Physical Exercises and Health

Lessons

Senior 4 · Physical Education

Physical Exercises and Health

4 lessons

Lesson 3 of 4

Effects of exercise on health: muscular development, speed, endurance and relaxation

25 min

The slow heart of a fit runner

At rest your body needs roughly 5 litres of blood pumped every minute, whether you are a long-distance runner or have never trained. Yet a trained endurance runner's heart may beat 15 or 20 times a minute fewer than an untrained pupil's. How can fewer beats move the same blood? The answer is one of many lasting changes that training makes to the body. In this lesson you look at four kinds of training (for muscle, speed, endurance and relaxation), what each one changes inside the body, and how each change protects your health.

Definition

Muscular strength: the greatest force a muscle group can exert in one effort. Muscular endurance: the ability of a muscle group to repeat or hold a contraction for a long time without tiring.

Hypertrophy: growth in the size of muscle fibres in response to training against resistance.

Speed: the ability to move the body, or part of it, quickly. It includes reaction time, the delay between a signal and the start of the movement.

Aerobic endurance: the ability to keep exercising for a long time using oxygen to release energy. Its best single measure is $\dot{V}\mathrm{O_{2}}\,\text{max}$ , the greatest rate at which the body can take up and use oxygen.

Stroke volume (SV): the volume of blood pumped by the left ventricle in one beat. Cardiac output $(Q)$ : the volume pumped per minute, $Q = SV \times HR$ .

Relaxation: deliberately lowering muscle tension, heart rate and mental arousal, for example by slow breathing.

What each kind of training changes

Training	Examples	Changes in the body	Health benefit
Muscular development	Push-ups, squats, lunges, resistance bands, supervised weights	Fibres thicken (hypertrophy); nerves recruit more fibres at once; tendons and bones become stronger and denser	Better posture, fewer joint and back injuries, stronger bones for life, easier daily tasks
Speed	Short sprints of 10 to 40 m with full recovery, reaction games, starts	Faster nerve signals and fibre recruitment; better use of fast-twitch fibres; more efficient technique	Quicker reactions in sport and in everyday emergencies, such as stepping out of the way of a vehicle
Endurance	Continuous running, cycling, swimming, long games, rope skipping	Larger, stronger heart (higher stroke volume); more capillaries and mitochondria in muscle; lower resting heart rate and blood pressure	Lower risk of heart disease, stroke, type 2 diabetes and obesity; less fatigue in daily life
Relaxation	Slow breathing, progressive muscle relaxation, gentle stretching, visualisation	The calming (parasympathetic) nervous system takes over: heart rate, breathing and muscle tension fall	Lower stress and anxiety, better sleep, faster recovery between sessions

Young people can train for strength safely when the technique is taught, the loads are increased gradually and an adult supervises. The WHO recommends muscle- and bone-strengthening activity on at least three days a week for everyone aged 5 to 17.

Worked example

Worked example 1: why a trained heart beats slower. At rest, Niyonzima's body needs a cardiac output of $5.0\,\mathrm{L\,min^{-1}}$ . Before a year of endurance training his stroke volume was 70 mL; afterwards it is 85 mL. Find his resting heart rate before and after, and the number of beats saved per day.

Rearrange $Q = SV \times HR$ : $HR = \frac{Q}{SV}$
Before (with $5.0\,\mathrm{L} = 5000\,\mathrm{mL}$ ): $HR = \frac{5000}{70} = 71.4\,\text{bpm}$
After: $HR = \frac{5000}{85} = 58.8\,\text{bpm}$
Beats saved per day: $(71.4 - 58.8) \times 60 \times 24 = 12.6 \times 1440 \approx 18\,000$

A bigger, stronger left ventricle pumps more blood per beat, so the heart can do the same job with fewer beats and more rest between them. This is why a falling resting heart rate is one sign that endurance training is working.

Speed and the energy systems

Muscles can only use one fuel directly: ATP. They rebuild it in three ways, which differ in speed and in how long they last. Speed training works mainly the first; endurance training mainly the third.

Speed improves partly through faster reactions. A simple ruler-drop test measures reaction time: a partner holds a ruler with its zero mark between your open finger and thumb and lets go without warning; you catch it. Because the ruler falls freely from rest, the distance $d$ it falls before you catch it gives the time:

d = \tfrac{1}{2} g t^{2} \quad \Rightarrow \quad t = \sqrt{\frac{2d}{g}}

Worked example

Worked example 2: reaction time from a ruler drop. Keza catches the ruler after it falls 20.0 cm. After three weeks of reaction games she catches it after 15.0 cm. Find both reaction times $(g = 9.81\,\mathrm{m\,s^{-2}})$ .

Before, with $d = 0.200\,\mathrm{m}$ : $t = \sqrt{\frac{2 \times 0.200}{9.81}} = \sqrt{0.0408} = 0.202\,\mathrm{s}$
After, with $d = 0.150\,\mathrm{m}$ : $t = \sqrt{\frac{2 \times 0.150}{9.81}} = \sqrt{0.0306} = 0.175\,\mathrm{s}$
Improvement: $0.202 - 0.175 = 0.027\,\mathrm{s}$ , about $\frac{0.027}{0.202} \times 100 = 13\%$ .

Notice that time grows only with the square root of distance: halving the reaction time would need the catch distance to fall to a quarter.

Relaxation: training the off switch

Hard training, exams and competitions all switch on the body's stress response: a faster heart, quick shallow breathing and tight muscles. Relaxation techniques switch it off, and like any skill they improve with practice.

Slow breathing. Sit or lie comfortably, one hand on the belly. Breathe in through the nose for 4 seconds so the belly rises, then out slowly for 6 seconds. One breath takes 10 s, so this gives $\frac{60}{10} = 6$ breaths per minute, about half a typical resting rate. A longer out-breath than in-breath slows the heart.
Progressive muscle relaxation. Tense one muscle group (feet, calves, thighs, hands, arms, shoulders, face) hard for about 5 seconds, then let go completely for 10 to 20 seconds and notice the difference. Work up the body.
Visualisation. With eyes closed, picture a calm place in detail, or rehearse a skill going well.

A cool-down that ends with five minutes of slow breathing brings the heart rate down faster and makes it easier to concentrate in the next lesson.

Video: "What makes muscles grow?" by Jeffrey Siegel, TED-Ed. Watch for how resistance training damages and then rebuilds muscle fibres, and why rest, sleep and food are part of the training.

Tip

Memory hook: $Q = SV \times HR$ . The output of a pump is the volume per stroke times the strokes per minute. Endurance training raises SV, so at rest the same $Q$ needs a lower HR.

Warning

Common mistakes. Training hard every day: muscles grow during recovery, so plan rest days. Sprinting for speed when already tired: speed work needs full recovery between efforts (several minutes for a short sprint). Thinking relaxation is "doing nothing": it is a trained skill with measurable effects. Signs of overtraining include a resting heart rate that stays higher than usual, poor sleep, lasting soreness and falling performance.

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Practice

Distinguish between muscular strength and muscular endurance, with one exercise for each.
Give two changes that endurance training makes to the heart or muscles, and one health benefit of each.
A pupil's cardiac output at rest is 4.5 L per minute and her stroke volume is 75 mL. Calculate her resting heart rate.
During hard exercise a runner's heart rate is 180 bpm and stroke volume 110 mL. Calculate the cardiac output in litres per minute.
In a ruler drop the ruler falls 25.0 cm before it is caught. Calculate the reaction time (g = 9.81 m s⁻²).
Which energy system supplies most of the energy for a 400 m race and for a 10 000 m race? Use the diagram.
Describe how to do progressive muscle relaxation.
A pupil trains with maximum-effort sprints every day, with 20 seconds of rest between them, and says his speed is getting worse. Analyse why, and suggest a better plan.

Show answersHide answers

Strength: the greatest force in one effort (e.g. one heavy squat). Endurance: repeating or holding a contraction for a long time (e.g. 30 push-ups or a long plank).
For example: bigger stroke volume, so a lower resting heart rate and less strain on the heart; more capillaries, so better oxygen supply and less fatigue; lower blood pressure, so less risk of stroke.
4500 ÷ 75 = 60 bpm.
180 × 110 = 19 800 mL per minute = 19.8 L per minute.
t = √(2 × 0.250 ÷ 9.81) = √0.0510 = 0.226 s.
400 m (about a minute): anaerobic glycolysis. 10 000 m: aerobic.
Tense one muscle group hard for about 5 s, then release fully for 10 to 20 s, noticing the change; move through the body from the feet to the face.
With only 20 s of rest the phosphocreatine stores are not rebuilt, so each sprint is slower and trains fatigue rather than speed; daily work also leaves no recovery. Better: short sprints (10 to 40 m) with several minutes of rest between them, two or three times a week, with rest days.

Quick check

Stroke volume rises from 70 mL to 80 mL, and resting cardiac output stays at 5.6 L per minute. What happens to resting heart rate?

Remember

Strength training thickens fibres and strengthens bones and tendons; endurance training enlarges the heart's stroke volume and lowers resting heart rate ( $Q = SV \times HR$ ).
Speed training improves reactions and fast-fibre recruitment; it needs full recovery between efforts.
Short all-out efforts use phosphocreatine, efforts of about 10 s to 2 min mainly anaerobic glycolysis, longer efforts mainly the aerobic system.
Relaxation (slow breathing, progressive muscle relaxation) lowers heart rate and stress and speeds recovery.

Sources and credits

Cardiac output, Bioenergetic systems, Muscle hypertrophy and Progressive muscle relaxation, Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
WHO guidelines on physical activity and sedentary behaviour, World Health Organization, 2020: the recommendation on muscle- and bone-strengthening activity.
Photos: Push-up session, Adewuyi Israel, and Metaphysical Yoga Meditation at Orji FlyOver, Owerri Imo State, Emmanuel Obiajulu, Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: "What makes muscles grow? - Jeffrey Siegel", TED-Ed, embedded from YouTube.
Energy-systems diagram, tables, worked examples, practice and quick check: written by Ganzaa. The pupils' figures are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1The slow heart of a fit runner

2What each kind of training changes

3Speed and the energy systems

4Relaxation: training the off switch

The slow heart of a fit runner

What each kind of training changes

Speed and the energy systems

Relaxation: training the off switch