Acute Responses to Exercise

Original Editor - Wanda van Niekerk based on the course by James Laskin

Top Contributors - Wanda van Niekerk, Jess Bell and Nupur Smit Shah  

Responses to Exercise[edit | edit source]

Exercise imposes demands on physiological systems in the human body, and these induce various responses. These responses are dependent on[1]:

  • type or mode of exercise
  • intensity of exercise
  • duration of exercise
  • frequency of exercise
  • environmental conditions[2]
  • emotional influences[3]

Cardiovascular System[edit | edit source]

  • Components[4]:
    • circulatory system
    • pulmonary system
  • Function[4]:
    • transport and exchange of respiratory gases (oxygen and carbon dioxide)
    • transport and exchange of nutrients and waste products
    • transport of hormones and chemical messengers
    • regulation of body temperature (thermoregulation)
    • regulation of fluid balance
    • regulation of blood pressure
    • maintenance of pH balance
    • prevention of blood loss through haemostatic mechanisms
    • infection prevention through white blood cells and lymphatic tissue

Read more: Cardiovascular System

Heart Rate[edit | edit source]

  • There is a linear increase in heart rate with workload and oxygen consumption (VO2) during dynamic exercise
  • Heart rate (HR) response may be influenced by[5]:
    • age
    • body position
    • fitness
    • type of activity
    • medication
    • environmental conditions
    • blood volume

Maximal Heart Rate[edit | edit source]

  • Maximum heart rate (HRmax) is achieved after all-out exertion to volitional fatigue.[6] It is the highest heart rate value an individual can achieve during an all-out effort to the point of exhaustion. It is verified by a plateau in heart rate despite increasing workload.[7]
  • HRmax trainability:
    • Although there are several resources stating that HRmax remains unchanged with training, there are also studies that report that HRmax is reduced after regular aerobic exercise in sedentary adults and endurance athletes.[8][9] [10][11]
    • HRmax can increase when regular aerobic exercise is stopped and there is also evidence suggesting that tapering or detraining increases HRmax.[8][12]
    • Whyte et al.[13], reported that HRmax was similar between elite aerobic and anaerobically trained athletes, however when these athletes were compared with age matched sedentary participants the HRmax of the athletes were significantly lower than the sedentary participants.
  • HRmax remains constant day to day and decreases slightly from year to year
  • HRmax can be estimated through age-based prediction equations, but note that there is a large standard error of estimate associated with these methods. It is also recommended that these equations should be applied to similar populations as the populations used when the equation was derived.[14]
    • HRmax = 220 - age in years (this is the most common and widely used formula)[15]
      • This formula is still used in clinical settings. However, it has been reported that there are deviations and over- and under-estimation of maximum heart rate in younger and older populations.[16]
    • HRmax = 208 - (o.7 x age in years) - this is a more precise formula, adjusted for people older than 40 years[17]
    • HRmax = 211 - (o.64 x age in years) - even more precise formula, adjusted for generally active people[7]
  • Assessment of HRmax:
    • Laboratory setting
      • Graded exercise test[16]
    • Field test
      • Running and cycling protocols
  • Measuring HRmax helps healthcare professionals determine cardiovascular compliance to exercise testing, exertion during exercise and exercise prescription.[16]


Steady-State Heart Rate[edit | edit source]

  • Steady state = "a condition in which the energy expenditure provided during exercise is balanced with the energy required to perform that exercise and factors responsible for the provision of this energy reach elevated levels of equilibrium"
  • Steady-state heart rate = optimal heart rate for meeting circulatory demands at a given sub-maximal workload/intensity
  • Steady-state heart rate is related to the efficiency of the heart and the balance between enhanced contractility and increased stroke volume. Thus, heart rate is kept as low as possible at any given workload - this is achieved through enhanced contractility and increased stroke volume
    • increased stroke volume results from an increase in venous return, this in turn increases the left-ventricular end-diastolic volume (preload). The increased preload stretches the myocardium, which causes it to contract more forcefully (enhanced contractibility) (in accordance with the Frank-Starling Law)


  • Steady-state heart rate helps predict functional capacity

Maximal Oxygen Consumption (VO2max)[edit | edit source]

  • VO2max is the gold standard assessment of aerobic capacity[20]
  • It is the product of maximal cardiac output (Q) and arteriovenous oxygen difference (a - vO2)[21]
  • It is the maximal ability of the body to consume (absorb, deliver and utilise) oxygen for the production of energy[20]
  • Read more: VO2 Max

VO2max = HRmax x SVmax x (a-vO2)max = Q x (a -vO2)max

Stroke Volume[edit | edit source]

  • Stroke volume = "the volume of blood pumped out of the left ventricle of the heart during each systolic cardiac contraction"[22]
  • End diastolic volume (EDV) is dependent on heart rate filling pressure and ventricular compliance[23]
  • End systolic volume (ESV) is dependent on afterload and contractility[23]
  • Stroke volume (SV) at rest: 60-100 ml/beat
  • Maximal stroke volume: 100-120ml/beat
  • Reaches a maximal value at approximately 50% of maximal effort

SV = EDV - ESV[22]


Regulation of Stroke Volume[edit | edit source]

  • End-diastolic volume (EDV)[23]
    • Volume of blood in the ventricles at the end of diastole ("preload")
  • Average aortic blood pressure[23]
    • The pressure that the heart must pump against to eject blood ("afterload")
    • Increased afterload = decreased stroke volume (usually in ill individuals and not in healthy athletic populations)
  • Strength of the ventricular contraction[23]
    • "Contractility"

End Diastolic Volume[edit | edit source]

  • Frank Starling mechanism
    • Greater preload results in the stretch of ventricles and a more forceful contraction[25]
  • Affected by:
    • Vasoconstriction
    • Skeletal muscle pump
    • Respiratory pump

Cardiac Output[edit | edit source]

  • Cardiac output is the amount of blood pumped by the heart in 1 minute[22]
  • Cardiac output (Q) = Heart rate (HR) X Stroke volume (SV)[23]
  • Rest: 5 L/min
  • Maximal: 20 L/min or more
  • Up to approximately 50% of maximum effort, HR and SV both contribute to an increase in Q[23]

Heart Rate, Stroke Volume, and Cardiac Output - Integrated Cardiac Response to Exercise[edit | edit source]

Consider this example: A person is in a reclining position, then moves to a seated position and lastly to standing. The heart responds to these positional changes as follows[23]:

  • In the reclining position, heart rate is at its lowest (e.g. 50 beats/min)
  • In the seated position, heart rate increases a bit more (e.g. 55 beats/min)
  • In standing, heart rate increases further (e.g. 60 beats/min)
  • Explanation[23]:
    • Standing up increases the venous pooling of blood in the most dependent parts of the body
      • With the positional changes from reclining to seated to standing, gravity causes pooling of blood in the legs, thus there is a reduced volume of blood returning to the heart.
    • This redistribution of blood causes a reduction in the intrathoracic blood volume returning to the heart. Through the Frank-Starling mechanism, this causes a reduction in the stroke volume (by 30% to 40%)
    • To compensate for the reduction in stroke volume, heart rate increases to maintain cardiac output
    • Stroke volume rises again when going back to a supine position, in response to increased venous return

Consider this next example: From standing, a person starts to walk, then jog at a moderate pace and lastly breaks into a fast-paced run. The heart will respond as follows:

  • Standing heart is the lowest (e.g. 60 beats/min)
  • Walking - heart rate increases (e.g. 90 beats/min)
  • Moderately paced jog - heart rate increases further (e.g. 140 beats/min)
  • Fast-paced run - further increase in heart rate (e.g. 180 beats/min)
  • Explanation[23]:
    • Heart rate increases proportionally as exercise intensity increases
    • Initial increase in heart rate (up to around 100 beats/min) is mediated by a withdrawal of parasympathetic tone
    • Increased activation of the sympathetic nervous system mediates further increases in heart rate
    • Stroke volume increases with exercise and, thus, further increases cardiac output

In untrained individuals in the initial stages of exercise, increased cardiac output is a result of increased heart rate and stroke volume. Stroke volume increases proportionally with increasing exercise intensity, but in untrained individuals, maximum stroke volume is usually reached at 40-60% of VO2max. After that, an increase in cardiac output is mostly the result of increased heart rate. However, highly trained athletes have the ability to increase stroke volume further.[23]

Blood Pressure[edit | edit source]

  • Systolic blood pressure (SBP)
    • During endurance exercise, SBP increases in direct proportion to the increase in exercise intensity[26]
  • Diastolic blood pressure (DBP)
    • Slight decrease or increase or no change
  • Read more: Blood Pressure

Arteriovenous Oxygen Difference (a-vO2)[edit | edit source]

  • Arteriovenous oxygen difference = the amount of oxygen extracted from the blood as it travels through the body[23]
  • Calculated as the difference between the oxygen content of arterial blood and venous blood
  • Increases with increasing rates of exercise as more oxygen is taken from blood
    • At rest: (20ml O2/100ml - 15ml O2/100ml) = 5ml O2/100ml
    • With maximal exercise: (20 ml O2/100ml - 5 mlO2 ml/100ml) = 15 ml O2/100 ml


Pulmonary Ventilation (Ve)[edit | edit source]

  • Ve = Respiratory rate (RR) x Tidal volume (TV)
    • TV = volume of air breathed in a single breath
  • At the onset of exercise, there is an immediate increase in ventilation
    • This is due to tidal volume expansion and an increase in breathing frequency[28]
  • As exercise progresses, increased metabolism generates CO2 and hydrogen (H). This stimulates the peripheral chemoreceptors in the carotid bodies and the central chemoreceptors to activate the respiratory centre in the brain.[23]
  • When exercise ceases, pulmonary ventilation returns to normal at a slower rate
  • Ventilatory response to light, moderate and heavy exercise[23]:
    • There is an initial steep increase, thereafter the ventilation rate plateaus at a steady-state value for light and moderate exercise
    • With heavy intensity exercise, the ventilation rate does not plateau (it continues to increase with heavy exercise)

Blood Flow[edit | edit source]

Blood flow patterns change from a resting state to exercise[23]:

  • At rest, 15-25% of cardiac output (Q) goes to skeletal muscle
  • With exercise, 85 -90% of cardiac output goes to skeletal muscle
  • There is a 5-fold increase in blood flow to the heart
  • There is a decrease in skin, renal, hepatic, and splanchnic blood flow during exercise
  • There are only small changes of blood flow to the brain

Transition from Rest to Exercise and Exercise to Recovery[edit | edit source]

  • There are intensity-dependent adaptations to HR, SV, Q, SBP, RR, Ve and VO2
  • Plateau in submaximal (i.e. below lactate threshold) exercise
  • Recovery depends on:
    • duration and intensity of exercise
    • training state of the individual

Prolonged Exercise and Cardiovascular Drift[edit | edit source]

With prolonged aerobic exercise at a steady state, stroke volume gradually decreases and heart rate increases. This maintains cardiac output. Arterial blood pressure also declines. The increase in heart rate during prolonged exercise is known as cardiovascular drift and it is due to dehydration, increased skin blood flow (rising body temperature) and a decrease in venous return.[23]

Cardiovascular drift = the upward drift of heart rate over time, coupled with a progressive decline in stroke volume and continued maintenance of cardiac output.


Summary of Acute Physiologic Exercise Responses[edit | edit source]

  • Exercise Pressor Response
    • Invoked by muscle contraction
      • Muscle receptors are stimulated by muscular distortion or metabolic byproducts - afferent signal transmission to central nervous system (CNS)
    • Sympathetic nervous system phenomena
      • Generalised vasoconstriction in non-active skeletal muscle
      • Increased myocardial contractility
      • Increased HR
      • Increased SBP
      • Increased and redistribution of Q
      • Response = exercise intensity
  • One major challenge to homeostasis posed by exercise is the increased muscular demand for oxygen
  • During heavy exercise, oxygen demands may be 15-25 times higher
  • Two significant adjustments of blood flow during exercise are:
    • increased cardiac output
    • redistribution of blood flow
  • Depends on:
    • Type, intensity and duration of exercise
    • Environmental conditions
    • Emotional influence
Table 1. Acute physiological responses to exercise
Cardiovascular system
  • Increased heart rate
  • Increased stroke volume
  • Increased cardiac output
  • Increased blood pressure (systolic)
  • Redistribution of blood flow to working muscles
  • Increased arteriovenous oxygen difference
Respiratory system
  • Increased breathing rate
  • Increased tidal volume
    • at low exercise intensities, tidal volume and breathing frequency increases proportionally
    • at high-intensity exercise, tidal volume plateaus, and further increases in ventilation is due to the rise in frequency of breathing
  • Increased ventilation
  • Increased pulmonary diffusion
  • Increased oxygen uptake
Muscular system
  • Increased motor unit and muscle fibre recruitment
  • Increased blood flow to the muscles
  • Increased arteriovenous oxygen difference
  • Increased muscle temperature
  • Increased muscle enzyme activity
  • Increased oxygen supply and use
  • Decreased muscle substrate levels

References[edit | edit source]

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