Every joint is a lever
A lever has three parts: a fulcrum (the joint), a force (the muscle pulling), and a resistance (gravity, a weight, the body). The position of these three parts relative to each other defines the lever's class — and the class defines whether the joint is built for speed or force.
First-class levers
Fulcrum sits between force and resistance. Like a seesaw. In the body, the atlanto-occipital joint (skull on top of spine) is a first-class lever — neck extensors pull on the back of the skull, the spine acts as the fulcrum, and the weight of the face is the resistance. The triceps extending the elbow is another example. First-class levers can favor either force or speed depending on where the fulcrum sits.
Second-class levers
Resistance sits between fulcrum and force. Like a wheelbarrow. Always favors force over speed. The classic example: standing calf raise. Ball of the foot is the fulcrum, body weight is the resistance, calf muscles pull up at the heel. Big load, small movement. This is why calves can handle massive weight relative to their size.
Third-class levers
Force sits between fulcrum and resistance. Like a fishing rod. Almost every limb movement is a third-class lever. Bicep curl: elbow is the fulcrum, bicep insertion is the force (close to the fulcrum), the weight in the hand is the resistance (far from the fulcrum). Always favors speed and range of motion over force. This is why a 200lb deadlift requires the muscle to generate far more than 200lb of internal force.
Why this matters for programming
A trainer who understands lever mechanics knows why:
- Long-armed lifters fight harder against barbells (longer resistance arm)
- A close-grip bench feels heavier than wide-grip with the same load
- Calves and quads can take huge volumes
- The "sticking point" of a movement is where leverage is worst
Translating this to client cueing
You don't have to teach a client about levers. But you use the knowledge:
- Cue elbows tight on a press (shorter resistance arm)
- Position the bar over mid-foot on squats (keeps the fulcrum aligned)
- Don't blame "weakness" when biomechanics make a movement harder for a given client
TL;DR
First-class = fulcrum in middle. Second-class = resistance in middle. Third-class = force in middle. Almost all body movement is third-class. Lever class explains why some exercises are inherently harder.