Whether the intent is to increase athletic performance or enhance fitness in the face of an ever-expanding chronic disease epidemic, a structured exercise program is essential.
The ability to monitor intensity in a training program is paramount, and can be aided by a multitude of methods ranging from simplistic (such as exercising based on ratings of perceived exertion) to complex (such as global positioning systems [GPS] and personal bicycle mounted power meters).
Within this spectrum, the heart rate (HR) monitor is a relatively uncomplicated method of monitoring exercise intensity by providing real-time measurements of cardiovascular responses to exercise.
The science behind HR monitoring
At the start of exercise, or when changing exercise intensity, a flood of information is sent from the periphery (e.g. muscle and lungs) to the brain indicating a need to modulate blood flow and oxygen supply to the muscle.
In response, HR is increased or decreased as necessary to match blood flow demands. During incremental exercise, HR will continue to increase until reaching maximum (HRmax) after which further increases in intensity have no influence.
HRmax is highly correlated with peak aerobic capacity (the highest level of oxygen-use during maximal exercise) and although the relationship between HR and aerobic metabolism is non-linear, HR is still a good indicator of aerobic intensity.
It should be noted that during exercise above aerobic maximum (high intensity interval training, or HIIT) HR is not a good indicator of actual intensity.
In addition to real-time monitoring of exercise intensity, HR provides an indication of overall fitness and changes in fitness over time. Normal resting HR is 60-100 beats a minute; but with training, the heart, as with any other muscle, becomes larger and stronger resulting in a greater release of blood per contraction.
Increased fitness enhances the parasympathetic nervous system (responsible for lowering heart rate) and this, coupled with a stronger heart, results in lower resting HR.
As fitness levels increase over time, changes in movement economy (the energy needed for a given workload), increases in overall aerobic capacity and the strength of the heart can lower HR at a given workload. Indeed, measurements of pre- and post-training HR responses to a set workload have been used to monitor fitness changes throughout the literature.
What information do you need to train with HR?
One of the more confusing areas of HR training is the determination of training zones.
Metabolically, three main zones exists, as highlighted by the existence of two metabolic thresholds measured via concentration of lactate (lactate threshold and onset of blood lactate accumulation [OBLA] respectively) or ventilation (ventilator threshold one and two respectively).
For simplicity, these can be referred to in terms of intensity as easy, moderate and hard. Exercise in the easy zone (below lactate threshold or ventilator threshold one) can be sustained for prolonged periods of time (60+ minutes) and makes up the majority of most training programs.
Indeed, recent research examining numerous endurance-based training programs showed up to 75% of all training is within this zone.
Exercise duration is reduced in the moderate and hard zones; but exercise in these zones is necessary for improvements in physiological variables consistent with fitness and health, as well as for athletic performance.
Most HR zones are prescribed as a percentage of HRmax (see table below). Using a maximal exercise test (which involves exercising under increasing workloads until exhaustion), HRmax and training zones can be precisely determined.
This can be expensive; thus, several methods to calculate HRmax are available. Of these, Stevens Creek’s (HRmax = 205 – [age/2]) and Fox and Haskell’s (HRmax = 220-age) equations correlate well with actual values; although caution must be taken as these are predicted values only.
Alternatively, recording heart rate during competition can provide accurate HRmax values.
It should be noted the HR zones presented below vary largely to accommodate individuals of all fitness levels, meaning individuals with higher fitness should use the upper end of the zones.
Limitations to training with HR
Heart rate is a global stress variable influenced by state-of-mind and environmental conditions. Significant elevations in HR occur during exposure to heat as the body increases HR to provide blood to the skin for cooling.
During prolonged efforts, especially in the “hard” training zone, HR can drift upward causing individuals to reduce their effort and thus under-train.
Conversely, fatigue can present as an inability to increase HR during exercise and can limit its use.
Finally, HR is influenced by some medications (such as beta-blockers) and individuals should be aware under such circumstances increasing HR beyond a certain point may not be possible. Indeed, attempting to do so may be dangerous.
All you need to do to increase your fitness is just listen – albeit intently – to your heart.