Background: The energy cost of transport (CoT; J.kg-1.min-1) displays a U-shaped curve during walking, which intersects with a linear running CoT. The bottom of the U represents the economical speed (ES) and the intersection between walking and running. CoT represents the economically optimal transition speed (EOTS). Tissue saturation index (TSI: (oxyhemoglobin (O2Hb)) ÷ (O2Hb + deoxyhemoglobin (HHb)) × 100) is a method of quantifying cellular respiration and oxygen consumption in local muscle tissue, and so could be said to assess a different part of the oxygen transport chain compared to whole body CoT measures. Furthermore, environmental factors such as heat and altitude influence a range of central and peripheral physiological responses and could influence CoT and TSI in different ways. However, to date no study has quantified CoT and TSI during exercise in heat and hypoxia.

Aim: To determine the influence of heat and hypoxia on both CoT and TSI, and to assess the impact these environmental conditions have on the relationship between CoT and TSI. An additional aim was to determine the influence environmental conditions have on markers of economy (ES & EOTS) during walking and running.

Methods: Using a repeated measures cross-over study design, 15 participants (Mean ± SD: age: 25.2 ± 3.8, weight: 70.1 ± 11.2kg, height: 176.7 ± 8.8cm) performed four experimental trials with eight walking stages (2.4, 3.1, 3.8, 4.5, 5.2, 5.9, 6.6, 7.3 km.h-1) and four running stages (7.3, 8.0, 8.7, 9.4 km.h-1), separated by 2-3 days. Each trial was performed in one of four environmental conditions (control: 0 m, 21.9 ± 1.3°C, heat: 0 m, 36.1 ± 0.7°C, moderate hypoxia (MH): 1800 m, 22.1±1.2°C, and severe hypoxia (SH): 3600 m, 23.1 ± 1.2°C) in a randomized order. The pulmonary oxygen uptake (VO2, mL/min-1) and carbon dioxide output (VCO2, mL.kg-1.min-1) was measured using a computerized breath-by-breath system. VO2 and VCO2 were subsequently used to calculate CoT. Local tissue oxygenation of the vastus lateralis muscle (TSI) was measured continuously using near infrared spectroscopy (NIRS).

Results: Walking CoT for SH and MH were both significantly (10.2%, P=<0.001, and 9.4%, P=0.036, respectively) elevated compared to control. Running CoT for SH and MH were also significantly (11.8%, P=0.0.21, and 9.8%, P=0.020, respectively) higher than running control. CoT for either running or walking in the heat was not significantly different than control. Running SH TSI was, overall, significantly lower (-14%, P=0.003) than running control. SH TSI each individual stage above W4.5 was significantly different to the respective control stage, but not significantly different at slower velocities. No significant difference was found in the ES or EOTS of any of the environmental conditions compared to control.

Conclusion: Out of all the environmental conditions, only SH resulted in significant differences of both CoT and TSI, in relation to control during walking and running. MH resulted in a significantly altered CoT, while Heat presented no significant difference in either CoT or TSI compared with control. Both hypoxic conditions significantly altered breathing variables and so it is concluded that the alveolar pulmonary exchange is a key factor in assessing CoT. Hypoxic levels need to be severe enough to alter the oxygen transport chain sufficiently, to thus influence cellular metabolism and alter TSI values simultaneously with CoT values. CoT and TSI displayed similar yet inverse relationships during walking. The highest TSI values often fell at the lowest CoT values (ES), demonstrating a clear relationship between the two measures. Economic markers were not shifted as a result of the environmental conditions, as CoT shifted only vertically within the study. These findings suggest that sufficiently severe hypoxia applied, during walking and running, can alter the entirety of oxygen transport chain and could thus be applied by practitioners to amplify adaptations in the efficiency of the oxygen transport chain.