Two curves that define every rep
Force production isn't constant. It changes with how fast a muscle is shortening and how long it is when contracting. These two relationships — force-velocity and length-tension — are the physiological reason exercises feel the way they do.
The force-velocity curve
Concentric (shortening) contractions: as velocity goes up, force goes down. You can move a 500lb deadlift slow; you can throw a baseball fast. You can't do both at once.
Isometric (no movement): maximum force production happens here.
Eccentric (lengthening): you can produce ~1.3× concentric max. This is why you can lower a weight you can't lift, and why eccentric overload is a real programming tool.
Training implication: if your client wants to get stronger, slow heavy lifts. If they want to get faster, lighter explosive lifts. You can't maximize both with the same set.The length-tension relationship
A muscle produces maximum force at its "optimal length" — usually slightly longer than resting length. Stretched too long, the actin-myosin filaments don't overlap enough to grip. Shortened too far, the filaments collide and can't slide further.
This is the physiology behind:
- Why the bottom of a squat feels weakest (muscles too long)
- Why lockout on a deadlift feels weakest (muscles too short)
- Why bicep curls feel hardest at 90° (peak length-tension)
How to use this in programming
For hypertrophy: train through the full range so muscles experience tension at multiple lengths. Recent research shows the lengthened portion of a movement produces the most growth — partial reps at the stretched end can be more effective than partial reps at the top. For strength: spend time at the sticking point. If a client always misses a bench at 4 inches off the chest, add pin presses from that height. For speed: train at the velocity you want to produce. Sprinters don't get faster by squatting heavy alone — they need explosive movements too.What to actually say to clients
You don't lecture them. You program with this in mind. When they ask "why does the bottom of the squat feel so hard?" — now you have the answer. When they want to peak strength, you slow them down. When they want to peak power, you speed them up.
TL;DR
Force-velocity: slower = more force, faster = less force, eccentric = most. Length-tension: muscles are strongest at slightly-longer-than-rest length. Both explain real lifting behavior and inform what reps × intensity to prescribe.