Stability of the K4b² portable metabolic analyser during rest, walking and running

This study investigated the stability of the measurement of respiratory variables during rest, walking and running using the K4b² portable metabolic analyser in ten active males (age 31 ± 11 years; V ̇O₂ peak 42.1 ± 2.6 ml · min^-1 · kg^-1). Following a 10 min rest, participants completed three discontinuous incremental exercise tests on a treadmill while walking (4, 5 and 6 km · h^-1) and running (8, 10, 12, 14 and 16 km · h^-1, or until volitional exhaustion). Participants completed 3 min of exercise at each speed, followed by a 3 min recovery after each stage above 10 km · h^-1. The respiratory variables were measured using either a laboratory-based metabolic cart as a reference method (Oxycon Pro, OP), a K4b² calibrated immediately before the test (K4b²), or a K4b² calibrated 1 h before the test (K4b²DEL). Compared to the OP, carbon dioxide production (V ̇CO₂) and V ̇_E were not different when measured by K4b² or K4b²DEL. There was no difference in V ̇O₂  between OP and K4b² tests (P = 0.19, mean difference = 38 ml · min^-1 and limits of agreement (LOA) = 208 to -285) although K4b²DEL overestimated V ̇O₂ (P = 0.05, mean difference = 84 ml · min^-1 and LOA = 302 to -469). These data suggest that a drift in measurement accuracy appears to cause the K4b² to overestimate V ̇O₂ in tests lasting longer than 1 h.