Training in Heat and Humidity on the Rowing Machine

It is 28 °C in the garage at seven in the morning and the 2k you just rowed was slower than the same piece in April. Nothing about your fitness changed. The room did. An erg is a heat-generating machine sitting in still air, so on a close summer morning the room is not scenery around the session. It is a load the session has to pay for.

The context

External thermal load depends on ambient temperature, humidity, wind speed and solar radiation ([12] Bergeron 2012). An erg room gives you the first two and none of the last two. On the erg the air is still unless you make it move, the machine turns your effort into heat in your body, and the ambient load is whatever the room happens to be that day: a garage with the sun on the door, a basement gym with no window, a boathouse loft with the radiator on since April.

Humidity changes the character of the problem rather than simply adding to it. Evaporation is the dominant route for shedding heat, and humid air slows it down; convection and radiation still carry part of the load, and air hotter than your skin can add heat instead of taking it away. In a crossover study that matched hot-dry and warm-humid conditions on wet-bulb globe temperature, evaporative heat loss ran at 450 W in the dry condition and 319 W in the humid one, with dry heat loss moving the other way: the hot-dry riders at 39.2 °C were taking on dry heat while the warm-humid riders at 34.2 °C were losing it ([9] Zhao 2026). On a humid day you keep sweating, keep losing fluid, and lose it for less cooling than you paid for.

Two athletes live with this: the one who trains indoors all winter and then meets a July race on the water, and the one who trains indoors in July because the river is too warm to row. Both are running an unintended heat adaptation, in a room nobody is measuring.

The core idea

Here is the part that surprises most rowers, and it comes from physiology rather than coaching folklore: the variable that costs you the split is your skin temperature, not your core temperature.

Hot skin, above roughly 35 °C, forces high skin blood flow so the body can dump heat, and that flow competes with the muscles for cardiac output. The human data show that hot skin alone impairs submaximal aerobic performance even in people who are not dehydrated, and conversely that performance is sustained with core temperatures above 40 °C as long as the skin stays cool to warm ([4] Sawka 2012). Read that twice: a core temperature of 40 °C is not what ruins your 2k. A hot skin is.

Dehydration is the second variable, and it sharpens the first. Body water losses above 2 per cent of body mass define hypohydration, and in hypohydrated people aerobic performance begins to be impaired once skin temperature exceeds 27 °C, with a further 1.5 per cent of impairment for every additional degree of skin temperature ([4] Sawka 2012). Look at that threshold in a still erg room: 27 °C is not extreme. It is roughly what your skin reaches in a warm room with no moving air.

The third variable is the one you control before you start: the position stand on fluid replacement is blunt that the goal of drinking during exercise is to prevent dehydration of more than 2 per cent of body weight, and because sweat rates and sweat electrolyte content vary considerably between individuals, replacement has to be customised ([1] American College 2007).

So the mechanism in one line: a still, warm, humid room raises skin temperature, hot skin steals blood flow from the legs, and dehydration makes the steal worse.

The how-to

1. Move the session out of the middle of the day. Earliest or latest slot you can manage. A room at 22 °C at six in the morning and 30 °C at six in the evening are two different rooms. If you cannot move the session, move the machine: out of the sun-splashed corner, away from the boiler, near the door. And wear the lightest thing you own: clothing and protective gear can measurably increase thermal strain and prompt premature fatigue ([12] Bergeron 2012).

2. Aim the fan at your face and leave it running. This is the biggest lever you have, and it is bigger than ice. In a climate chamber at 30 °C and 50 per cent relative humidity, ten fit cyclists at 95 per cent of ventilatory threshold lasted 28 ± 12 minutes with neither precooling nor airflow. Airflow of roughly 4.8 m/s lengthened that by 30 ± 23 minutes, about 109 per cent ([6] Morrison 2014). Chest-deep immersion added 16 ± 15 minutes, and the two combined were no better than the fan alone ([6] Morrison 2014). Put the fan at face height, a metre or two in front, and run it for the warm-up too. Aim it at your face and forearms rather than your feet: the mechanism is skin temperature, and that is the cheapest skin you have.

3. Treat pre-cooling as the second lever, not the first. If the room is hot and you have no fan, pre-cooling does work. A meta-analysis of 27 randomised controlled trials, 18 of them above 26 °C, found pre-cooling improved performance by 6.6 per cent in the heat against 1.4 per cent in moderate temperatures, with the largest gains in time trials at 4.2 per cent ([5] Wegmann 2012). Cold drinks and cooling packs produced the biggest effects, a cooling vest a moderate one, and water application the smallest ([5] Wegmann 2012). The same review cautioned that firm conclusions cannot yet be drawn, so treat this as a real but modest lever, and remember that a working fan removes most of the need ([6] Morrison 2014).

4. Replace what you sweated, fluid and sodium. Weigh in naked and dry before the session and again afterwards, then write the change down. Your sweat rate is roughly the mass you lost plus the fluid you drank, divided by the hours, and individual sweat rates are estimated exactly this way ([1] American College 2007). One hot session gives you a starting estimate for that day's conditions, not a constant to carry through the summer; until you have it, keep the loss under 2 per cent of body weight ([1] American College 2007). Then make sure the replacement carries sodium: active athletes in hot weather can lose 4 to 10 litres of water and 3,500 to 7,000 mg of sodium in a day, and both must be replaced to get back to euhydration ([7] Shirreffs 2011). For a long hot session that means an electrolyte mix rather than plain water. Do not over-drink either: exercise-associated hyponatremia comes from drinking ahead of thirst, and it arrives wearing heat illness's clothes — nausea, headache, vomiting and confusion ([11] Altieri 2026). Thirst is a reasonable guide; a schedule that forces fluid down regardless is not.

5. Give the body a fortnight. The consensus on training in the heat is unambiguous: heat acclimatisation is the single most important intervention available, and it should comprise repeated exercise-heat exposures over one to two weeks ([2] Racinais 2015). The adaptations are concrete: improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain and better fluid balance, with the response set by the intensity, duration, frequency and number of exposures, and by whether the heat is dry or humid ([3] Périard 2015). You do not need a heat chamber: two weeks of normal sessions in the hot room, properly hydrated, is the protocol, with the caveat that the adaptation is specific to the conditions you trained in ([3] Périard 2015).

6. Hold the effort, not the split. In a cohort of 2,560 people, Borg's rating of perceived exertion correlated strongly with heart rate at r = 0.74 and proved a valid tool for monitoring and prescribing exercise intensity regardless of gender, age or exercise modality ([8] Scherr 2013). In the heat the same split is a more expensive version of itself, so the rower chasing it runs out of session early. Pick an effort instead, around 13 to 15 on the Borg scale for a hard aerobic piece, and accept the split that comes with it. Write the target down first; in the last 500 m you will not be able to judge it.

7. Know where the session stops. This is the part that matters most. Exertional heat stroke is defined as a rectal temperature above 40 °C with signs of organ system failure, most often central nervous system dysfunction, and in some cases rectal temperature is the only on-site difference between severe heat exhaustion and heat stroke ([10] American College 2007). Early recognition and rapid cooling reduce the morbidity and mortality ([10] American College 2007). So the line you draw is not a number you cannot measure. It is neurological: confusion, stumbling, slurred speech, or a partner saying your words do not make sense. Stop, get someone, cool fast. Expect cramps too, since exercise-associated muscle cramping is more prevalent in hot and humid conditions and usually settles with rest and fluid and salt ([10] American College 2007). Expect fatigue early; exhaustion comes faster as heat stress climbs ([10] American College 2007).

The trade-offs

Three honest limits. The first is that almost none of this literature is about rowers: the physiology comes from treadmill protocols and cycling in climate chambers, and the heat-illness guidance from team-sport and mass-participation events. There is no meaningful body of peer-reviewed work on heat and humidity in indoor rowing specifically. This article adapts other sports' evidence to a machine you cannot add wind, shade or boat speed to, and whose room is a different environment every summer.

The second is that these are laboratory protocols. The airflow result came from cycling at 95 per cent of ventilatory threshold in a chamber at 30 °C and 50 per cent relative humidity, with a measured 4.8 m/s of fan ([6] Morrison 2014). The humidity result came from ten non-acclimatised men, on a bike, for an hour ([9] Zhao 2026). A garage in July is not a climate chamber, and nobody has tested a 2k on an erg at 28 °C and 70 per cent humidity.

The third is that the measurements you take are imperfect and the thresholds are population averages. Sweat rate varies considerably between individuals, which is exactly why replacement has to be personalised rather than generic ([1] American College 2007). And the index most sports use, wet-bulb globe temperature, does not capture everything: the researchers who matched two environments on it concluded that ambient temperature, humidity and workload all have to be considered alongside it when choosing a heat-mitigation strategy ([9] Zhao 2026). The practical version for a rower: the room thermometer is a rough proxy, and the fan is not optional.

What to do tomorrow

Four things, in order.

1. Weigh in before your next session and again straight after it, and write the number on the wall by the erg. One session gives you a starting estimate for that day's conditions, not a constant: sweat rate moves with the temperature, the humidity, how long you row, what you wear and how acclimatised you are. Repeat the weigh-in when the room changes and update the number.
2. Put the fan at face height and leave it running for the whole piece, warm-up included. If you own one fan, that is where it goes.
3. Fill a second bottle with an electrolyte mix and leave it beside the erg. Water is fine for half an hour in a cool room; it is not the whole answer for an hour in a hot one.
4. Decide your stop rule before you start and say it out loud to whoever else is in the room. Confusion or stumbling ends the session, not the last piece.