In a previous post I discussed how we can use limb loaded resistance to increase the inertia of the limb whilst running and therefore provide additional resistance and hence a sport specific strength stimulus. Let’s take this a step further.
Figures A and B have the same mass added to the thigh 400 gms (14 oz). However, in Figure B 400 grams has been placed further down the thigh, which is called distal loading. This shift down the thigh has increased the muscular effort required to move the limb as compared to the more proximal (closer to the hip joint) mid femur loading in Figure A. The reason for this is that the rotational inertia of the limb has been increased, this mechanical concept important to understand, so as progressive and safe overloading of wearable resistance (WR) can take place.
Here's the why… The formula for rotational inertia is I = mr2 where I = rotational inertia; m = mass; and, r = distance from axis of rotation. By placing mass (m) on the thigh, the inertia has been increased, however, the placement of this load, as in the distance from the axis of rotation, which is the hip joint in this case, also has a significant effect. In fact, the effect of placement is greater than adding mass, as any change in distance from the axis of rotation is exponential (r2). That is, placement has more of an influence on rotational inertia (muscular effort) as any distance change is squared.
Training Insight: In terms of limb loading, initially place loads close (proximal loading) to the axis of rotation (e.g. hip or knee joint) to minimise the effects of rotational inertia. As you adapt to the WR and gets stronger, then move the loads distally (away from the joint) to increase the rotational inertia and therefore the muscular effort/strength stimulus for every step. Definitely has applications for return to training/play and performance.