Muscles carry out all the body’s movements and are attached to bones by tendons. The tendons are made of strong, connective tissue that can stretch to help deal with the forces produced during movement.
Muscles can only pull, they cannot push. They therefore work in pairs or teams that work in opposition to each other. When one set of muscles contracts, the other relaxes to bend a joint. They swap roles to straighten the joint again. For example, contraction of the biceps bends the elbow, while contraction of the triceps straightens it as the biceps relaxes. Muscles can only “push” indirectly, via levers.
Bending a joint is known as flexing. Flexing decreases the angle between two bones. When joints can move forwards and backwards, such as the shoulders, flexion means forward movement. When sitting down, both your hips and your knees flex.
Extension is the opposite of flexion and increases the angle between two bones. When joints can move forwards and backwards, such as the hips, extension means backwards movement. When standing, both your hips and your knees extend.
A lever allows movement to occur around a point called a fulcrum. A first class lever has the fulcrum in the middle. A second class lever places the load between the effort and fulcrum. In a third class lever, the effort occurs between the load and the fulcrum – like using a pair of tweezers.
Neck muscles work like first class levers. When they contract, they force your chin up on the opposite side of the fulcrum (a joint between your skull and spine).
The calf muscle can act as a second class lever by pulling when the foot is on the ground. The foot then bends at the base of the toe so the entire weight of the body is raised on tiptoe.
The biceps acts as a third class lever. Pulling close to the fulcrum – the elbow – it moves the bones only a little, but creates a lot of movement for the hand at the end of the lever. A small effort translates onto a big movement.
“The plantaris muscle sits just above the back of the knee and pulls on the heel bone via a tendon 50cm (20in) long. The Achilles is the strongest and thickest tendon. The Achilles tendon is strong enough to support more than 10 times your body weight when running”.
Muscles pull on bones via tendons. However, the tendons can be very long, and the muscles far from the joints they are operating. Amazingly, there are no muscles at all in the fingers. All of their movement is made by remote control – by muscles in the hand and arm.