Anaerobic Threshold

Physiology at Firstbeat

Seeking the Steady-state

In order to individualize training intensity, tools for carefully controlling cardiovascular and metabolic stress are needed. Anaerobic threshold is a metabolic threshold characterized as the highest workload at which the body is able to achieve a steady-state condition, so that the lactic acid accumulation and removal are in balance. Therefore, anaerobic threshold is important to long-term performance ability as it represents the highest performance intensity that can be tolerated for relatively long periods. When the threshold is exceeded, accumulation of lactic acid in muscles may cause fatigue in a brief period of time.

Currently available methods for estimating the anaerobic threshold can not be easily used every day, as they require specific exercise protocols with incremental exercise intensity and/or invasive blood lactate (lactic acid) samples. Even partaking an exercise test may be too far from a “comfort zone” of a novice exerciser.

Therefore, Firstbeat developed a method for detecting anaerobic threshold non-invasively with freely performed real-life exercises on daily basis outside of laboratory conditions. The Firstbeat’s algorithms for detecting the threshold use information on relationsip between heart rate and speed as well as HRV, and utilize sophisticated data modeling methods for calculating the individual intensity for the threshold.

Anaerobic threshold reflect’s person’s fitness level and can be used to predict for example his/her marathon performance. Heart rate level and/or pace corresponding to anaerobic threshold are relevant training parameters, and help to choose the right training intensity.

In practice, if a person’s anaerobic threshold pace for running is for example 4:00 min/km, he/she is able to run with that speed with a constant lactic acid level of about 4 mmol/l for prolonged periods. If the person increases pace to, for example 3:50 min/km, a steady state can not be achieved anymore. Instead the person’s lactic acid level starts to accumulate until fatigue takes place.


Energy Pathways in Endurance Sports

Physiologically, when exercise intensity is increased gradually from rest, at certain level anaerobic energy pathways usually start to noticeably activate and support the aerobic energy system in producing cellular energy (ATP). When anaerobic energy pathways are activated, ATP can be formed faster through glycolysis. Glycolysis means a lactic acid fermentation process in which glycogen/glucose are rapidly converted to ATP. However, this results in increased lactic acid formation in the muscles.

Until anaerobic threshold, the lactic acid can be metabolized by the body without continuous accumulation. If exercise intensity is increased above the threshold, aerobic energy production capabilities of the working muscles may have difficulties in matching the exercise energy requirements, and anaerobic energy production increases rapidly. Consequently, lactic acid starts to accumulate into the muscles and blood stream. This happens due to the body’s inability to remove lactic acid  fast enough by means of oxidation and glucose re-formation. Up to the anaerobic threshold intensity also the bicarbonate in blood and muscles buffers the rise in body’s acidity. At intensities above anaerobic threshold depletion of body’s bicarbonate stores leads to abrupt increase in respiration rate and ventilation. Firstbeat’s algorithm detects increased ventilator activity through heart rate variability analysis.

From a physiological point of view, another trace in the detection of anaerobic threshold is body’s exertion level that increases rapidly above the anaerobic threhold. Firstbeat algorithm tracks increased exertion level by analyzing cardiorespiratory drift during exercise. Cardiorespiratory drift (principally the increases in oxygen consumption and HR) is caused by e.g. lactic acid and adrenaline accumulation and increased body temperature, all of which emerge rapidly above AnT intensity.

Thus, in addition to HRV analysis, also cardiorespiratory drift is analyzed when tracking individual’s highest steady state intensity – i.e. the anaerobic threshold!