The Heart's Response to Exercise
Two of the key functions of the cardiovascular system are to:
1. Transport nutrients, hormones, gases and waste to and from our cells.
2. Regulate our body temperature and maintain our bodies fluid balance.
When we exercise a greater demand is placed on these functions as working muscles require more oxygen and nutrients than normal, they produce more waste products and generate more heat.
The degree of the cardiovascular response is determined by the demands placed on it by the training stimulus, the greater the demand the greater the response. The cardiovascular system is essentially made up of two parts - the heart (cardio) and the blood vessels (vascular). On the this page we'll focus our attention on the heart's responses to exercise.
Cardiac output (Q), heart rate (HR) and stroke volume (SV) responses:
Cardiac output refers to the total quantity of blood that is ejected by the heart and is usually measured in litres per minute. Heart rate refers to how often the heart beats and is also meaured per minute. Stroke volume refers to the amount of blood that is ejected by the heart with each beat. So cardiac output is quite simply the product of heart rate and stroke volume.
Heart rate increases in a linear fashion to increases in the intensity of exercise. This is illustrated in the adjacent graph, showing how the heart rate (in beats per minute – bpm) increases to match the incremental demands of walking, jogging and running.
It is also worth noting that heart rates start to rise prior to any type of exercise – just the thought of exercise is enough to trigger a heart rate response.
This initial response serves simply to prepare the body for activity and is controlled by the sympathetic division of the autonomic (involuntary) nervous system.
Stroke volumes also rise as a person starts to exercise and continue to rise as the intensity of the activity increases. This is shown in the adjacent stroke volume graph as the increases between standing, walking and jogging. This increase is primarily due to a greater volume of blood returning to the heart.
You will also notice that stroke volumes are higher when lying, and to a lesser degree sitting, as opposed to standing. This is because it is much easier for blood to return to, and fill the heart when a person is lying and sitting as the effect of gravity on blood flow is not as great when in these positions.
The increase in stroke volume only continues up to a point however. Once the intensity of the exercise exceeds 50-60% of an individual’s maximum heart rate their stroke volume ceases to rise, as shown on the graph as the similar stroke volumes for jogging and running.
This is primarily because the increase in heart rate that has also occurred does not allow enogh time for the heart to fill anymore between each heart beat.
Cardiac output increases in a linear fashion to increases in the intensity of exercise, up to the point of exhaustion. This happens as a direct consequence of the heart rate and stroke volume responses to the intensity of exercise.
The increase in cardiac output at intensities up to 50-60% of a person’s maximum heart rate is attributable to increases in heart rate and stroke volume.
As the intensity of exercise exceeds 60% of a person’s maximum heart rate the increase in cardiac output is solely attributable to increases in heart rate.
Just how high can heart rate go before it reaches its maximum rate?
There is an easy formula commonly used in the fitness industry to estimate a person’s maximum heart rate (HR Max).
HR max is shown on the adjacent graph. The heart rate is shown to increase in a linear fashion to increases in intensity (treadmill speed in this case) up to a point (approximately 185bpm on the graph) where HR max is reached and it increases no further.
The intensity required to attain maximum heart rates is relative for all people. For example an unfit person may reach their HR max jogging at 8km/h while a fit person may reach their HR max running at 20km/h.
It’s important to bear this in mind when designing programmes for clients – what may seem to be an easy intensity for you (e.g. jogging at 10km/h on a treadmill) could quite literally ‘kill’ someone new to exercise!
Heart rates and exercise intensity
Because heart rates (and the intensities required to achieve certain heart rates) vary so much between people many trainers use the ‘Rating of Perceived Exertion’ (RPE) scale to measure and set exercise intensity with clients.
The RPE scale (shown adjacent) is a simple 1-10 scale where the client rates the intensity of the exercise according to how hard it feels to them.
If you were aiming to take a new, unfit client through a low-moderate intensity cycling workout and they rated the workout at 7 it would indicate that you’d exceeded the intensity you planned (and may well have exceeded your client’s capacity to cope)!
The RPE scale is an easy ‘client friendly’ way of measuring intensity according to the client’s feedback and providing direction to clients regarding the intensity they should train at.
What direction do you think a client would find easier to understand – ‘jog at an intensity of 3/10 that you feel is ‘light’ for 30 minutes’, or ‘jog at a point where your heart rate is between 120-135 bpm for 30 minutes’?
Heart rate response to aerobic training
If the intensity of the exercise remains constant (i.e. 50% of a person’s maximum heart rate, or an RPE of 5 throughout) then the heart rate will rise until it reaches what is known as ‘steady state’ where it stays relatively constant as the cardiovascular system meets the demands placed on it by the exercise.
Steady state is illustrated on the adjacent graph at the point where the heart rate flattens after an initial rise in the first few minutes of exercise. For steady state to be achieved and maintained the intensity of the exercise must remain constant.
The graph also shows how heart rates return to resting levels after exercise finishes. The more intense the exercise is the longer it will take for heart rate to return to its resting rate.
If the intensity of the exercise fluctuates then heart rates will also fluctuate. We see this where work periods of high intensity exercise are interspersed with periods of lower intensity exercise. As the intensity increases so does the heart rate and as the intensity drops so does the heart rate.
The following graph shows how a person’s heart rate fluctuates throughout an 11 mile run encompassing a variety of terrain.
You will notice that at the 4.5 mile mark the runner increased their speed significantly resulting in the largest increase in heart rate. Drops in heart rate relate to reductions in pace and easier (downhill) parts of the run.
Heart rate response to anaerobic training
Training for strength, speed and power focuses on energy coming from the anaerobic energy systems.
Work periods, or ‘sets’ are typically short (5 - 30 seconds), intensity is very high (8-10/10 RPE) and rest periods are long in comparison (≥ 2-3 minutes).
Because of the short work period and the use of energy from anaerobic pathways, heart rates don’t rise significantly and thus show only moderate rises during each work period.
They also return to near resting levels during each rest period and return to normal levels within a few minutes after the cessation of the workout.
With these types of training the cardiovascular system functions largely to replenish the anaerobic energy systems and as such is only minimally stimulated.
The heart rate response does become greater as the duration of each work period/set increases (≥ 30 seconds) and or the recovery period shortens (≤ 1minute).
We see this with training for muscular hypertrophy, muscular endurance and anaerobic fitness in particular, where there is a greater demand for the cardiovascular system to remove the accumulation of waste products (CO2 and lactate).
During these types of training heart rates rise and peak at the end of each work period/set.
The peaks will be larger for training oriented on muscular endurance and anaerobic fitness (longer work periods and less recovery time between each work period/set).
As there is less recovery time between work periods/sets with muscular endurance and anaerobic fitness training, heart rates tend to rise incrementally throughout the workout as well as having peaks at the end of each set.
Because of this it takes longer (20-40 minutes) for heart rate and stroke volume to return to normal resting levels at the end of the workout.
This is due to a greater demand placed on the cardiovascular system to shunt greater quantities of blood out of working muscles, return blood to the vital organs and clear the accumulation of waste products (lactate & CO2).
This prolonged elevation of heart rate post exercise is known as ‘EPOC’ (excessive post-exercise oxygen consumption). Heart rates essentially stay elevated for longer after these types of training in order to metabolise the lactate that has accumulated and return the body to homeostasis.
There is an added bonus with exercise that causes the heart rate to stay elevated for longer, more calories are burned as a consequence.
So a muscular hypertrophy workout and to a greater degree muscular endurance and anaerobic fitness workouts can be considered the workouts that keep giving even after they’ve finished – and are certainly beneficial for those wanting to get rid of unwanted fat stores!
