In his book Daniels’ Running Formula, Dr. Jack Daniels describes a number of key principles for training. Among these is the principle of diminishing returns. In essence, this principle states that upon undertaking a new training stimulus, an athlete will experience a relatively rapid acquisition of fitness as their body (perhaps specifically, their cellular metabolism) responds to the stress of the stimulus. Over time, however, as the athlete’s cells favorably adapt to the stimulus, their ability to continue to adapt favorably decreases as there is only marginal opportunity to continue to adapt since complete biological optimization is nearer (Daniels, 2014).
Working as a coach to young, novice endurance runners, I experience this principle with them as athletes new to training often experience sensational improvement in their early stages of training and racing. At the beginning of a season (or macrocycle), I will conduct a baseline fitness test. This is usually a 1.5-mile time trial (I use this distance because longer distances are usually unmanageable for newer runners who end up walking portions of the trial, and using shorter distances don’t usually reveal an athlete’s aerobic ability as well). After four to six weeks of training, the athletes will retest and predictably decrease their 1.5-mile run time by minutes. The sense of accomplishment this yields is invigorating and pushes athletes to continue to train. As the athletes continue to train and compete, their improvements in run time become smaller and smaller. This trend continues as an athlete continues from season to season, year to year. For example, for some of the varsity juniors and seniors I coach, a two- or three-second personal record in the 1600m run can be a major feat, since they have smaller and smaller margins for improvement.
What are some physiological causes for these diminishing returns on an athlete’s workload investment? In a publication of Sports Medicine from March of last year, Graham Holloway states that this decline in adaptation comes from, at least in part, a decrease in the production of reactive oxygen species (ROS) by mitochondria during exercise (Holloway, 2017). When an athlete undergoes aerobic training, the mitochondria in their contracting muscle fibers break down chemical fuel in order to regenerate ATP in a series of reactions that are part of an electron transport chain within the mitochondrial cristae membranes. These reactions utilize oxygen as the final electron acceptor in the electron transport chain (this is the reason for oxygen intake during exercise). Byproducts of these reactions are reactive oxygen species (ROS). These ROS molecules can act as inducers of metabolic adaptations which improve the ability of the working muscle fibers to utilize oxygen and regenerate ATP. With increased adaptation to aerobic workloads over time, however, the muscle fibers accumulate less ROS and the adaptations reach an optimized level where further adaptation is increasingly unlikely.
Holloway also states that decreased mitochondrial sensitivity to ADP is another factor that contributes to the diminishing returns of increased cellular workload. As the biogenesis of mitochondria within muscle fibers increases, it attenuates the power of mitochondria (Holloway, 2017). Scientists are currently working on finding ways to push these limitations further in order to enhance aerobic performance. One potential solution to decreased mitochondrial sensitivity to ADP is an increased intake of sodium nitrates, a chemical found in high concentrations in beetroot juice.
While the principle of diminishing returns will always hold true, athletes and coaches can look to new biochemical discoveries that demonstrate the causes of certain diminishing returns, and may continue to push those limitations further into the range of what the body is potentially capable of doing with the help of biochemical solutions to these biochemical limitations.
Daniels, J. T. (2014). Daniels' running formula (3rd ed.). Champaign, IL: Human Kinetics.
Holloway, G. (2017). Nutrition and training influences on the regulation of mitochondrial adenosine diphosphate sensitivity and bioenergetics. Sports Medicine, 47, 13-21.