What is "harder" for a hamstring curl?

[hamstringquestion]

TL;DR

hamstring curl for short legged or long legged person... Which is harder?

Two friends. One is shorter, one is taller. They get on the same hamstring curl machine.

Both adjust the pad where pressure is applied so that it's just above their ankles as shown in the gif below. The taller friend with longer legs has the pad extended further away from the machine. The shorter person with the shorter legs has it closer to the machine.

Is it "harder" for the tall person or the short person to do the same amount of weight? Shorter person says it's easier for the tall person because of the leverage (like using a breaker bar extension with a wrench).

Taller person says it's easier for the shorter person because of where the hamstring is in relation to the work being done (if you were holding a 1 foot long pole at one end and a 5 lb weight is attached on the other and you rotate it up at a similar angle to the leg curl, it would get more difficult the longer the pole is to raise that same 5 lb weight).

What is correct?

Harder for the long leg or the short leg??

 

[Orodruin]

Assuming that the axis of rotation for the machine coincides with that of the knees, it does not really matter how long the legs are. What is needed is a particular torque and therefore the relevant question in terms of the force generation of the leg is the moment arm of the muscle, ie, how far from the knee joint the muscle attaches to the bones of the lower leg. This is not necessarily directly tied to the length fo the lower legs.

 

[Drakkith]

Shorter person says it's easier for the tall person because of the leverage (like using a breaker bar extension with a wrench).

This particular aspect is called mechanical advantage, and is actually worse for someone with a longer leg (assuming the attachment point for the tendon is located at the same distance from the joint for both people). The leg is a class 3 lever, and a person with a longer leg will have the load placed further from the fulcrum (joint), reducing mechanical advantage.

This in itself doesn't mean it is 'easier' for a shorter person, as there are various other factors to take into account, but it is part of it.

 

[Orodruin]

The leg is a class 3 lever, and a person with a longer leg will have the load placed further from the fulcrum (joint), reducing mechanical advantage.

The load from the weights is generally not applied directly to the contact point of the leg and pad. It is usually applied to a winch through a line attached to the weight. Since the winch remains the same radius regardless of the placement of the pad, the torque generated will be the same in both cases. Ther relevant mechanical advantage is the tendon-joint distance vs the winch radius and leg length is irrelevant (apart from being possibly correlated to tendon-joint distance).

 

[Drakkith]

Since the winch remains the same radius regardless of the placement of the pad, the torque generated will be the same in both cases. Ther relevant mechanical advantage is the tendon-joint distance vs the winch radius and leg length is irrelevant (apart from being possibly correlated to tendon-joint distance).

I understand the torque needed to lift the weight is the same in both cases since that depends on the winch radius and the chosen weight, but surely the leg length and pad-bar length matters too. Right? Assuming the two lengths don't end up cancelling each other out, that is. A longer leg would apply less force on the pad, but the longer pad-bar length would give better advantage. Vice-versa for a short leg.

Is my thinking incorrect?

 

[Orodruin]

My point is that the lengths should cancel out (at least approximately) as long as the knee joint is more or less on the axis of rotation of the machine. The longer leg will require less force on the pad, bit it still translates to a need to generate the same torque - which is generated by the hamstring muscle at the tendon connections to the lower leg. So the relevant anatomy for how much force the hamstring needs to generate is tendon-joint distance, not leg distance per se.

Of course, this is all neglecting everything but the machine load.