The Effect of Acute Passive Heating on Microvascular Oxygen Delivery, Exercise Tolerance and Recovery

Abstract

The purpose of this study was to determine the effect of 1 bout of passive heating on oxygen delivery during knee extension exercise and neuromuscular recovery. We tested the hypotheses that: 1) 1 bout of passive heating would result in a decrease in the diffusive oxygen delivery (total‐[heme]) and an increase in the perfusive oxygen delivery (deoxy‐[heme]) in the exercising muscle, 2) peripheral fatigue would recover at a faster rate post heating and 3) passive heating would increase time to exhaustion.MethodsTwo men (183.9 ± 5.6 cm; 74.2 ± 10.8 kg) and two women (160.5 ± 0.7 cm; 53.8 ± 0.3 kg) performed intermittent isometric knee extension tests to exhaustion at 40% MVC pre and post 1 bout of passive heating. Absolute concentrations of deoxy‐[heme] (perfusive oxygen delivery) and total‐[heme] (diffusive oxygen delivery) of the vastus lateralis muscle were measured continuously via frequency‐domain multi‐distance near‐infrared spectroscopy (OxiplexTS, ISS) during exercise. Neuromuscular measurements of central (as measured by maximal voluntary contraction force, MVC; and voluntary activation, VA) and peripheral fatigue (as measured by potentiated twitch force, Qtw) were made every 30 s pre exercise and immediately after task failure for a total of 6 sets each. The passive heating protocol consisted of immersion up to the shoulder in a 40°C hot tu b until rectal temperature reached 38.5°C or increased by 1°C for 60 minutes.ResultsTime to exhaustion for pre and post passive heating was 553 ± 296 sec and 808 ± 491 sec, respectively (p=0.227), but 3 of the 4 subjects showed an increase in time to exhaustion post heating. From baseline to the last 60 seconds of exercise there was no significant difference in the increase in deoxy‐[heme] for pre (47.5 ± 30.8 μM) and post passive heating (38.5 ± 21.5 μM; p=0.526). Likewise, the increase in total‐[heme] during exercise was not significantly different following passive heating (p=0.117) (pre and post passive heating Δ total‐[heme] was 20.8 ± 19.8 μM and 8.3 ± 13.4 μM, respectively). There was no difference in baseline MVC pre to post heating (p=0.841), but MVC was significantly decreased following exercise for both pre and post passive heating (p<0.001). % change in VA from pre to post exercise was not different between pre and post heating (p=0.892). Qtw was not significantly different post exercise between pre to post heating. However, Qtw began to recover faster post compared to pre heating following exercise to exhaustion (30 sec vs. 90 sec, respectively).ConclusionThe trend of increased time to exhaustion after 1 bout of passive heating cannot be explained by increased perfusive or diffusive oxygen delivery. Passive heating led to a faster onset of recovery of peripheral fatigue (Qtw) following exercise to exhaustion, but the indices of central fatigue were not different pre vs. post heating.