Decreased Heat rate = improved performance?! Beware of the shortcut (PART 1/2).

A decrease in Heart Rate (HR) at a given intensity is often reported as a positive adaptation of the athlete showing an improvement in performance or fitness level.

Beware of this kind of shortcut, which can be totally false and above all be at odds with the popular belief of a decline in CF. It is important to have a more holistic approach. There is a link between CF evolution, exercise intensity, perceived effort and the level of training load.

A drop in CF at a given intensity (HRexe) can also be the first signal of an athlete’s state of fatigue or more precisely a form of functional overreaching that will cause a temporary drop in performance, no-functional overreaching causing stagnation or even a regression in performance, or more seriously an overtraining (cf. Diagram below) for which the athlete will have to observe long months of recovery before regaining an initial state at the level of his homeostasis (balance of the functioning of the organism).

Innov Triathlon Triathlon | Continuum fatigue
A drop in CF during the recovery phase (HRR) during a High Intensity Interval Training (HIIT) session can also be a sign of fatigue.

Longitudinal monitoring of the cardiac response combined with load monitoring and, above all, the taking into account of perceptual signals (difficulty of effort, quality of sleep, daily stress level, daily fatigue level, mood profile, etc.) makes it possible to set up intelligent training that will optimize the adaptation of the organism to a higher level. The analysis of cardiac variability may also be interesting to explore the autonomic nervous system in a non-invasive way. This HRV monitoring is important but should be done last. Simple clues are often more relevant.

Here is a concrete example from an athlete:

Comparison of cardiac response over 2 identical sessions.

Type of session: 4x (5′ @105-110%CP/ r 5′ @50-60%CP)
Discipline: Bike (Home-Trainer)
Context: 1st session realized with a good shape. 2nd performed with a state of fatigue after a run session in LTH.
Innov Training | Session comparison
We can see that if we only retain the drop in CF (-12 bpm) for an identical power, we pass completely by the impact of the session on the organism that generated a higher level of perception of the difficulty of effort (+2.5).
In conclusion:

A decrease in HRex/HRR should not systematically be seen as an improvement in the athlete’s performance. It can be synonymous with a state of fatigue that sets in or persists.
The evolution of the cardiac response must be taken into account in a global and inseparable whole taking into account various parameters (Charge, Perception of effort and fatigue, mood, Period of preparation, HRV index).
A drop in CF may indicate a decrease in nerve control and an imbalance in the Autonomous Nervous System (SNA).
Monitor your heart response regularly using submaximal tests.
Decreased performance, both in competition and training, is a relevant factor in detecting fatigue.
To go further:

Aubry & al (2015). The development of functional overreaching is associated with a faster heart rate recovery in endurance athletes. Plos One.
Lambert & al (2004). Variation in heart rate during submaximal exercise: implications for monitoring training. Journal of Strength and Conditioning Research.
Lambert & al (2009). Day to day variation in heart rate at different levels of sub maximum exertion: implications for monitoring training. Journal of Strength and Conditioning Research.
Lambert & al (2011). A novel submaximal cycle test to monitor fatigue and predict cycling performance. Br J Sports Med.
Lucia & al (2000). Heart Rate and performance parameters in elite cyclists: a longitudinal study. Med Sci Sports Exercise.