Running on the hypogravity treadmill AlterG® does not reduce the magnitude of peak tibial impact accelerations

Abstract

Summary Background Hypogravity treadmills like the AlterG® have become a popular exercise tool especially in return-to-sports and return-to-competition rehabilitation programmes for distance runners, triathletes, footballers and further sports disciplines. By applying a slight positive air pressure in a chamber around the lower limbs, the effective bodyweight force is reduced, thereby reducing gravity load to the musculoskeletal system. So far it was unclear in how far that hypogravity unloading also affects peak accelerations at the tibiae, a common site of running-related overuse injuries. Material and Methods Fifteen well-trained male runners (peak oxygen consumption of 60 ± 4 ml min-1 kg-1) completed three incremental treadmill tests until volitional exhaustion in randomised order, two of which on the hypogravity treadmill AlterG® at 80% and 60% effective bodyweight, respectively, and the third on a conventional treadmill, i.e. at 100% gravity. Triaxial accelerations at the distal portions of both tibiae were captured throughout the three (hypo-)gravity conditions and all running speeds, along with the physiological cost of running in terms of heart rate, oxygen consumption and rating of perceived exertion. Results Mean peak tibial accelerations at impact and active push-off amounted to 12.9 ± 2.3 g0, 12.6 ± 1.9 g0 and 12.5 ± 2.3 g0 for 12–18 km h-1 at 100%, 80% and 60% effective bodyweight, respectively (g0 ≈ 9.81 m s-2). They were not reduced by hypogravity unloading within measurement uncertainty (p = 0.668). However, they exhibited a clear dependence on running speed (p elevated on the AlterG® (p Conclusions Distal tibial accelerations are not reduced in hypogravity running on the AlterG® treadmill in terms of absolute speeds, and are elevated in terms of relative speeds, i.e. at running speeds demanding the same physiological effort. Thus, precaution should be taken by clinicians and coaches when planning rehabilitation programmes for athletes with a recent injury background at the distal tibiae. The cause for the ineffectiveness of hypogravity running in reducing tibial acceleration load is most probably a subconscious energetic optimisation in human running motor control: For energetic reasons, the leaps of each step become flatter under hypogravity instead of steeper, whereas the latter would have intuitively been expected because of the lower effective acceleration of free fall. Level of Evidence: Ib