Muscle types
There are three types of muscle in the body:
- smooth muscle – found in the internal organs and blood vessels – this is involuntary
- cardiac muscle – found only in the heart – this is involuntary
- skeletal muscle – attached to the skeleton – this is voluntary
Involuntary muscles are not under our conscious control which means we can’t make them contract when we think about it.
Voluntary muscles are under our conscious control so we can move these muscles when we want to.
Muscle fibre types
There are two different types of skeletal muscles:
- slow twitch, also known as type I – oxidative
- fast twitch, also known as type II – glycolytic
Each type of muscle fibre has different characteristics which are shown in the table:
| Type I | Type II | |
| Speed of contraction | Slow | Fast |
| Force produced | Low | Medium/high |
| Resistance to fatigue | High | Medium/low |
| Colour | Red | White |
| Energy system | Aerobic | Anaerobic |
Endurance cyclists need a high percentage of type I fibres so that their muscles can work for the duration of a race without getting tired. These muscles are red in colour because of the amount of capillaries that transport the oxygenated blood to the working muscles.
Sprinters need a high percentage of type II fibres which allow their muscles to contract very quickly. Fast muscle contractions give runners power and enable them to maintain a high speed over a short distance. However, this type of muscle tires very quickly, meaning sprinters are not able to run at this speed for very long.
Voluntary muscles
The main muscles of the human body are shown here.
Muscles and movement
Muscles cause movement by contracting across joints. Muscles are attached to the skeleton by tendons in two places:
- the origin
- the insertion
The origin is the end of a muscle which is attached to a fixed bone. The insertion is the other end of the muscle that is attached to the bone which moves.
Muscular contractions
Muscular contractions are defined as the change in the length of the muscle under contraction. Muscles contract in different ways to produce a range of movements:
- Isotonic contraction – involves the muscle producing tension and controlling the speed of the muscular contraction. This movement can be a concentric or eccentric muscular contraction.
- Isotonic concentric contraction – this involves the muscle shortening. The origin and insertion of the muscle move closer together and the muscle becomes fatter.
- Isotonic eccentric contraction – this involves the muscle lengthening whilst it is under tension. The origin and the insertion move further away from each other. An eccentric contraction provides the control of a movement on the downward phase and it works to resist the force of gravity.
Left: Concentric contraction – the biceps are contracting concentrically to move the weights upwards; Right: Eccentric contraction – the biceps are contracting eccentrically to lower the weights against resistance
- Isometric contraction – this involves a muscle producing tension but staying the same length. This occurs when the body is fixed in one position.
Here, the muscles are contracting isometrically to hold this gymnast in the crucifix position and are not changing length. When the isometric contractions end, isotonic contraction will occur.
Muscle pairs
Muscles transfer force to bones through tendons. They move our bones and associated body parts by pulling on them – this process is called muscle contraction.
However, muscle contraction cannot act to push the bone back into its original position, and because of this, muscles work in ‘antagonistic muscle pairs’. One muscle of the pair contracts to move the body part, the other muscle in the pair then contracts to return the body part back to the original position. Muscles that work like this are called antagonistic pairs.
In an antagonistic muscle pair, as one muscle contracts, the other muscle relaxes or lengthens. The muscle that is contracting is called the agonist and the muscle that is relaxing or lengthening is called the antagonist.
For example, when you perform a bicep curl, the biceps will be the agonist as it contracts to produce the movement, while the triceps will be the antagonist as it relaxes to allow the movement to occur.
The biceps contracts and raises the forearm as the triceps relaxes
The triceps contracts and lowers the forearm as the biceps relaxes
Antagonistic muscle pairs
The following groups of muscles are antagonistic pairs:
| Biceps | Triceps |
| Hamstrings | Quadriceps |
| Gluteals | Hip flexors |
| Gastrocnemius | Tibialis anterior |
| Pectorals | Latissimus dorsi |
Antagonistic muscle pairs in action
Preparation and contact phase in football
In the preparation phase, when a footballer prepares to kick a football, their hamstrings contract to flex the knee while the quadriceps lengthens to allow the movement. The hamstrings are the agonist and the quadriceps are the antagonist.
In the contact and recovery phase, the quadriceps contract to extend the knee while the hamstrings lengthen to allow the movement. The quadriceps are the agonist and the hamstrings are now the antagonist.
Muscle hypertrophy
When a person takes part in resistance exercises such as weight training, their muscle tissue is placed under stress. This causes micro-tears in the muscle fibres. The body responds by repairing the muscle fibres and making them larger.
When a muscle gets bigger, the process is called hypertrophy.
