Estimate the maximum force available from "your own"?

Question

Quadriceps
Biceps
Masseter

Show your assumptions and calculations
(What kind of equations & formula's are used to solve this)

Answer 1

Please see the picture in ref 1.
The most of the force in this exercise is exerted by the quadriceps. To model these muscles in great detail is somewhat complex but here is the basic principal well illustrated by ref 3.

Imagine a pulley of radius R and a bar of length L

The pulley is a socket (femur to tibia) joint.
Length L is the distance from the weight pressure point to the center of the socket. Also let A be the angle from the tibia to vertical.
We have
Moment about the center of the joint is

L W sin (A) = R F
so the force exerted by Rectus Femoris (the center muscle) is
F= L W sin(A)/R
F= mg sin(A) [L/R]
m - mass of the object you are trying to lift.

Note that as the angle increases the muscle has to work harder.

Use similar analysis for other muscles and groups.