumps blood automatically.

  • Smooth muscle: Controls internal organs and blood vessels automatically.

For throwers, skeletal muscle produces and transfers force through the kinetic chain.

It has four main properties:

  • Excitability (ability to respond to stimuli),

  • Contractibility (ability to contract),

  • Extensibility (the ability of a muscle to be stretched without tearing)

  • Elasticity (ability to return to its normal shape).

Through contraction, the muscular system performs three essential functions:

  • Motion - walking, running etc.

  • Heat production - maintain normal body temperature

  • Maintenance of posture - standing, sitting etc.

Motion

To understand how the muscles combine with the skeleton to provide motion, we must look at the movement's basic mechanics. The main framework of the body is covered by muscle, whose function is to permit movement. We know that to move or lift a load against another force, it is easier to use levers, and it is this principle that the musculoskeletal system adapts and that we must examine.

The parts that are used in a lever are as follows:

  • Lever - nearly always the bone

  • Fulcrum - the pivot point of the lever, which is usually the joint

  • Muscle Force - a force that draws the opposite ends of the muscles together

  • Resistive Force - a force generated by a factor external to the body (e.g. gravity, friction etc.) that acts against muscle force

  • Torque - the degree to which a force tends to rotate an object about a specified fulcrum

Agonist, Antagonist, Fixator & Synergist Muscles

Muscles can only exert a pulling force, so work in pairs. When we move a limb one muscle, the agonist muscle, also known as the prime mover, causes the movement and an antagonist muscle works in opposition to the agonist muscle. For example: In a biceps curl (bending the arm up from the side of the body) - the biceps is the agonist muscle causing the movement, and the triceps are the antagonist muscle working in opposition to the biceps.

The fixator in a movement is the muscle stabilizes the origin of the agonist and the joint that the origin spans (moves over-prime movers) to help the agonist function most effectively with fewer efforts. For the bicep curl, this would be the rotator cuff muscles, the ‘guardians of the shoulder joint’. Most fixator muscles are found working around the hip and shoulder joints. Other muscles, known as synergist muscles, stabilize muscle movements to keep them even and control the action so that it falls within a range of motion which is safe and desired.

A synergist is another muscle, besides the agonist, that assists the movement of a joint indirectly The most common example is a when groups of muscles are wired together to create a specific movement, each phase of the walking cycle requires certain muscles to “link together” to enable humans to be able to repeat that one phase of the walking cycle repeatedly without having to “think” about it.

This obvious has a major benefit for athletics and the good news is it can be trained. Where possible utilize exercises which use multiple joints and multiple planes of movement. These are sometimes referred to as compound exercises and they enhances the nervous system's ability to properly recruit muscle synergies improving coordination.

Heat production

Muscle contractions produce heat, and as much as 70% of body heat is produced by energy produced in muscle tissue. Blood is an essential element in temperature control during exercise, taking heat from the body's core and working muscles and redirecting it to the skin when the body is overheating. When the body's internal heat reaches too low, a level of thermoreceptors in the skin relays a message to the hypothalamus in the brain. In response to this signal, the skeletal muscles contract and relax involuntarily (shivering), increasing muscle activity to generate heat. In turn, muscles are also responsive to exterior heat - cold air improves muscle tone, and hot conditions have a relaxing effect on muscles.

Maintenance of posture

As well as enabling movement, muscles also maintain posture and body position. Sensory receptors in the muscles monitor the muscles' tension and length and provide the nervous system with crucial information about the location of the body parts, therefore enabling posture to be maintained. Muscles are never entirely at rest, nor do they have to shorten in length when they contract. The tension or tone produced due to these contractions between opposing muscle groups helps us remain in a static position, even when we are asleep.

Muscle Origin & Insertion

Each muscle end is attached to a bone, and these connections are known as the origin and insertion. The muscle's origin is attached to the immovable bone, and the muscle's insertion is attached to the movable bone.

How do muscles contract?

A muscle contraction starts from a motor neuron located in the brain. Once stimulated, the motor neuron sends a signal to the muscle fibres (a long, tubular muscle cell), causing them to contract. This contraction is also what generates movement in different parts of the body. The combination of muscle fibres and a single motor neuron is called a motor unit. The number of muscle fibres innervated by the same motor neuron varies greatly depending on the function of the muscle.

Skeletal muscles consist of thousands of muscle fibres. Their main function is to convert chemical energy into mechanical output in the form of muscle movement. However, not all muscle fibres in a muscle are fired simultaneously. By altering the firing rate and activating muscle fibres with different metabolic properties, the body is able to use energy more efficiently. This also allows each muscle to contract with varying levels of force. This phenomenon is known as multiple motor unit summation.

Motor units are always recruited in an order from smallest to largest. This phenomenon is known as the size principle. Slow motor units are naturally smaller in size, which means they have the lowest activation threshold. Therefore, these motor units are the first ones to be recruited during muscle movement.

Large motor units are activated when slow motor units are unable to produce enough force. Because of this, it takes a relatively long time (0,5s-2,5s) for a muscle to reach its maximum strength output. Since large motor units contract with more force, they also fatigue sooner. Hence why they are only recruited when needed. This means training including heavy resistance training, sprinting or other activities that fatigue muscles fast are beneficial to power production.

Muscle Spindles and the Stretch Reflex

Inside most skeletal muscles are sensory structures called muscle spindles. Each spindle contains specialised muscle fibres surrounded by sensory nerve endings. The primary nerve ending winds around the spindle in a spiral shape, which is why it is called an annulo-spiral ending.

Muscle spindles monitor:

  • The current length of a muscle.

  • How quickly that length is changing.

  • The position and movement of the associated joint.

When the muscle is lengthened quickly, its muscle spindles send signals to the spinal cord. The spinal cord can immediately send a signal back telling it to contract. This automatic response is called the stretch reflex. It happens very quickly and does not require a conscious decision.

The stretch reflex helps the body:

  • Resist sudden, uncontrolled lengthening.

  • Maintain posture and joint stability.

  • Respond quickly to changes in position.

  • Coordinate movement without consciously controlling every muscle.

Why Is This Important for Athletes?

Fast sporting movements often involve a muscle being stretched immediately before it contracts. This is part of the stretch-shortening cycle and can help the muscle produce force quickly.

During a throw, the muscles of the legs, hips, trunk, shoulder and arm must lengthen and contract in a precisely timed sequence. Muscle spindles provide constant information about these movements and help the nervous system adjust muscle activity, balance and joint stability.

However, this does not mean that an athlete should deliberately force a muscle into an extreme stretch. A movement that is too fast, poorly controlled or beyond the athlete’s usual range may cause a strong protective contraction and increase strain on the muscle or tendon.

A progressive warm-up is therefore important. Dynamic movements gradually prepare the muscles, muscle spindles and nervous system for the speed and range required during throwing. Regular technical practice also teaches the nervous system to recognise and control those positions more efficiently.

When stretching for mobility, avoid aggressive bouncing or forcing the joint. Move gradually, remain within a comfortable range and allow the muscle time to relax. The aim is to develop movement that can be controlled—not simply to achieve the greatest possible stretch.