Why shorter people can lift more.
If there are any workout buddies out there that include one short person and one tall person, you may find that the short person will find squatting the same weight a lot easier than the longer limbed counterpart. There is a reason for this! Not necessarily to do with muscular strength. The reason is simply limb length. Looking at the biomechanics of the body, your limbs and muscles attached to them create leavers around the joints. The further away the load is from the pivot point (fulcrum) the more effort is needed to lift the load (or create angular velocity around the joint). If we take a simple example of the bicep curl… as in the image below the pivot is the elbow, the load is acting downward at the distal end of the forearm (in the hand) and the effort is coming from the bicep from the insertion point on the radius bone as shown in the picture below.
Mechanical Advantage (MA)
Mechanical advantage = Effort arm / Resistance Arm
(distance from pivot to effort) (distance from pivot to load)
You are said to have mechanical advantage if the result of the above equation is 1 or more, but with this type of leaver (the effort between the pivot and load, (3rd class leavers)) it is always going to by less than 1. In the above pictures the left (shorter person) MA will equal 0.05, but the right picture (taller person) the MA will equal 0.035. This means with the same force applied to both leavers the left example will create more angular velocity around the joint (i.e. lift the load quicker).
As most leavers in the human body are this type of leaver the same principle can be applied to most concentric exercises. This type of leaver is better designed for create velocity around a large range of motion at a joint, i.e. for running and throwing, and not very efficient for force production. 2nd class leaver systems where the load is between the fulcrum and the effort (e.g. lifting a wheelbarrow) are better suited for force production but only around a small range of motion.