As muscles grow, their force direction becomes more favorable for strength

We previously discussed the importance of muscle attachment points, as a muscle attached further from a joint generates more rotational force (torque) than one close to the joint. In terms of physics this due to the lengthening of so-called “moment arms”. The further you push from the hinge of a door, the longer the moment arm is said to be.

The moment arm also depends on how the force is applied to the door. Pushing straight at the door surface would mean more force translated into closing the door compared to using the same force at an angle. For force generated by muscles generally doesn’t pull bones in an optimal direction (i.e. 90 degrees out from the bone surface).

However, as muscles grow in size, the angle by which they pull the bones increases towards 90 degrees, meaning that the same force translates into more torque at the joints, making you proportionally stronger compared to the gains in force generating capacity of the actual muscle. In terms of physics, we say that the muscle moment arms tend to get longer as the muscle grows.

This process occurs at most joints, but not all. So while you cannot lengthen your muscle moment arms by changing your muscle attachment points, you can train for hypertrophy and lengthen your moment arms that way!

Source:

Vigotsky AD, Contreras B, Beardsley C. (2015) Biomechanical implications of skeletal muscle hypertrophy and atrophy: a musculoskeletal model. PeerJ 3:e1462