Blog By Dr John Cronin
Some of you out there might think that introducing wearable resistance (WR) to your running with either trunk, arm, thigh or calf loading, increases the potential for injury, because you are adding load to the body so the forces you experience at foot strike are going to be greater. Well, let me allay your injury concerns and in fact in a later article I will explain how you can use WR to improve your injury resistance to common running injuries.
Before I get sidetracked, let’s go back to this initial concern of the additional weight potentially causing greater impact forces at foot strike and, therefore, heightening the chance of injury. These forces at foot strike are called ground reaction forces (GRF) and are measured by force plates that are embedded in the ground (Figure 1A) or within a treadmill Figure 1B).
Figure 1: In ground (A) in treadmill (B) force plates.
These force plates measure GRF in three planes of motion – mediolateral, anterior-posterior (horizontal) and vertical as shown in Figure 2. As you can see from the diagram the vertical GRFs (vGRF) are by far the biggest forces and the ones we are concerned with in terms of adding load and causing injury. These vertical forces are a function of how much you weigh and the acceleration due to gravity. If you weigh more you will have a greater vGRF and if you (or more specifically your centre of mass – which is near your navel) drop from a higher distance the effects of gravity will have a greater influence on these forces.
So theoretically with the addition of WR you have additional mass and therefore greater vGRFs. However, this is not the case and there is research that has shown the addition of quite substantial masses (e.g. 10% body mass there has been a reduction in the vGRF)1,2. How can this happen? Remember your vGRF is made up of two components, even though we are increasing the mass of the runner the rise and fall of their centre of mass is less as I have tried to show in Figure 3. The runner’s centre of mass on the right does not rise as high because of the vest loading, and therefore the effects of acceleration due to gravity are less.
Figure 3: Effects of vest loading on the centre of mass of the runner.
Now let me give you another couple of insights as why we don’t need to worry about these vGRF. First, the dominant loading we use with runners is calf or shank loading as shown in Figure 4. Now we usually use loads of 1 to 2% body mass on the shank so if you weigh 70 kg then that is between 700 to 1400 grams of loading, which equals 350 to 700 gms on each calf sleeve. This is very light loading and so therefore not a lot of additional mass on the runner and hence not a great change in the vGRF. The real game changer with limb loading, however, is that the overload we are exposing the muscles to is a rotational overload (see Figure 5) and therefore the effects on vGRF will be minimal.
Figure 4: Calf sleeves for running.
Hopefully you have not found this too academic and you understand why using WR has minimal effects on your vGRF at foot strike. The take home message is that limb loading is a rotational overload which has fascinating implications for the strengthening of the muscles around the hip and knee. In fact, I believe WR has an important role in improving resistance to common running injuries, however, that is another story so please stay tuned. Until then, slap those calf sleeves on and enjoy the benefits of resisted running.
Figure 5: Diagram showing the differences between vertical (vest loading) and rotational overloads at the hip (shorts) and knee (calf sleeves) Hip.
1 Cross, MR, et al (2014). Effects of vest loading on sprint kinetics and kinematics. J Strength Cond Res 28(7): 1867–1874.
2 Couture et al (2018) Effects of upper and lower body wearable resistance on spatio-temporal and kinetic parameters during running. Sports Biomechanics, https://doi.org/10.1080/14763141.2018.1508490
Dr. John Cronin is recognised internationally as one of the world’s leading sports scientists. He is a Professor of Strength and Conditioning at Auckland University of Technology’s Sports Performance Research Institute New Zealand. As our Head of Research, Dr. Cronin oversees all EXOGEN® wearable resistance research globally.
The Sports Performance Research Institute New Zealand (SPRINZ) is New Zealand’s number one rated sports research institute with a growing global reputation. SPRINZ is a group of dynamic and innovative researchers producing applied research in improving human health, sports performance and long-term athletic development.