So what's inertia and can you use it for rehab and/or to simply get stronger for running? Inertia is the resistance of a body to change in motion and is a function of mass. For example, if 400 grams is placed on the calf as in the runner in the picture, then the inertia of the calf and leg has been increased as the limb is 400 gm heavier and therefore requires more muscular effort to accelerate and decelerate. It also needs to be remembered that the inertia or overload that the wearable resistance provides needs to be thought of in terms of load relative to limb mass and not body mass. So that 400 gm has different inertial qualities depending if it is placed on the trunk, thigh or calf.
Training Insight: By simply making your body/limb heavier, greater muscular work/strength is needed to move the body/limb at the same velocity as unloaded i.e. strength training whilst you run. The magnitude of this overload/inertia is increased depending on where the mass is placed i.e. load relative to limb mass. Understanding the weak musculature in the runner can help determine where to add the additional mass. For example, if the muscles across the hip need strengthening, then thigh loading would be the preferred training option. Training loaded and competing unloaded would seem an intuitive training method to improve running performance.