Movement in the body is produced by a system of levers. These series of levers work together to produce coordinated action, some by actual movement (dynamic) and others by stabilization (static).
F x FA = R x RA Force x Force Arm = Resistance x Resistance Arm
Initial Example Change insertion 1 cm Change point of resistance 1 cm
F x 2 cm = 10 kg x 9 cm 2 F = 90 kg F = 45 kg
F x 3 cm = 10 kg x 9 cm 3 F = 90 kg F = 30 kg
F x 2 cm = 10 kg x 8 cm 2 F = 80 kg F = 40 kg
Resistive force (R) is initially relatively short [close to fulcrum (A)].  As motive force (F) acts on lever, resistive arm becomes longer requiring progressively greater motive forces throughout movement. 



Also see Tension Potential and its impact on force production.