Force, Velocity And The Curve Between Them

The relationship between how much force a muscle produces and how fast it shortens is one of the more reliable findings in muscle physiology. It constrains what any load can train.

Force falls as shortening speed rises

When a muscle shortens quickly, the individual force-generating attachments between its filaments have less time to form and produce less force overall.

The consequence is a curved relationship: maximum force occurs when the muscle is not shortening at all, and force approaches zero as speed approaches its maximum.

This is why a heavy load moves slowly and a light one moves fast, and why the two cannot be combined regardless of intent.

Lengthening contractions sit outside the curve

When a muscle produces force while being lengthened, it can generate more force than it can in a static hold, which is the opposite of what shortening does.

The mechanism involves both the behaviour of the cross-bridges under stretch and the contribution of elastic structural proteins within the fibre.

This is why a lifter can lower more weight than they can raise, and why lowering phases are used deliberately in some training approaches.

Power sits at neither extreme

Power is force multiplied by velocity, so it is zero at maximum force with no movement and zero at maximum speed with no load.

The peak lies somewhere between, at a moderate load moved with intent, which is the basis for the loads used in explosive training.

Where that peak sits differs between movements and between individuals, which is why a single prescribed percentage for power work is an approximation.

Training shifts parts of the curve

Heavy training tends to raise the high-force end of the curve, improving what can be produced at slow speeds without changing much at the fast end.

Light, fast training tends to raise the high-velocity end, improving performance at speed with less effect on maximal force.

The specificity of these shifts is why athletes needing both qualities train across the range rather than at a single point on it.

Why this constrains programme design

Choosing a load is implicitly choosing a position on the curve, and therefore choosing which end of the relationship the session develops.

A programme built entirely from heavy slow work will produce a strong lifter who cannot express that strength rapidly, and the reverse also holds.

Recognising that the load is a proxy for the speed being trained makes exercise and load selection a more deliberate decision than picking a comfortable weight.