Stimulus to Fatigue Ratio

The idea behind SFR is simple: every exercise generates a stimulus (which drives adaptation, like hypertrophy or strength) and fatigue (which requires recovery). The ratio between these two factors determines how 'efficient' an exercise is in your program. Not all exercises that stimulate a muscle do so at the same recovery cost.

Practical example: leg press and barbell squat both stimulate the quads. But the squat generates much more systemic fatigue (back, nervous system, joints) compared to the leg press. If your goal is to maximize quad volume while recovering well between sessions, the leg press has a better SFR for that specific muscle.

Careful: a high SFR doesn't mean 'better exercise' in absolute terms. The squat has advantages the leg press doesn't (global strength, stabilization, athletic transfer). SFR is a tool for making informed decisions, not a final verdict. It becomes especially useful when training volume is high and fatigue management gets critical.

In practice, machines and cables tend to have a better SFR than free barbells for hypertrophy, because they isolate the target better without overloading support structures. Keep this in mind when programming volume: high-SFR exercises let you accumulate more productive sets without burying yourself under fatigue.