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Human Physiology · Nervous system and exercise

Physiology of Exercise

Exercise is the physiotherapist’s main medicine. This final chapter brings the body systems together: how energy is supplied, how the heart, lungs and muscles respond to a session and adapt to training, and how to dose aerobic and resistance exercise.

10Sections
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10Questions

Part 9 · Nervous System and Exercise · Chapter 47 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the three energy systems and their time courses.
  • Explain oxygen deficit, steady state and excess post-exercise oxygen consumption.
  • Describe acute cardiovascular and respiratory responses to exercise.
  • Explain VO2max, METs and the link between fitness and survival.
  • Describe adaptations to aerobic and resistance training.
  • Apply dose principles to exercise prescription.

Why it matters

Nearly every physiotherapy plan includes exercise. Understanding its physiology lets you choose the right type, intensity and volume, and recognise abnormal responses.

This chapter links back to muscle, heart, lungs, blood, metabolism and the nervous system.

Energy for movement

Figure 1 · Three energy systems

Relative contribution of phosphocreatine, anaerobic glycolysis and aerobic metabolism by effort duration Phosphocreatine dominates the first few seconds of all-out effort, anaerobic glycolysis peaks around 30 to 60 seconds, and aerobic metabolism supplies most energy after about two minutes. 050100Duration of all-out effort (log scale)% of energy supply5 s30 s2 min10 minPhosphocreatineAnaerobic glycolysisAerobic
All three always work together; duration and intensity change the mix. Choose intervals and rests to target the system you want to train. Schematic.
SystemFuelRateCapacity
PhosphocreatineStored PCrVery fastSeconds
Anaerobic glycolysisGlycogen to lactateFastAbout a minute or two of hard work
AerobicCarbohydrate and fat with oxygenSlowerHours

Figure 2 · Oxygen uptake at the start and end of exercise

Oxygen uptake kinetics showing oxygen deficit, steady state and excess post-exercise oxygen consumption At the start of constant exercise, oxygen uptake rises over a few minutes, creating an oxygen deficit met by phosphocreatine and glycolysis. It then reaches a steady state. After exercise, oxygen uptake stays raised for a while, the excess post-exercise oxygen consumption. oxygendeficitsteady stateexcess post-exerciseoxygen consumptionexerciseOxygen uptakeTime (minutes)
Fitter people reach steady state faster and recover sooner. Warm-ups shorten the deficit. Schematic.

Acute responses

Figure 3 · Acute responses to exercise

Typical cardiovascular and respiratory responses from rest to maximal exercise Heart rate rises from about 70 towards its maximum; stroke volume rises then plateaus; cardiac output rises from about 5 to 20 to 25 litres per minute; ventilation from about 6 to over 100 litres per minute; systolic pressure rises while diastolic changes little; muscle blood flow rises greatly. Heart rateabout 70 → up to (220 − age)Stroke volumerises, then plateausCardiac outputabout 5 → 20–25 L/minVentilationabout 6 → 100+ L/minSystolic pressurerises with intensityDiastolic pressurelittle change in dynamic exerciseMuscle blood flowrises up to about 20-foldTypical values for a healthy young adult at maximal dynamic exercise
Redistribution matters. Blood shifts from gut and kidney to working muscle and skin. Typical textbook values.

Maximal oxygen uptake (VO2max) is cardiac output multiplied by the oxygen extracted by muscles. It is the best single measure of aerobic fitness.

One MET is the resting oxygen uptake, about 3.5 mL/kg/min. Fitness can be expressed in METs.

Kodama and colleagues pooled 33 cohort studies with 102,980 people [1]. Each 1-MET higher fitness was associated with 13% lower all-cause mortality [1].

Figure 4 · Fitness predicts survival

Risk ratios for all-cause mortality by cardiorespiratory fitness Each 1-MET higher fitness level was associated with a 13 per cent lower risk of death. Compared with intermediate and high fitness, low fitness carried risk ratios of 1.40 and 1.70. RR 0.87Per 1-MET higher fitnessRR 1.40Low vs intermediate fitnessRR 1.70Low vs high fitnessRR 1.033 studies102,980 peopleLow fitness:under 7.9 METs1 MET is about1 km/h fasterrunning speed
Fitness is a vital sign. Small gains in aerobic capacity are associated with meaningful falls in mortality [1].

Training adaptations

SystemAerobic trainingResistance training
HeartLarger stroke volume; lower resting heart rateModest wall thickening
MuscleMore mitochondria and capillariesLarger fibres; more strength
Nervous systemBetter efficiencyEarly strength gains from better recruitment
MetabolismMore fat use; better insulin sensitivityBetter insulin sensitivity; more lean mass
BoneSmall effect unless impactGreater loading benefit

Milanović and colleagues pooled 28 controlled trials in healthy adults [2]. Endurance and interval training both produced large VO2max gains, slightly greater with intervals [2].

Schoenfeld and colleagues found a graded dose-response between weekly sets and muscle growth [3].

Figure 5 · Training adaptations follow the dose

Aerobic capacity gains with endurance and interval training and muscle growth with resistance training volume Endurance training raised maximal oxygen uptake by 4.9 and interval training by 5.5 millilitres per kilogram per minute compared with no exercise. Each additional weekly set of resistance training added about 0.37 per cent muscle growth, and higher volumes gave 3.9 per cent more than lower volumes. +4.9Endurance training+5.5Interval trainingV̇O2max gain vs no exercise (mL/kg/min) [2]Resistance training volume [3]+0.37%muscle growth per extra weekly set+3.9%higher versus lower volumegraded dose-response
Both aerobic methods work; intervals add a little more [2]. More sets build more muscle, in a graded dose-response [3].

Practical skill · dosing exercise

Use FITT: frequency, intensity, time and type. Set aerobic intensity by heart rate, perceived exertion or talk test. For strength, progress load and sets gradually. Review response and adjust.

Principles of training

PrincipleMeaning
OverloadAdaptation needs a stimulus beyond the usual
SpecificityAdaptations match the type of training
ProgressionIncrease load as the body adapts
ReversibilityGains are lost with detraining
IndividualityResponses vary between people
RecoveryAdaptation happens between sessions

Clinical relevance for the physiotherapist

  • Measure fitness. Walk tests and METs predict outcomes [1].
  • Choose methods. Continuous and interval training both raise VO2max [2].
  • Dose strength. More weekly sets give more muscle growth [3].
  • Monitor. Watch heart rate, blood pressure, symptoms and recovery.
  • Progress. Apply overload, specificity and recovery.

Assessment and application

SituationInterpretationAction
Sedentary adult, peak about 6 METsLow fitness, higher risk [1]Progressive aerobic programme
Time-poor patient wants fitness gainsInterval training effective [2]Supervised interval sessions if safe
Older adult with low muscle massNeeds resistance volume [3]Progress weekly sets gradually
Blood pressure falls during rising workloadAbnormal responseStop and refer

Common mistakes

  • Prescribing exercise without intensity or progression.
  • Ignoring fitness as a risk marker [1].
  • Assuming only long continuous sessions improve fitness [2].
  • Too few sets to grow muscle [3].
  • No recovery between hard sessions.

Exam preparation

Long answer

Describe the cardiorespiratory responses to exercise and the effects of training.

  • Energy systems (Figure 1).
  • Oxygen kinetics (Figure 2).
  • Acute responses (Figure 3).
  • VO2max and fitness [1].
  • Training adaptations [2, 3].

Short notes

  • Oxygen debt and EPOC.
  • VO2max.
  • MET.
  • Principles of training.

Viva and practical

  • “What is one MET?” — about 3.5 mL O2/kg/min.
  • “Does interval training improve VO2max?” — yes, about 5.5 mL/kg/min [2].
  • “How does volume affect hypertrophy?” — graded dose-response [3].
  • OSCE: prescribe an aerobic programme using FITT.

Current evidence

Exam and traditional teaching

Exercise is good for health; prescription details are secondary.

What the evidence adds

Fitness strongly predicts survival [1].

Interval and continuous training both work, and resistance volume drives hypertrophy [2, 3].

What the student should understand

Exercise is a dose-dependent therapy. Prescribe it with the same precision as any medicine.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. Each 1-MET higher level of cardiorespiratory fitness was associated with an all-cause mortality risk ratio of:
  1. (A) 0.13
  2. (B) 0.87
  3. (C) 1.13
  4. (D) 1.87

Answer: (B) The pooled risk ratio was 0.87 per 1-MET [1].

Q2. Low fitness was defined in that meta-analysis as below about:
  1. (A) 3 METs
  2. (B) 7.9 METs
  3. (C) 15 METs
  4. (D) 20 METs

Answer: (B) Low fitness was under 7.9 METs [1].

Q3. Compared with high fitness, low fitness carried a risk ratio for all-cause mortality of about:
  1. (A) 0.70
  2. (B) 1.07
  3. (C) 1.70
  4. (D) 7.0

Answer: (C) Risk ratio 1.70 [1].

Q4. A 1-MET increase corresponds roughly to running faster by:
  1. (A) 0.1 km/h
  2. (B) 1 km/h
  3. (C) 10 km/h
  4. (D) 1 m/s

Answer: (B) 1 MET corresponds to about 1 km/h higher running speed [1].

Q5. Endurance training improved VO2max versus no exercise by about:
  1. (A) 0.5 mL/kg/min
  2. (B) 4.9 mL/kg/min
  3. (C) 49 mL/kg/min
  4. (D) No change

Answer: (B) The effect was 4.9 mL/kg/min [2].

Q6. High-intensity interval training improved VO2max versus no exercise by about:
  1. (A) 5.5 mL/kg/min
  2. (B) 0.55 mL/kg/min
  3. (C) 55 mL/kg/min
  4. (D) It reduced it

Answer: (A) The effect was 5.5 mL/kg/min [2].

Q7. Compared with endurance training, interval training gave:
  1. (A) Much smaller gains
  2. (B) A possibly small additional gain of about 1.2 mL/kg/min
  3. (C) Identical gains
  4. (D) Losses

Answer: (B) HIT had a possibly small beneficial effect of 1.2 mL/kg/min over endurance training [2].

Q8. Each additional weekly set of resistance training increased muscle growth by about:
  1. (A) 0.037%
  2. (B) 0.37%
  3. (C) 3.7%
  4. (D) 37%

Answer: (B) Each set added about 0.37% [3].

Q9. Higher versus lower weekly training volume produced a muscle gain difference of about:
  1. (A) 0.39%
  2. (B) 3.9%
  3. (C) 39%
  4. (D) None

Answer: (B) The difference was 3.9% [3].

Q10. The relationship between resistance training volume and hypertrophy was described as:
  1. (A) Inverse
  2. (B) Graded dose-response
  3. (C) Absent
  4. (D) Random

Answer: (B) A graded dose-response relationship [3].

Quick review

Key takeaways

  • PCr for seconds, glycolysis for about a minute, aerobic beyond.
  • Oxygen deficit at start; EPOC after.
  • Cardiac output rises about four- to five-fold at maximum.
  • Each 1-MET higher fitness: 13% lower mortality [1].
  • Endurance and interval training both raise VO2max [2].
  • More weekly sets, more muscle growth [3].

Further reading

SourceWhy it earns its place
Kodama S et al. — JAMA, 2009Fitness and mortality [1]
Milanović Z et al. — Sports Med, 2015Interval versus endurance training [2]
Schoenfeld BJ et al. — J Sports Sci, 2017Resistance volume and hypertrophy [3]

References

  1. Kodama S, Saito K, Tanaka S, et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis. JAMA. 2009;301(19):2024–2035. doi:10.1001/jama.2009.681 PMID 19454641 Meta-analysis of cohort studies
  2. Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials. Sports Medicine. 2015;45(10):1469–1481. doi:10.1007/s40279-015-0365-0 PMID 26243014 Systematic review and meta-analysis
  3. Schoenfeld BJ, Ogborn D, Krieger JW. Dose-response relationship between weekly resistance training volume and increases in muscle mass: a systematic review and meta-analysis. Journal of Sports Sciences. 2017;35(11):1073–1082. doi:10.1080/02640414.2016.1210197 PMID 27433992 Systematic review and meta-analysis

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Reviewed by the Physiotherapist India Team. · Human Physiology contents