Context: Treadmills that unload runners via a differential air pressure bladder (DAP; e.g. AlterG Anti-Gravity Treadmill) are commonly used to reduce effective body weight in a clinical setting, however, the relationship between the level of unloading and tibial stress is currently unknown.
Objective: To determine the relationship between tibial impact acceleration and level of body weight (BW) unloading during running.
Setting: University motion-analysis laboratory.
Participants: 15 distance runners (9M, 6F; 20.4 ± 2.4 years; 60.1 ± 12.6 kg).
Main Outcome Measures: Peak tibial acceleration and peak-to-peak tibial acceleration was measured via a uniaxial accelerometer attached to the tibia during a 37-min continuous treadmill run that simulated reduced BW conditions via a DAP bladder. The trial began with a 10-min run at 100% BW followed by nine 3-min stages where BW was systematically reduced from 95% to 60% in 5% increments.
Results: There was no significant relationship between level of BW and either peak tibial acceleration or peak-to-peak tibial acceleration (p>.05). Both heart rate and step rate were significantly reduced with each 5% reduction in BW level (p<.01).
Conclusions: Although ground reaction forces are reduced when running in reduced BW conditions on a DAP treadmill, tibial shock magnitudes are unchanged as an alteration in spatiotemporal running mechanics (e.g. reduced step rate) may nullify the unloading effect.