The purpose of athletic training is to induce adaptations to the muscles through increased work load. Adaptations can be neurological, which involve improving muscle and muscle fiber coordination, or physiological, which involve improving circulation and increasing muscle mass. While neurological adaptations can be made in several days or weeks, physiological adaptations require much more time to occur.

Stress, Recovery, and Training Intensity

To induce physiological adaptations, athletes much repeatedly stress the muscles and then allow for a period of recovery. Precision control of training intensity distribution, therefore, is considered essential to maximizing physiological adaptations. This point is exemplified in the training patterns of elite endurance athletes, who exercise at high volumes and loads relative to their sport, according to a study by the University of Stirling.

The study further notes that these athletes spend more than 80% of their training time working out at zone-one intensities far below the lactate threshold; the remaining 20% or less of their time is spent working out at or above the lactate threshold in zones two and three. The significance of this is considered to be due to the recovery capabilities of training in the various zones.

While recovery capability does not differ greatly between zones two and three, it is greatly enhanced when training in zone one. Furthermore, training at high intensities, namely those found in zone three, have been shown to lead to greater improvements in physiological adaptation and endurance performance, reports the University of Stirling.

This suggests that an ideal training regimen involves approximately 80% of lower-intensity zone-one training and 20% of higher-intensity zone-three training, with very little training in zone two. To ensure athletes meet these requirements, stress and recovery must be measured to a high degree of precision.

Measuring Training Intensity with Precision

Heart rate is perhaps the most commonly sampled training-intensity metric. Though it is universal to all types of sports, heart rate is not a very accurate measure of stress and recovery levels for physiological adaptation. Optimum heart rate stress levels for an individual athlete may differ by sport, such as between cycling and running. Moreover, during anaerobic training activities such as weight lifting, heart rate may vary only very slightly with very different anaerobic exertion levels. Finally, because heart rate is a lagging indicator of the body’s true metabolic state, it has limited relevance as a precision measurement of stress and recovery.

Devices that measure external load, such as bicycle power meters, provide more accurate measurements of training stress and recovery. The power meter works by tracking the propulsion forces on a bike. Given its accurate power measuring capability, many athletes believe the power meter has some predictive value on racing performance. The only drawback with the power meter is its limitation to a bicycle.

Another option is muscle oxygen measurement. Muscle oxygen provides an accurate snapshot of an athlete’s metabolic state in real time; because it is a localized measurement, athletes can monitor the effect of each particular exercise on individual targeted muscles.

Regardless of the metric used, precise measurement of training intensity is needed to ensure stress and recovery induces physiological adaptions. Doing so will confirm the athlete is working within the training intensity zones needed to efficiently improve endurance performance.