---
title: "Physical Activity and Health"
book: "High School Biology"
subject: biology
language: en
chapter: 10
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/10-physical-activity-and-health
---

# Chapter 10 — Physical Activity and Health

Two students are sixteen. One walks to school, plays basketball twice a week and sleeps eight hours; the other is driven, spends the evening seated in front of a screen, and sleeps six. At sixteen they look similar. At fifty their doctors will see two different people: the first with the heart, bones, muscles and blood sugar of someone twenty years younger, the second with a list of conditions that begins with the word "chronic". The three previous chapters have shown what a working body does; this one is about what regular work does to it — and what abuse, in the form of overtraining or drugs, undoes.

## 10.1 What training changes

**Definition 10.1 (Physical activity and training).**

*Physical activity* is any movement of the body produced by the muscles that raises [energy expenditure](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-expenditure) above rest: walking, climbing stairs, housework, sport. *Training* is physical activity repeated with the aim of improving a capacity. The body responds to a repeated demand by *adapting*: enlarging, strengthening or re-equipping the structures the demand loads — provided the demand is followed by the rest in which the adaptation is built.

**Proposition 10.2 (Adaptations to endurance training).**

Repeated exercise of long duration and moderate intensity, over months, produces:

- a larger, stronger heart: the stroke volume rises, the resting rate falls (from 70 to 50 or below), the maximal output and the $\dot V\!\mathrm{O_2}$ max increase by 10% to 30%;
- more capillaries and more [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in the [muscle fibres](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) , which burn a larger share of fat and produce less lactic acid at a given power;
- more red blood [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) , hence more oxygen carried per litre;
- larger [glycogen](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-glycogen) stores, and a body that regulates its blood sugar and its blood pressure more easily.

**Proof.** *Admitted at this level.* ∎

![Resting heart rate of two groups of beginners over six months of running. The fall reflects a growing stroke volume: the same 5\, L/ min at rest in fewer beats. Adaptation is proportional to the demand, and slows as the body approaches its new state.](https://one-course.com/images/onecourse/chapters/biology-2/g10-sport-and-health/fig-6c25b6d582ca.svg)

*Resting heart rate of two groups of beginners over six months of running. The fall reflects a growing stroke volume: the same $5\,\mathrm{L}/\mathrm{min}$ at rest in fewer beats. Adaptation is proportional to the demand, and slows as the body approaches its new state.*

**Example 10.3 (The numbers of adaptation).**

A sedentary 17-year-old with a $\dot V\!\mathrm{O_2}$max of $40\,\mathrm{mL}/\mathrm{min}$ per kilogram runs three times a week for six months: $48$, and a resting heart rate down from 72 to 58. His muscle biopsy shows 40% more [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and 30% more capillaries per fibre. His fibres are the same in number and the same in type; they have been re-equipped.

**Proposition 10.4 (Adaptations to strength training).**

Repeated contractions against heavy loads thicken the [muscle fibres](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) (more myofibrils in each) and improve the nervous command that recruits them; strength can double in a year while the number of fibres does not change. The [tendons](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) and the bones loaded by the training thicken too, more slowly. Bone, in particular, is built and maintained in response to the loads it carries: an active adolescence sets the bone mass that will be drawn down in old age, and a limb immobilised in a cast loses bone within weeks.

**Proof.** *Admitted at this level.* ∎

## 10.2 Activity and health

**Proposition 10.5 (Effects of regular activity on health).**

Regular [physical activity](#def-g10-sport-and-health-training) — the equivalent of an hour a day of moderate activity for adolescents, half an hour for adults — lowers the risk of the commonest chronic diseases: heart and artery disease, type 2 diabetes, several cancers, osteoporosis, depression. It does so through the adaptations above: a heart and vessels that work at lower pressure, muscles that draw glucose from the blood readily, bones that keep their mass, a body mass kept within a healthy range. The effect is large — comparable to that of not smoking — and it does not require sport, only movement.

**Evidence.** Studies following tens of thousands of people for decades find that those in the most active fifth of the population die, at a given age, at about half the rate of those in the least active fifth, with heart disease accounting for most of the difference; the relation holds after allowing for diet, smoking and social conditions, and it is graded — each step up in activity brings a step down in risk. Trials in which sedentary people are assigned to a walking programme show falls in blood pressure and blood sugar within months. ∎

![Risk of death over a follow-up of twenty years, relative to the least active fifth, in a large adult population (rounded from several studies). Each step of activity lowers the risk; the largest gain is the first, from none to some.](https://one-course.com/images/onecourse/chapters/biology-2/g10-sport-and-health/fig-44bfbfc5c318.svg)

*Risk of death over a follow-up of twenty years, relative to the least active fifth, in a large adult population (rounded from several studies). Each step of activity lowers the risk; the largest gain is the first, from none to some.*

**Example 10.6 (Blood sugar and muscle).**

A muscle at work takes glucose from the blood without needing the hormone insulin, and a trained muscle stays more sensitive to insulin between sessions. A brisk walk after a meal lowers the peak of blood glucose that follows it; a sedentary body, asked to store the same sugar without moving, needs more insulin each year — the road to type 2 diabetes, whose mechanism is the subject of the final year.

## 10.3 Too much, too fast, too wrong

**Proposition 10.7 (Overtraining and injury).**

Adaptation is built during rest. A demand repeated without enough recovery — more sessions, or heavier ones, than the tissues can repair between them — produces the reverse of adaptation: persistent fatigue, a rising resting heart rate, falling performance, disturbed sleep, frequent infections, and, in the tissues that repair slowest ([Chapter 9](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#ch-g10-muscles-and-joints)), tendinitis and stress fractures. The commonest cause of injury in adolescents is not the sport but the rate at which its load is increased.

**Proof.** *Admitted at this level.* ∎

**Example 10.8 (The rule of ten per cent).**

A runner who increases her weekly distance by no more than 10% a week rarely hurts herself; one who doubles it in a fortnight often does. Bone and [tendon](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) adapt over months, muscle and heart over weeks: the slow tissues set the pace at which the load may rise.

**Proposition 10.9 (Doping).**

*Doping* is the use of substances or methods that raise performance by acting on the body’s regulation, in place of the adaptation training would build. Each one imitates an adaptation and carries the cost of bypassing it:

- the hormone erythropoietin (EPO), or transfusion of one’s own stored blood, raises the number of red blood [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) — and thickens the blood until it can clot in the vessels of the heart or brain;
- anabolic steroids, derived from testosterone, thicken [muscle fibres](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) — and disturb the body’s own hormones, the liver, the heart and the mood;
- stimulants mask fatigue — and remove the warning that protects the heart from overheating and exhaustion;
- growth hormone, and drugs that hide the others, complete the list.

[Doping](#prop-g10-sport-and-health-doping) is forbidden in competition because it is unfair; it is dangerous because every one of these substances overrides a regulation that exists for a reason.

**Proof.** *Admitted at this level.* ∎

**Example 10.10 (EPO in numbers).**

Red blood [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) make about 45% of the volume of normal blood. EPO abuse has pushed athletes to 55% and beyond, a blood so thick that the heart, slowed by sleep, cannot move it: several young cyclists died in their sleep in the years before tests existed. [Training](#def-g10-sport-and-health-training) at altitude raises the same figure by two or three points, over weeks, under the body’s own control.

## 10.4 The everyday rules

**Method 10.11 (Training safely).**

1. *Progress slowly* : increase the load by about a tenth a week, and let bone and [tendon](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) set the pace.
2. *Warm up and cool down* : ten minutes each, for the reasons of [Chapter 9](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#ch-g10-muscles-and-joints) .
3. *Drink* : an hour of hard exercise loses one to two litres of sweat; replace it, with some salt, before thirst says so — a loss of 2% of body mass already lowers performance.
4. *Eat for the effort* : [carbohydrates](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) before and during long efforts, [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) for repair afterwards ( [Chapter 7](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#ch-g10-exercise-and-energy) ); no supplement replaces a varied diet.
5. *Rest* : sleep is when adaptation is built; a rising resting heart rate is the earliest sign of too little.
6. *Listen to pain* : sharp pain in a [joint](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-joint) or [tendon](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) is a signal, not an obstacle.

![An aid station on a marathon course. At a sweating rate of over a litre an hour, the water lost must be replaced during the effort; waiting for thirst means running already dehydrated.](https://one-course.com/images/onecourse/chapters/biology-2/g10-sport-and-health/img-284c8f832824.jpg)

*An aid station on a marathon course. At a sweating rate of over a litre an hour, the water lost must be replaced during the effort; waiting for thirst means running already dehydrated.*

**Example 10.12 (Hydration in numbers).**

A $60\,\mathrm{kg}$ runner sweats $1.2\,\mathrm{L}$ per hour in warm weather. In a two-hour run she loses $2.4\,\mathrm{kg}$, 4% of her mass: her blood volume falls, her heart rate rises by ten beats to keep the output, her body temperature climbs, and her pace drops by a tenth. Drinking half a litre an hour would have halved the loss.

**Remark 10.13 (The body keeps the account).**

Every adaptation of this chapter is a structure the body builds in response to use, and every one of them is dismantled again when the use stops: a month in bed loses a fifth of the muscle, a year of inactivity loses most of the endurance gains. The account is kept continuously, and the balance at fifty is the sum of the deposits made at sixteen, twenty-six and thirty-six.

## 10.5 Exercises

**Exercise 10.1 ★.**

List four adaptations of the body to endurance training.

**Solution of Exercise 10.1.**

A larger heart with a bigger stroke volume and a lower resting rate; more capillaries and [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in the [muscle fibres](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle); more red blood [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); larger [glycogen](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-glycogen) stores (and better regulation of blood sugar and pressure).

**Exercise 10.2 ★.**

Why does the resting heart rate fall with training, although the resting [cardiac output](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#def-g10-heart-lungs-effort-output) does not change?

**Solution of Exercise 10.2.**

The stroke volume has grown, so the same $5\,\mathrm{L}/\mathrm{min}$ is delivered in fewer beats: $Q = f \times V_s$ with a larger $V_s$ needs a smaller $f$.

**Exercise 10.3 ★.**

What is the recommended daily amount of [physical activity](#def-g10-sport-and-health-training) for an adolescent? Does it have to be sport?

**Solution of Exercise 10.3.**

About an hour a day of moderate activity. No: walking, cycling to school, stairs and housework all count — what matters is movement that raises [energy expenditure](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-expenditure).

**Exercise 10.4 ★.**

Define doping and give two examples with the regulation each one overrides.

**Solution of Exercise 10.4.**

The use of substances or methods that raise performance by overriding the body’s regulation instead of building an adaptation. EPO overrides the control of [red-cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) number; stimulants override fatigue, the warning that protects the heart.

**Exercise 10.5 ★.**

Name three signs of overtraining.

**Solution of Exercise 10.5.**

Persistent fatigue, a rising resting heart rate, falling performance (also disturbed sleep, frequent infections, [tendon](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) or bone pain).

**Exercise 10.6 ★★.**

From the heart-rate figure, compare the fall after 12 weeks in the two groups, and say what it shows about the relation between demand and adaptation.

**Solution of Exercise 10.6.**

After 12 weeks: $72 - 60 = 12$ beats for four sessions, $72 - 64 = 8$ for two. Adaptation is proportional to the demand — more sessions, more change — and in both groups it slows as the new state is approached.

**Exercise 10.7 ★★.**

A runner’s resting output is $5.0\,\mathrm{L}/\mathrm{min}$. Her resting heart rate falls from 72 to 54 over six months. By what factor did her stroke volume change?

**Solution of Exercise 10.7.**

$V_s = Q/f$: from $5000/72 \approx 69\,\mathrm{mL}$ to $5000/54 \approx
93\,\mathrm{mL}$, a factor of $72/54 \approx 1.33$.

**Exercise 10.8 ★★.**

From the relative-risk figure, by what fraction is the risk of death lower in the most active fifth than in the least active? Which single step brings the largest gain?

**Solution of Exercise 10.8.**

$1 - 0.55 = 45\%$ lower. The first step, from least active to low (1.00 to 0.80), brings the largest gain.

**Exercise 10.9 ★★.**

A $55\,\mathrm{kg}$ student plays a two-hour tennis match in summer, sweating $1.0\,\mathrm{L}$ per hour and drinking nothing. What fraction of her mass has she lost, and what are three consequences?

**Solution of Exercise 10.9.**

$2.0\,\mathrm{kg}$, i.e. $2.0/55 \approx 3.6\%$ of her mass. Her blood volume falls, her heart rate rises to keep the output, her body temperature climbs, her performance drops; cramps become likely.

**Exercise 10.10 ★★.**

A beginner runs $10\,\mathrm{km}$ a week. Following the rule of ten per cent, how many weeks does it take to reach $20\,\mathrm{km}$ a week? (Compute week by week, or note that $1.1^7 \approx 1.95$.)

**Solution of Exercise 10.10.**

$10 \times 1.1^7 \approx 19.5\,\mathrm{km}$, so $20\,\mathrm{km}$ is reached in the eighth week: about two months.

**Exercise 10.11 ★★.**

Explain why muscle strength can double in a year of training while the number of [muscle fibres](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) stays the same.

**Solution of Exercise 10.11.**

Each fibre thickens by adding myofibrils, and the nervous system learns to recruit more fibres at once and in better synchrony; both raise the force. Fibre number is fixed in adulthood.

**Exercise 10.12 ★★★.**

An athlete’s blood has 45% red [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); after EPO abuse, 56%. Using [Chapter 8](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#ch-g10-heart-lungs-effort), estimate the gain in oxygen carried per litre and hence in $\dot V\!\mathrm{O_2}$max, and explain why the gain is bought at the risk of a fatal clot.

**Solution of Exercise 10.12.**

Oxygen per litre rises by $56/45 \approx 1.24$, about 24%; with the same [cardiac output](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#def-g10-heart-lungs-effort-output) and extraction, $\dot V\!\mathrm{O_2}$max rises by up to the same fraction. But blood with 56% [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) is much thicker; slowed by a sleeping heart it can clot in a coronary or cerebral vessel.

**Exercise 10.13 ★★★.**

[Training](#def-g10-sport-and-health-training) at altitude for three weeks raises red [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) from 45% to 48%; EPO raises them to 56% in the same time. In what sense is the first an adaptation and the second not, although the molecule involved is the same?

**Solution of Exercise 10.13.**

At altitude the body itself raises its EPO in response to a real lack of oxygen, by a controlled amount, and lowers it again when the lack ends: the regulation is at work. Injected EPO imposes a number the regulation would never choose, with no lack to justify it and no brake.

**Exercise 10.14 ★★★.**

Explain, using [Example 10.6](#ex-g10-sport-and-health-sugar), why doctors prescribe walking to patients whose blood sugar is beginning to rise, and why the prescription works better than a drug that only lowers the sugar.

**Solution of Exercise 10.14.**

Working muscle takes up glucose without insulin, and trained muscle stays more sensitive to it; walking therefore lowers the blood sugar directly and reduces the insulin needed at every meal. A drug lowers the sugar but leaves the cause — insensitive, unused muscle — in place, and does nothing for the heart, bones or mass that walking also improves.

**Exercise 10.15 ★★★.**

"A month of intensive training before the exam season will set me up for the year." Discuss with the notions of adaptation, rest, and the time scales of the different tissues.

**Solution of Exercise 10.15.**

Adaptation is built during rest between repeated, gradually increased demands: a month of intensive load is more likely to produce overtraining and injury than fitness, since [tendon](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#def-g10-muscles-and-joints-muscle) and bone adapt over months, not weeks. And the gains dismantle within weeks of stopping: nothing is "set up" for a year by a single burst. Regular moderate activity through the year does what the month cannot.

## 10.6 Problem: Two Students, Thirty Years

**Problem 10.1.**

Weekend problem — an active and a sedentary sixteen-year-old followed through training, a season of overtraining, and the accounts their bodies keep to middle age

Alex and Sam are sixteen, both $62\,\mathrm{kg}$, both with a resting heart rate of 72 and a $\dot V\!\mathrm{O_2}$max of $42\,\mathrm{mL}/\mathrm{min}$ per kilogram. Alex starts running three times a week; Sam does not.

**Part I — Alex trains.** After six months Alex’s resting heart rate is 56 and his $\dot V\!\mathrm{O_2}$max $50\,\mathrm{mL}/\mathrm{min}$ per kilogram; his maximal heart rate (200) has not changed.

1. His resting [cardiac output](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#def-g10-heart-lungs-effort-output) is still $4.8\,\mathrm{L}/\mathrm{min}$ . Compute his stroke volume before and after.
2. Compute his $\dot V\!\mathrm{O_2}$ max in litres per minute before and after, and the percentage gain.
3. Assuming his maximal oxygen extraction ( $150\,\mathrm{mL}$ per litre) has not changed, compute his maximal [cardiac output](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#def-g10-heart-lungs-effort-output) before and after, and his maximal stroke volume before and after.
4. Which of the adaptations of [Proposition 10.2](#prop-g10-sport-and-health-endurance) explains the gain in stroke volume, and which one would explain a gain in extraction?
5. Alex’s weekly distance went from $6\,\mathrm{km}$ to $24\,\mathrm{km}$ in six months. Was the rule of ten per cent respected? Show the calculation.

**Part II — Alex overtrains.** In the spring Alex doubles his distance in two weeks to prepare a race. Three weeks later: resting heart rate 64, poor sleep, a cold, a pain in the shin, slower race times.

6. Name the condition and list the signs from the chapter that match.
7. What is the most likely cause of the shin pain, in the vocabulary of [Chapter 9](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#ch-g10-muscles-and-joints) , and why that tissue?
8. Why does the resting heart rate rise, when training had lowered it?
9. Propose a three-week plan and justify each element with [Method 10.11](#met-g10-sport-and-health-rules) .
10. A teammate offers a "supplement" that removes tiredness. Which category of [Proposition 10.9](#prop-g10-sport-and-health-doping) does it belong to, and what regulation does it override?

**Part III — Sam sits.** At thirty Sam is $80\,\mathrm{kg}$, with a resting heart rate of 78 and a $\dot V\!\mathrm{O_2}$max of $32\,\mathrm{mL}/\mathrm{min}$ per kilogram; Alex, still active, is $64\,\mathrm{kg}$, 54 and $48$.

11. Compute both men’s $\dot V\!\mathrm{O_2}$ max in litres per minute. Which absolute value is larger, and why is the per-kilogram figure the one that matters for daily life?
12. Climbing four flights of stairs costs each man about $12\,\mathrm{mL}$ of oxygen per kilogram per minute above rest. What fraction of his maximum does each use? Who arrives out of breath?
13. Sam’s doctor finds a blood sugar slightly above normal. Explain, with [Example 10.6](#ex-g10-sport-and-health-sugar) , the link with his inactivity.
14. The doctor prescribes half an hour of brisk walking a day. Using the relative-risk figure, estimate the reduction in long-term risk if Sam moves from the least active fifth to the moderate one.
15. Sam objects that he has no time for sport. Answer him with [Definition 10.1](#def-g10-sport-and-health-training) .

**Part IV — The account at fifty.**

16. Bone mass peaks around twenty-five and then declines by about 1% a year. If Alex’s peak was 10% higher than Sam’s because of his active adolescence, how many years of decline does that margin represent?
17. After a fall at fifty, Sam fractures a wrist and Alex does not. Relate this to question 16 and to [Proposition 10.4](#prop-g10-sport-and-health-strength) .
18. Alex broke his training for a year at forty after an injury and lost most of his endurance gains, then rebuilt them. What does this show about the nature of an adaptation?
19. List, for Sam at fifty, three conditions of [Proposition 10.5](#prop-g10-sport-and-health-health) his inactivity has made more likely, and the mechanism of one of them.
20. State the result: the two numbers (resting heart rate, $\dot V\!\mathrm{O_2}$ max per kilogram) that separate the two men at thirty, and the one habit, stated in hours per week, that produced the difference.

**Solution of Problem 10.1.**

**1.** $4800/72 \approx 67\,\mathrm{mL}$ before; $4800/56 \approx
86\,\mathrm{mL}$ after.

**2.** $42 \times 62 \approx 2.6\,\mathrm{L}/\mathrm{min}$; $50 \times 62 =
3.1\,\mathrm{L}/\mathrm{min}$: a gain of 19%.

**3.** Maximal output $\dot V\!\mathrm{O_2}/0.150$: from $17.4$ to $20.7\,\mathrm{L}/\mathrm{min}$; maximal stroke volume at 200 beats: from $87$ to $103\,\mathrm{mL}$.

**4.** The larger, stronger heart explains the stroke volume; more capillaries and [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in the fibres would explain a gain in extraction.

**5.** A factor of 4 in 26 weeks is an average of $4^{1/26}
\approx 1.055$, about 5.5% a week — within the 10% rule ($1.1^{26}
\approx 12$ would have allowed far more).

**6.** Overtraining: rising resting heart rate, disturbed sleep, infection, falling performance, pain in a slow-repairing tissue.

**7.** A stress injury of the bone of the shin (a stress fracture or inflammation of its surface): bone adapts slowest of all tissues, and the load was doubled in a fortnight.

**8.** Recovery is incomplete from one session to the next; the body is in a state of persistent strain, and the adaptation that lowered the rate is being reversed.

**9.** A week of near rest and sleep (adaptation is built during rest); then two weeks at half the previous distance, no speed work, rebuilding by a tenth a week; the shin seen by a doctor and rested until painless (listen to pain); drink and eat for repair.

**10.** A stimulant: it masks fatigue, the warning that protects the heart from exhaustion and overheating.

**11.** Sam $32 \times 80 \approx 2.6\,\mathrm{L}/\mathrm{min}$; Alex $48
\times 64 \approx 3.1\,\mathrm{L}/\mathrm{min}$. Alex’s is larger even in absolute terms; and daily life — stairs, walking, carrying oneself — moves the body’s own mass, so the figure per kilogram is the one that decides how it feels.

**12.** Sam $12/32 \approx 38\%$ of his maximum, Alex $12/48 =
25\%$. Sam, working above a third of his ceiling, arrives out of breath.

**13.** Unused muscle takes little glucose from the blood and loses its sensitivity to insulin; the same meals need more insulin each year and the blood sugar begins to rise.

**14.** From 1.00 to 0.70: about 30% less risk.

**15.** [Physical activity](#def-g10-sport-and-health-training) is any movement raising [energy expenditure](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-expenditure): walking to work, stairs instead of the lift, housework — half an hour a day of it needs no sport and no equipment.

**16.** At 1% a year, a 10% margin is ten years of decline.

**17.** Sam’s peak was lower and he has been losing bone for twenty-five years; his wrist has crossed the threshold at which a fall breaks it, while Alex’s bone, built under load in adolescence and maintained since, has ten years in hand.

**18.** An adaptation is a structure maintained by use, not a permanent acquisition: it is dismantled when the demand stops and rebuilt when it resumes.

**19.** Heart and artery disease, type 2 diabetes, osteoporosis (also some cancers and depression). Diabetes: inactive muscle loses its sensitivity to insulin, the blood sugar rises and the pancreas is overworked.

**20.** Resting heart rate 54 against 78; $\dot V\!\mathrm{O_2}$max $48$ against $32\,\mathrm{mL}/\mathrm{min}$ per kilogram; about three hours a week of running, kept up for fourteen years.