Muscles

Muscles can only pull. When a muscle contracts, it gets shorter and pulls on a bone through a tendon. It cannot push, so muscles work in antagonistic pairs: when one contracts, its partner relaxes, and then they swap.

Pair	This one contracts to…	Its partner contracts to…
Biceps and triceps	Biceps (front of the upper arm): bend the arm at the elbow	Triceps (back of the upper arm): straighten it
Hamstrings and quadriceps	Hamstrings (back of the thigh): bend the leg at the knee	Quadriceps (front of the thigh): straighten it
Calf and shin muscles	Calf muscle: pull the heel up, so you stand on tiptoe	Shin muscle: pull the toes up

Muscles get the energy to contract from respiration in their cells. Regular exercise makes them stronger.

Biomechanics. Bones are levers, joints are pivots and muscles supply the force. Forces are measured in newtons (N) with a force meter; a grip meter measures how hard your hand can squeeze. For a fair test, change one thing (such as which hand), keep everything else the same, and repeat each reading to find the mean. Bigger muscles can usually pull with bigger forces.

Stretch The biceps is attached very close to the elbow, so it must pull with a force several times bigger than a weight held in the hand. Moments balance: muscle force × its distance from the pivot = weight × its distance from the pivot. There are three kinds of muscle: skeletal muscle moves bones, cardiac muscle is the heart, and smooth muscle is in the walls of the gut and blood vessels.