How muscles create movement

Muscles in the body are responsible for generating movement, particularly voluntary muscles that you can control consciously. These muscles usually work across a joint, where they are attached to bones by strong, fibrous tissues called tendons. Because muscles can only pull in one direction and not push, they must operate in pairs to allow for smooth, controlled movements.

Key terms in antagonistic muscle action

Antagonistic muscle action refers to the way muscles work together in opposing pairs to produce movement. One muscle in the pair contracts to drive the action, while the other relaxes to allow it.

Essential terms

• Origin - The point where a muscle attaches to a stationary bone.
• Insertion - The point where a muscle attaches to a moving bone.
• Agonist - The muscle that contracts to create the main movement; it is also known as the prime mover.
• Antagonist - The muscle that relaxes during the movement.
• Antagonistic pairs - Pairs of muscles that work in opposition; they create movement when one (the agonist) contracts and the other (the antagonist) relaxes.

Types of muscle contractions

Muscle contractions are the processes by which muscles generate force to produce movement or maintain position. There are two main categories: isotonic contractions, where the muscle length changes, and isometric contractions, where it does not.

Isotonic contractions

Isotonic contractions involve a change in muscle length as they contract.

There are two different types of isotonic contractions:

• Concentric contraction - This occurs when the muscle shortens as it contracts.
• Eccentric contraction - This happens when the muscle lengthens as it contracts.

Isometric contractions

Isometric contractions take place when muscles generate force but remain the same length.

Origin and insertion points of major muscles

Each muscle has specific attachment points that determine how it influences movement at a joint.

Attachment points for key antagonistic pairs

Antagonistic muscle pairs in action

Antagonistic pairs switch roles depending on the movement required.

Examples in sporting activities

• Shoulder press - When lifting the weight upwards, the deltoid contracts as the agonist to raise the arms. To lower the weight, the latissimus dorsi contracts and the deltoid relaxes.
• Leg extension and flexion - To straighten the leg at the knee, the quadriceps contracts as the agonist. To bend the leg back, the hamstring contracts as the agonist.
• Running - During the knee extension phase of a stride, the quadriceps acts as agonist, while the hamstrings lengthen and work as antagonist.
• Pull-up - As you pull your body upwards, the biceps shortens (concentric) as the agonist. The triceps acts as the antagonist by lengthening (eccentric).
• Plank exercise - Core muscles, such as the abdominals, contract isometrically to hold the body in a straight position, maintaining posture without changing length.