Well trained runners who undertook 5 weeks of hot water immersion (5 sessions per week, 45 minutes per session, most sessions immediately following exercise) saw increased hemoglobin mass, total blood volume, and heart pumping capacity.
By the end of the 5 weeks, hemoglobin mass had increased by an average of 33 grams and VO2max had increased by 2.7 ml/min/kg. Treadmill speed at maximal exhaustion increased by 0.8 km/hr.
Running economy, treadmill speed at lactate threshold (4 mmol/L), and heart rate at lactate threshold did not change.
Improvements were similar to those found in studies using only active heat training.
The control groups did not see any of the above improvements.
Methodologies
10 runners, 9 men and 1 woman, were split into two groups. The athletes were well trained but not elite (VO2max of 56.5 to 72.5 mL/min/kg; 5–11 hours per week of training)
One group undertook 5 weeks of hot water immersion (5 x 45-minute sessions per week, starting at 40°C/104°F). The control group did nothing during those times.
After 5 weeks, the groups switched protocols (ie. the first control group switched to hot water immersion, and vice versa).
Most (75%) immersions took place immediately following exercise, when core temp was already elevated.
There was extensive pre/post testing of cardiac, blood, and performance markers.
There were no changes to training volume or training intensity.
CORE’s Hot Take
This study offers more support that heat training is an excellent substitute for altitude training for those competing at low elevation.
Most (75%) of the hot water immersion sessions were ‘exercise-enhanced’ – meaning they were done immediately following exercise. This is a critical point for two reasons:
It is unknown if the adaptation arose from the hot water immersion alone, or because exercise-induced hyperthermia was extended. In other words, it is unknown if adaptations would have arisen if immersion was started while core temp was at baseline.
The total Heat Training Load from a 45-minute session started from baseline core temp is approximately half of that started immediately after exercise (when core temp is already elevated). See CORE’s Passive Heat Training Calculator for examples.
Total immersion time was 3:45 hours/week. CORE’s Sweatlabs Team commented that, "Most age-groupers are time-crunched, and their performance is limited by total training hours. They’d likely see greater overall performance gains by spending that extra 3:45 hours/week on the trainer or treadmill doing active heat sessions. They’d get more cardio and muscular training in addition to the heat-induced gains."
Sitting in a post-exercise hot bath is vastly different from a ‘leisurely bath on a Saturday night’. Many find it more uncomfortable than active heat training. Study participants on average ranked their thermal discomfort as a 6 (on a scale where 1=comfortable, 10=extremely uncomfortable), with the range being 4 to 8.
Moving Forward
This study provides valuable information and raises interesting questions. We’d like to see follow-up studies to answer the following questions:
How does adding 3–4 hours/week of passive heat training compare to adding 3–4 hours/week of active heat training?
What is the minimum dose of exercise-enhanced passive heat training required for hemoglobin and performance gains?
Does passive-only heat training (ie. starting from baseline core temp) give these same hemoglobin and performance gains?
Do women get the same gains from exercise-enhanced passive heat training (only 1 of 10 study participants was female).
Conduct your own thermal research with CORE Calera®
