Have you ever wondered why that guy who’s much smaller than you can naturally curl more than you?
Well it all depends on the insertion point of the bicep onto the forearm and the length of your forearm in comparison to your frame.
First, we must understand a basic mechanical concept called moments. The easiest example of moments to understand is that of a seesaw, to keep a seesaw straight 2 people of the same weight must sit an equal but opposite distance away from the middle (the pivot point) - But what happens if person A is twice the weight of person B? Well It may come intuitively, especially if you played on the seesaw a lot as a child, but person A will have to sit at half the distance from the middle to keep the seesaw straight. To put this more scientifically, the clockwise moment caused by person A is balanced by the anticlockwise moment of person B.
So how do we calculate moments?
Well, a moment is equal to the force applied about an axis, multiplied by the distance from the axis perpendicular (right angle) to the direction the force is being applied. Moments are typically calculated in Newton-meters, meaning the distance should be measured in meters and the force measured in newtons (1kg = 2.2lbs=9.81N)
So how does this work in the bicep?
Here’s a picture of the bicep showing where the distal bicep tendon inserts onto the forearm, with the distance of the insertion point from the elbow (pivot point) and the length of the forearm clearly labelled.
Now this may not be clear to see because the pivot point is at the end of the system and not the middle, but this set up is the exact same as the seesaw in the earlier example but in this case, it is the dumbbell that is creating the clockwise moment and the bicep that is creating the anticlockwise moment as it is pulling the forearm upwards and in the opposing direction of the dumbbell.
So, let’s see if we can create an equation to represent this setup when the arm is fixed in the position above.
Imagine the bicep being in an isocentre state, so that it is neither concentric or eccentrically contracting, this means the clockwise and anticlockwise moments for the forearm must be equal therefore
Rearranging this to find out the force produced by the bicep we get:
Now we get to the point where the title starts to make sense and we talk about genetics.
Genetics is the main factor determining where your distal bicep tendon inserts onto your forearm. The range of which is between 2cm and 6 cm, this is almost exclusive to body size, however, taller people with longer limbs are more likely to find themselves at the higher end of this range but even then, there is still a large variation. Using the equation above it is then clear to see that if someone with the same forearm length curled the same weight, the force the bicep produced can be 3 times less for someone with a 6cm insertion point compared to a 2cm one. This also means if they produce the same bicep force they can curl 3 times the weight.
This is easier to see if you compare the scenario to a door on a hinge - the closer to the hinge you push, the more force you must apply to get it to open, but the further away you push, the easier it is to open.
The equation also shows that if the forearm is shorter, the less bicep force is required to lift the weight, so now you should be asking yourself why smaller framed people with shorter limbs can’t curl massive weights?
Well, this is simple. smaller framed people also have smaller biceps (and smaller bicep potential) and thus can’t produce the same bicep force as a larger framed individual (this is obviously relative to training as someone with a small frame who has been lifting for a while can produce more bicep force than an untrained large framed person).
But what if they’re the same frame but the forearm is small in comparison to the rest of the frame?
Well the average forearm length for males and females is 275mm +/- 18mm. So let’s take someone with 257mm forearms (the smallest of this range) – yes, they can lift more if they can produce the same bicep force but only 6.5% more than the average as the length has only decreased by 6.5%. This means the insertion point has a much more dramatic effect on how easily you can curl a certain weight than forearm length does.