From your first 5K to Khardung La, it's all physiology.

A long-standing rule of thumb from exercise physiology: running on the flat costs about 1 kcal per kilogram of body mass per kilometre (≈ 4.2 kJ/kg/km), and the figure changes surprisingly little with speed. In oxygen terms, that's about 200 mL of oxygen per kg per km; each litre of oxygen you use releases about 5 kcal from food, so the two rules agree. Individual runners vary by roughly ±10–20%: that spread is running economy.

What changes your economy

• Training: years of running, plus strength and plyometric work, generally improve economy.
• Technique and springiness: tendons such as the Achilles store and return elastic energy each stride (Ch 2).
• Shoes: lighter shoes cost less to swing, and studies of some modern foam-and-plate racing shoes report a few per cent better economy for many runners. The size of the effect varies by person.
• Hills, wind and surface: uphill costs extra; trails and sand cost more than smooth roads; a headwind adds air drag, which is small for most runners at everyday paces but grows with speed squared.

Walking always keeps one foot on the ground. Running has a flight phase, when both feet are off it. Over a whole stride you don't accelerate up or down on average, so the ground's average push must equal your weight (Newton's second law). But the ground only gets to push while your foot is down, so during contact the force has to be much larger.

Springs in your legs

When your foot lands, tendons (especially the Achilles) and the arch of the foot stretch and store elastic energy, then release much of it as you push off, like a bouncing ball. That's a big part of why running is cheaper than its forces suggest. Fatigue, very stiff or very soft surfaces, and long downhills change how well this works.

Cadence and step length

Speed = cadence × step length. At 5:00/km (200 m per minute) and 170 steps per minute, each step is about 1.18 m: about 36,000 steps in a marathon. Many coaches suggest a slightly quicker cadence for runners who overstride (landing with the foot far ahead of the body), and studies find that modest cadence increases reduce loading at the knee and hip. There's no single "correct" footstrike: changing how you land mostly shifts load between the knee and hip and the calf and Achilles, so change gradually.

VO₂ max is the most oxygen your body can take up and use per minute, usually in mL per kg per minute. It depends on how much blood your heart pumps and how well your muscles extract oxygen. Many untrained adults are in the 30s to 40s; elite endurance runners can exceed 70. Training raises it, especially in the first months; age gradually lowers it.

The oxygen a pace needs is simply economy × speed. With 200 mL/kg/km:

Thresholds: how close can you run to the ceiling?

As effort rises, there's a point where lactate starts building up in the blood faster than your body can clear it. Above this lactate threshold, fatigue accumulates quickly. Trained runners can hold a higher share of their VO₂ max before they reach it, so they can race longer at a higher percentage. Very roughly, short races are run near the top of the range and marathons well below it.

Pacing

Because oxygen cost rises with speed and anything above threshold accumulates fatigue, an even pace (or a slightly faster second half) is usually the most economical way to race. Starting too fast borrows from later and spends carbohydrate quickly (Ch 4). Heat and altitude lower the pace you can hold at the same effort (Ch 5, Ch 6).

Muscles run on ATP, and they only hold a few seconds' worth, so three systems keep refilling it:

Why the wall arrives

A well-fed runner stores very roughly 400–500 g of glycogen in muscle and about 100 g in the liver: around 2,000 kcal. A 70 kg runner's marathon costs about 2,950 kcal (Ch 1), and at race pace a large share of it comes from carbohydrate. Fat stores are enormous, but fat releases energy more slowly and needs more oxygen per kcal. When glycogen runs low, often around 30–35 km, the pace you can hold drops sharply: the wall.

Two levers delay it: start with full stores (carbohydrate-rich meals in the days before), and take carbohydrate during the race. Sports-nutrition guidance commonly suggests about 30–60 g of carbohydrate per hour for long races, and up to about 90 g per hour for some athletes using mixed sugars. Practise race-day fuelling in training: guts adapt too.

What drives oxygen into your blood is its partial pressure: about 21% of the total air pressure. Total pressure falls with height, so every breath carries fewer oxygen molecules, even though the percentage hasn't changed.

What your body does about it

Within hours you breathe faster and deeper, and your heart rate rises at any given effort. Over days, your kidneys rebalance your blood chemistry so you can keep breathing harder. Over weeks, your body makes more red blood cells. That's acclimatisation, and it takes time whatever your fitness: the Ladakh organisers stress it is "not determined by individual fitness levels". Even acclimatised, your aerobic ceiling is lower up high: as a rough rule of thumb, VO₂ max falls by several per cent per 1,000 m above about 1,000 m (the Lab uses 7%), so the same pace feels much harder.

Altitude sickness

Acute mountain sickness (AMS) usually begins within hours to a day of arriving high: headache with nausea, fatigue, dizziness or poor sleep. Mild AMS often settles with rest and no further ascent. Two rarer conditions are emergencies: HAPE (fluid in the lungs: breathlessness at rest, cough, falling exercise tolerance) and HACE (swelling of the brain: confusion, unsteadiness, drowsiness). Both need immediate descent and medical care. Flying straight into Leh puts you at 3,500 m in an hour, with no gradual climb, which is exactly why the organisers ask for 7–15 days on the ground first.

Muscles turn only around a fifth to a quarter of their energy into movement; the rest becomes heat. Our 70 kg runner at 10 km/h burns about 700 kcal per hour ≈ 810 W, so roughly 650 W of heat: like a small room heater inside you. Your body sheds it mainly by evaporating sweat, which takes about 2.4 MJ per kilogram.

Humidity is the real enemy

Evaporation needs dry air to carry vapour away. On a humid coastal morning, you sweat just as hard but cool less, so your core temperature and heart rate climb at the same pace. Indian city races start early in the cooler months for this reason: the Vedanta Delhi Half Marathon is in October and the Tata Mumbai Marathon in January. Heat adaptation (training gradually in warm conditions over a week or two) increases sweat rate and lowers heart rate in the heat.

Hydration: neither too little nor too much

Losing a lot of body water raises heart rate and body temperature. But drinking far more than you sweat, especially plain water over many hours, can dilute blood sodium (hyponatraemia), which is dangerous too. Weigh yourself before and after long training runs to learn your own sweat rate, and drink to a plan. At high altitude the air is cold and very dry, so you lose extra water just by breathing hard; the Ladakh organisers make hydration packs mandatory for ultra runners.