## Blog By Dr John Cronin

So how can light loads of 200-400 gm (7 to 14 oz) be called resistance strength training? How can the RPE be 25-30% (previous post) harder when sprinting with such loads as compared to an unloaded sprint? A lot of the explanation comes down to a concept called rotational inertia. So if you want to learn how rotational inertia contributes to rotational overload buckle up, the wheels are about to spin.

Inertia

Inertia is the resistance of a body to change in motion and is a function of mass. For example, if you place 400 grams on your thigh as in Figure 1A, then your thigh is 400 gm heavier and therefore requires more muscular effort to accelerate and decelerate. Have a look at Figure 1B, where there is 400 grams placed on the thigh once more. So it weighs the same as Figure 1A. However, do you think it is requires more, less or the same muscular effort to get it going compared to A? It requires more as that loading has greater rotational inertia. Let me explain.

Rotational Inertia

So instead of inertia, rotational inertia is what we are really interested in when talking rotational overload with limb loaded wearable resistance (WR), and it is important to understand, if you are to overload with WR safely and effectively, so tune into his next paragraph. The formula for rotational inertia is I = mr2  where I = rotational inertia; m = mass; and, r = distance from axis of rotation. So let’s take the thigh as an example, we know the thigh has mass and therefore requires rotational force (torque) to move it. The larger the thigh mass the more muscular effort (torque) is required by the hip flexors and extensors. So by simply adding more WR to the thigh we increase the rotational inertia of the thigh, which means more muscular effort or turning forces/torques are required at the hip joint. But let’s not forget the second part of the rotational inertia formula (r2), which translates to where we put the mass is really important. In fact this has more of an influence on rotational inertia (muscular effort) as any distance change is squared. Figure 1: A 400 gm/14 oz load attached mid thigh (A) and distal thigh (B) Figure 2: Effects of mid and distal thigh loading on rotational inertia

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