How hard should I train, and what do I need to drink?

Two formulas are in common use and they disagree. Fox (220 − age) is the famous one; Tanaka (208 − 0.7 × age) came from a meta-analysis and fits the data better, especially over 40. At 25 they differ by about 1 bpm; at 65 by nearly 8. Both carry a standard deviation of 10 to 12 bpm, so treat either as a starting point rather than a fact.

As a share of maximum heart rate: zone 1 is 50–60% (recovery), zone 2 is 60–70% (aerobic base), zone 3 is 70–80% (tempo), zone 4 is 80–90% (threshold) and zone 5 is 90–100% (VO2 max). Most endurance training should sit in zone 2, which usually feels easier than people expect.

Karvonen works from heart-rate reserve — maximum minus resting — instead of maximum alone, then adds resting heart rate back. For a fit person with a low resting rate, Karvonen zones come out noticeably higher than plain percentage-of-max, which is why the two methods can disagree by 10 bpm or more.

They are two names for the same scale. RPE 10 means 0 reps left in the tank, RPE 9 means 1, RPE 8 means 2, and so on down. Adding your target reps to your reps in reserve gives the total reps you could have done, and that maps onto a percentage of your one-rep max.

5 reps with 2 in reserve is a 7-rep-to-failure effort, which lands around 78 to 80% of a one-rep max on standard charts. The estimate is a population average: at the same 1RM, some people manage far more reps than others at the same percentage.

No. The number came from the name of a 1960s Japanese pedometer, not from research. The measured association with lower mortality plateaus around 7,500 to 8,000 steps a day in adults, and around 7,500 in older women. The largest gains come at the bottom of the range — moving from 3,000 to 6,000 does more than 8,000 to 10,000.

It depends heavily on style. By MET values, yin and restorative sit around 2, hatha 2.5, vinyasa and ashtanga around 4, and hot yoga around 5 — roughly the cost of a brisk walk. For a 165 lb person, an hour of vinyasa is about 300 calories and an hour of restorative about 150.

Much less than it feels. Heat raises heart rate and sweat rate sharply, but the extra energy cost is modest. The weight you lose in a hot class is water and returns with the next drink. Perceived effort in heat is a poor proxy for calories burned.

The ACSM position stand puts it at roughly 400 to 800 mL per hour during exercise, adjusted for sweat rate and conditions. For sessions under an hour, drinking to thirst is enough. Beyond an hour, add 300 to 600 mg of sodium per litre to reduce the risk of exercise-associated hyponatraemia.

Weigh yourself naked and towelled dry before and after a session, and add back anything you drank. The difference divided by the hours gives your sweat rate. It varies up to three-fold between people and rises sharply with heat, so measuring beats any table.

Not at habitual intakes. Controlled trials show moderate coffee consumption produces no meaningful difference in hydration status compared with water in regular drinkers. Coffee counts toward daily fluid intake. The FDA points to about 400 mg of caffeine a day — roughly four cups — as a level not generally associated with negative effects in healthy adults.

Replace 125 to 150% of the weight you lost, spread over the following 4 to 6 hours rather than in one go. That is about 1.25 to 1.5 litres per kilogram lost. Include sodium — plain water alone drives some of it straight back out as urine.