Kinetic chain

The body moves as a chain of segments joined at the joints. In a throw, a kick or a punch, the big, heavy segments near the ground start the movement, and each one hands its speed on to the next, lighter one. The feet push against the floor, the hips turn and move forward, the trunk turns with them, the shoulder goes forward and the arm extends last.

Each link adds its own speed on top of the speed it inherits. Roughly, the speed of the fist is

$$v_\text{fist} \approx v_\text{hips} + v_\text{trunk} + v_\text{shoulder} + v_\text{arm}$$

with each term the extra speed that segment adds relative to the one below. This is why an “arm punch” is weak. With the hips and trunk left out, only the last term is left. The timing matters too: if a link starts too early or too late, its speed is wasted rather than passed on.

Coaches call the whole thing “putting the body behind the punch”. In a jab it shows up as a short step, the hips turning and driving forward, and the lead shoulder rising to cover the chin.

In this project

The jab reference asks for a 16 cm lead step and a 45° turn of the hips. In training, the fighter first learnt the arm without the chain: its hips were 11 cm behind the reference’s at impact. Rewards for the hips’ position at impact brought that down to 4.9 cm. On targets beyond full reach the hips stayed 9 to 10 cm behind, and the jab was slow there.

In the speed work of 18 October, the cross broke the chain at the shoulder: the hips were in place at full extension, but the chest and rear shoulder were 6–8.5 cm behind the reference’s, and the fist fell 1–2 cm short. The trunk’s motors were nowhere near their limits; they just weren’t asked. A reward for the punching shoulder keeping up through the impact brought the worst lag down to 2–6.5 cm, and the cross started landing in the air (see Faster punches).