The Muscular System

Muscle tissue 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.

Stretch Reflex

Within the Triceps, and all muscles, a unique muscle fibre known as the annulo-spiral receptor. This receptor is sensitive to the rate and extent the Triceps are being stretched. As the Triceps lengthens, this receptor sends a signal proportional to the amount and rate of stretch to tell the Triceps to contract. It is a safety mechanism to prevent the Triceps from being overstretched.