The human body, muscle by muscle

The pectoralis major draws the arm toward the midline and pushes it forward. It is the key muscle in any pressing movement: bench press, push-ups, or simply shoving a heavy door. Its upper, middle and lower fibres share the load depending on the angle.

The lats pull the arm down and back: they are the muscle of pull-ups and rows. They are central to any movement where you pull a load toward you, and they give width to the upper body.

The trapezius elevates, depresses and retracts the shoulder blades, and supports the neck. It stabilises the shoulder in nearly every upper-body movement and is key to standing tall.

The lower-back muscles (spinal erectors) keep the back straight and extend the torso. They protect the spine whenever you lift and are the foundation of a solid core.

The deltoid raises the arm to the front, side and back through its three heads (anterior, lateral, posterior). It is the muscle of "broad" shoulders and stabilises almost every arm movement.

The biceps bends the elbow and rotates the forearm (supination). It works whenever you pull or lift something toward you and assists the back in pull-ups.

The triceps extends the elbow: it is the pusher. Present in every press and dip, it makes up the largest share of the arm's volume — more than the biceps.

The forearms control the wrist and fingers: this is grip strength. A strong grip limits fatigue on every pull and the deadlift, and shows whenever you carry heavy loads for long.

The rectus abdominis flexes the trunk forward and, above all, braces the torso. Strong abs protect the back, transfer force between upper and lower body, and stabilise every heavy movement.

The obliques rotate and side-bend the torso and, crucially, resist rotation to stabilise the spine. They shape the waist and are essential in any throwing or striking sport.

The quadriceps straightens the knee: it stands you up out of a squat, drives you up stairs and absorbs every stride. With four heads, it is one of the most powerful muscle groups in the body.

The hamstrings bend the knee and extend the hip. They power running and jumping, and decelerate the leg on every stride. Often neglected, they balance the quads and protect the knee.

The gluteus maximus extends the hip and drives the body forward and up. It is the most powerful muscle in the body: it dominates sprinting, jumping and lifting, and stabilises the pelvis with every step.

The adductors pull the thigh inward and stabilise the pelvis and knee. They work deep in the squat and lunges and are essential for changing direction.