Using power measurements as a stress and response tool serves as a unique training guidance, although insight and awareness of your own cardio-respiratory system is just as unique! A well tuned perception of effort (proprioceptive) or session RPE (Relative Perceived Exertion) at current endurance fitness levels, also allows heart rate monitoring to provide good physiological feedback to quantify the relative intensity of rides.
Training with power alone ignores the fact that we are not ‘machines’, however integrating multiple data channels including heart rate, provides a more holistic view of your overall physiology at work. As you become fitter, your heart rate will decrease (as less oxygen is required or the amount of blood pumped per stroke of the heart is greater, known as bradycardia) for a given power output or a given speed on a particular stretch of road or same climb. This happens both short term during acute super-compensation and long term during chronic adaptations. As your fitness improves your training zones shift and although your maximum heart rate will not really change much (dependent on age and status of training), your speed and power will change in relation to your heart rate. This is a basic phenomena of the threshold principle. Recovery also improves as lactate clearance and distribution is positively affected, so greater volumes of training above threshold power can be tolerated. To accurately determine this, precise testing performed under controlled conditions will help you understand how quickly you are improving and also how well you are able to maintain the improved performance and understand the cyclical periodisation needed to reach your next level.
Accurate interpretation of heart rate data alone can be difficult as heart rate is subject to variation caused by independent variables; dehydration, quality of sleep and fatigue, body temperature (cardiac drift) and position on the bike. This makes the approximation between intensity levels as a measure of training load vary greatly within an individual. However the average heart-rate can be a fair representation of exercise volume and training response. The Training – Impulse (TRIMP) model relies on quantitating either Fraction of Heart Rate Reserve (FHRR) and adapted more recently for power data, Bike Score (a predecessor of TSS) and has been well established as a valuable training tool by Banister, 1991. The maximum heart rate reserve is key to the relationship measuring heart rate with other data channels. The upper limit or maximal heart rate remains mostly static, although not always justifiably or accurately measured (except for determination of Fraction Heart Rate Reserve), while the resting heart rate and hence the heart rate for a sub-maximal stress will fall, as a riders fitness improves and cardiac output increases. As recovery of heart rate after exercise is faster in more physically active people (Bunc et al, 1988) and decreases after an acute increase in training load (Borresen and Lambert 2007), heart rate recovery has the potential to be a useful tool and sensitive marker of tracking changes in training status. Dr. Stephen Cheung has written a brief synopsis of Lamberts research findings on the use of HR recovery and variability, here.
The impulse-response model is limited by efforts above Maximum Oxygen Consumption (MOC) /Maximal Aerobic Power (MAP), that have a large anaerobic capacity component which is not captured by maximal heart rate data. This is generally why training with a power-meter is a superior method. Even for power output measurements performed on the road ‘in the field’, weighting still needs to be placed on the higher, more physiologically demanding values. Normalised Power (NP) as utilised in Training Peaks has been recently devised to take into consideration the non-linear nature of human physiology and in fact follows lactate kinetics, modelled to the fourth power. These have been introduced for quantitating a training impulse for power, Training Stress Score (TSS). Bike Score similarly relies on the critical power parameter to calculate a standardised training load value for each ride and is synonymous with TSS. However both critical power and functional threshold power are fundamentally different from the physiologically relevant lactate threshold power and heart rate parameters, of which clearance can be derived, and more importantly recovery can be quantitated easily.
The beauty of using both power and heart rate data, is that one is able to track changes in power:heart rate ratios for prescribed interval sessions so the correct number of intervals at the right range of intensity are performed before the body responds adversely to the stress. This is usually reflected by aerobic decoupling indicated by a drift in heart rate of greater than 5-10% (unless the training is predominantly anaerobically driven, then heart rate is irrelevant) as the cardio-respiratory system fails to adequately provide oxygen and fuel to the working muscles and become more reliant on lactate from anaerobic metabolism. Likewise, a similar drop in power of 5-10% in the later intervals will suggest that training to that zone or intensity is no longer productive.
Effective energy-system based intervals are conveniently derived from critical power calculations which synchronise with actual threshold data to provide personalised sessions for either the turbo or on the road. Factoring in subjective feedback from your own perceived exertion, muscular sensations and degree of fatigue are also critical aspects to consider during and after training. These may be considered along with the bio-feedback data to help estimate the amount of recovery needed and steer your next sessions within the phase or specific training period.