Metabolic responses to submaximal incline unweighted running

PURPOSE: The purpose of this study is to examine the metabolic changes to running on an unweighted treadmill system at 80% (80BW), 90% (90BW) and 100% (100BW) body weight. The second purpose was to find a speed and an incline that will offset the reductions of the metabolic cost of running at a decreased body weight. METHODS: 17 healthy, active participants (8 females, 9 males) performed incremental tests to exhaustion at 100BW, 90BW, and 80BW on an unweighted treadmill system (Lightspeed Lift). Participants then ran at 70% vVO2max at 90BW and 80BW at an incline of 0% (BW-0), 2% (BW-2), and 7% (BW-7) for 5 minutes each. Heart rate (HR), oxygen consumption (VO2), respiratory exchange ratio (RER), blood lactate, and stride length were measured during all sessions. RESULTS: Speed at 70%vVO2max significantly increased as unweighting increased (F2,32 = 37.7; p< 0.001, eta2=0.70). HR and VO2 were not significantly different between 100BW, 90-0, 90-2, 90-7, and 80-7 (p>0.05), but were both significantly lower at 80-0 (HR: MD=13.1 bpm, p=0.002; VO2: MD=6.1 mL/kg/min, p=0.011) and 80-2 (HR: MD=10.7 bpm, p=0.011; VO2: MD=5.3 mL/kg/min, p=0.025). RER was not significantly different between 100BW, 90-0, 90-2, 80-0, and 80-2 (p>0.05), but was significantly higher at 90-7 (MD= -0.04, p=0.010) and 80-7 (MD= -0.03, p=0.027). Blood lactate was not significantly different between 100BW and 90-7 (p >0.05), but was significantly lower at 90-0 (MD=1.3, p=0.016), 90-2 (MD=1.4, p=0.007), 80-0 (MD=1.9, p=0.000), 80-2 (MD=1.8, p=0.000), and 80-7 (MD=1.3, p=0.003). Stride length was not significantly different between 100, 90-0, 90-2, and 90-7 (p >0.05), but was significantly longer at 80-0 (MD= -0.35 m, p=0.001), 80-2 (MD= -0.35 m, p=0.001), and 80-7 (MD= -0.33 m, p=0.002). CONCLUSION: Unweighting reduces the metabolic cost of submaximal running. Increased speed and incline offset the reductions at decreased body weights. Running at 90-0, 90-2, 80-0, and 80-2 will keep RER unchanged compared to 100BW. Running at 90-0, 90-2, 90-7, and 80-7 will keep HR and VO2 unchanged compared to 100BW. Running at 90-7 will keep blood lactate unchanged compared to 100BW. Running at 90-0 and 90-2 will keep HR, VO2, RER, and SL unchanged compared to 100 BW. Athletes using 90BW training to increase training volume or rehabilitate from injury should consider a zero to two percent increase in incline during training to offset the cardiometabolic cost reduction of unweighting. Any greater unweighting or incline change may produce a training response that is not intended. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.