Levers determine how heavy the same weight feels throughout a range of motion. Our bodies evolved logically. Muscles operate strongly when leverage helps the least and operate less well when leverage helps the most. This truth has caused a lot of confusion because it seems to contradict our experiences.
When leverage is best, we think we have grown stronger, when in reality we have grown weaker and leverage compensates for the difference.
A lever is a rigid or semi-rigid body that exerts force on any object impeding its tendency to rotate. This occurs when subjected to a force that does not pass through its pivot point. It functions as a simple machine that adjusts the force and speed for a movement.
The three classes of levers depend on the arrangement of the force, axis of rotation, and resistance:
- Force-Axis-Resistance (FAR)
- Axis-Resistance-Force (ARF)
- Axis-Force-Resistance (AFR)
Force-Axis-Resistance (FAR) is a first-class lever. This strikes a balance compared to the other levers. Elbow extension is an example.
Axis-Resistance-Force (ARF) is a second-class lever. The muscle and resistive forces act on the same side as the fulcrum. This allows for less muscular force compared with the resistive force. Plantarflexion of the ankle is an example.
Axis-Force-Resistance (AFR) is a third-class lever. These levers are built for range of motion and speed, with most of the levers in the human body in this class. Elbow flexion is an example.
Mechanical advantage is the tradeoff between distance and force for the same amount of work.
A mechanical advantage of less than 1 requires greater muscle forces versus resistive forces, requiring more force in return for allowing more distance.
A mechanical advantage greater than 1 allows muscle force to be less than the resistive force, multiplying your force output by demanding more distance.
Think of playing on a seesaw. When one kid, despite his weight, positions further away from the center, the other kid is more likely to rise. The weights of each kid did not change. The outputs changed as they moved further or closer to the center of the seesaw.
A kid further from the center has a greater mechanical advantage, and a kid closer to the center has a lesser mechanical advantage.
Force, Torque, and Moment Arms
Torque equals the magnitude of the force times the length of the moment arm. The moment arm refers to the perpendicular distance from the line of action of the force to the fulcrum. Increasing the length of the moment arm results in the weight feeling heavier.
This explains why the weights listed on machines can deceive you. You may lift a lot or far less weight than you think you can handle but they do not display the mechanical advantage. A lateral raise with even light dumbbells feels difficult due to the long moment arm.
It also shows why the weights other people use cannot compare to your own, given everyone’s unique body mechanics.
Muscles evolved to produce force when needed the most. For most normal movements, this means when your leverage is worst. This concept may go against your initial thought process.
Tension is the main stimulus for more size and strength. Tension, or the squeeze you feel when a muscle contracts, develops best at middle lengths. The length-tension relationship establishes this fact.
The parts that form sites for contraction must be close enough to form connections that create tension. Fewer connections form when these parts lie too closely or too far apart. When too close, they overcrowd like a ball of yarn. When too stretched, they fail to reach each other. Muscles work best with the right amount of overlap.
Imagine the bench press. At the lockout, you feel strong because your leverage is ideal, since the shoulders and elbows do not bend much. The muscles in this position bunch up and form less connections though.
The bench press feels hardest when your elbows are furthest away from your shoulders. Your muscles function strongly here despite the mechanical disadvantage and the longer moment arm. They form the most connections here and therefore the most tension.
Levers Support Free Weight, Compound Exercises
You must not fool yourself – and you are the easiest person to fool.
– Richard Feynman
We want the weight to feel heaviest precisely where our muscles operate most strongly. This actually occurs when leverage is worst. A good free weight, compound exercise achieves this goal.
Any variable resistance made possible through a machine, chains, or resistance bands make an exercise less effective. They make the weight feel heavier in positions that the muscles operate weakly. Isolation exercises also tend to extend the moment arm too greatly when muscles are weak.
Levers support relying on basic, compound movements. Free weight barbells and dumbbells allow us to battle against gravity as intended. Overload a few simple movements with free weights to grow stronger and more muscular while staying safe.