A Synopsis of Bioenergetics and Human Performance

"I don't sell cars, I sell engines. The cars I throw in for free because something has

to hold the engine in.”

- Enzo Ferrari

Introduction

Bioenergetics is the study of energy flow through living organisms. With regards to the human body, bioenergetics relates to how energy is produced and utilized by cells and tissues. It’s a complex and fascinating area of physiology that is vital to human health and performance.

The interactions between the anaerobic and aerobic energy systems, and how they unify to supply adenosine triphosphate (ATP) to meet the demands placed on the body are essential components to be understood. It enables the practitioner to develop more effective physical preparation programming schemes. Afterall, outside of a select few, most sports and professions have a blend between neurological and metabolic demands.

The 3 Energy Systems Summarized

ATP provides energy for many processes such as sarcomere contractions and nerve impulse propagation within living organisms, making it a requirement for movement and exercise. It can be derived from one aerobic or two anaerobic pathways:

Aerobic Respiration: Requires oxygen and happens through a series of metabolic pathways in the body that oversees macronutrient catabolism leading to glucose, fatty acids, and amino acids, creating ATP that is used to power cellular functions including muscular contractions.

Glycolysis: Does not require oxygen, however is dichotomous and can be linked to aerobic respiration…

Fast glycolysis is the anaerobic breakdown of glucose into ATP and pyruvate. This process occurs rapidly and is used during high-intensity, short-duration activities when the demand for energy is immediate and oxygen availability is limited. Fast glycolysis produces ATP quickly but also generates lactate as a byproduct.

Slow glycolysis still produces ATP and pyruvate yet, then in the presence of sufficient oxygen, pyruvate is oxidized into Acetyl CoA to enter the Citric Acid Cycle. slow glycolysis is utilized during lower(-ish) intensity and longer duration activities where oxygen availability remains. This can be considered a transition that is flux with aerobic respiration as per an individual’s respiratory exchange ratio.

ATP-PCr System: Does not require oxygen and provides immediate energy (~10s +/- 5s) for short bursts of high intensity muscular contractions that lead to movement (E.g., Olympic Weightlifting or 15m accelerations). It relies on stored ATP and phosphocreatine (PCr) within the muscle cell cytosol. Creatine kinase breaks the PCr phosphate bond allowing for the addition of a third phosphate to ADP, yielding 1 ATP.

Figure 1: An overview of the 3 energy systems and how they interact with one another at various intensities.

3 Systems with 1 Job, ATP Production

Contrary to many teachings, these systems do not operate like a light switch. First and foremost, aerobic respiration is the preferred method of ATP production. Without it for minutes and we would not survive. When the demand for oxygen delivery supersedes availability and the capacity to deliver, anaerobic pathways takeover as a buffer. This does not mean that the body shifts to 100% reliance on anaerobic (fast) glycolysis and/or the ATP-PCr system. These systems constantly interact with one another and intertwine to meet the demands placed upon the organism. Examples of this are how aerobic respiration is linked to PCr resynthesis between the mitochondria and the cytosol, the Cori Cycle, and even slow glycolysis itself.

Imagine these systems as a speedometer (Figure 1). The markings on the gauge display an aerobic threshold, anerobic threshold, VO2 max, and an anaerobic speed/power reserve. The more effort exerted the needle then displays the energy system that is predominantly being leveraged. With respect for local (peripheral) and central demands on the organism. As effort fluctuates the needle then simply demonstrates how these pathways interact with one another. Always returning to an aerobic respiration idle.

The question then is, how do we train and manipulate these thresholds to prepare for the physical demands upcoming? This may have an individual prepping to run 800m as fast as possible, training for intermittent bouts in a chaotic team sport, or a mission critical prerequisite of being able to cover long distances slowly while under fueled and carrying 40kg of additional kit. Furthermore, after profiling an individual a sequential approach may be necessary to be rewarded with the desired outcome.

Below are guidelines on how to “raise the basement”, manipulate the anaerobic threshold, "raise the ceiling", and manipulate the anaerobic speed/power reserve.

Addressing the Aerobic Threshold AKA "The Basement"

The aerobic threshold, also known as the aerobic-anaerobic transition or the first lactate threshold, is a physiological concept used to describe the point during exercise where the body's energy production begins to gradually shift to producing ATP without the presence of oxygen (I.e., Glycolysis). Where the popular “zone 2 cardio” lies. Because of this, blood lactate begins to slowly accumulate when the threshold is passed (at ~2mmol, or ~1mmol above resting reading) due to the organisms’ aerobic constraint and inefficiencies producing ATP anaerobically.

Several studies have investigated the aerobic threshold and its significance in exercise physiology, mainly looking at steady state long slow distance/duration exercise or aerobic intervals. One such study by Casado and colleagues (2023) probed said literature and the relationship between lactate guided training and their effect on various physiological and physical determinants in 1500m and 5000m runners. It was found that lactate guided intervals and LSD sessions with associated pacing provided the opportunity for an increase in high-intensity sessions (when compared to traditional “Zone 4” sessions), achievement of internal load targets, improved management of internal load at altitude, and objectivity in periodization and monitoring capabilities. Demonstrating that the basement can be raised by way of steady state LSD and/or intervals that fluctuate slightly above the Aerobic Threshold and slightly below.

Example: "Raise the Basement" Intervals (5 x 2min @ RPE 5-6 w/ 2min rest inter-set)

HR scroll from individual w/ Polar H10

Addressing the Anaerobic Threshold

The anaerobic threshold is a physiological marker that represents the point during exercise where the body's demand for oxygen exceeds its ability to supply it. As a result, there is a rapid inflection of blood lactate accumulation (~4mmol+), which can be used as an indicator of the intensity at which the anaerobic threshold occurs. Slightly below this marker is where Maximal Aerobic Speed/Power, Critical Speed/Power, and Maximal Lactate Steady State lay.

At this point Aerobic Respiration cannot keep up with the demand for oxygen and Slow Glycolysis is relied upon heavily for musculature contractions. Lactate begins to accumulate due to the inability of pyruvate to enter the Citric Acid Cycle and the rapid abundance of hydrogen ions accumulating. This only lasts 5min-10min and is unsustainable as our muscle cell pH drops becoming more acidic and inhibiting various functions. With the inability to push this into the blood for transport to be used as a fuel source through Aerobic Respiration and gluconeogenesis, alongside molecular signaling responsibilities.

Obviously a relatively satisfactory “basement” is required here, but through exposure to Anerobic Threshold Intervals positive adaptation can allow lactate kinetics to improve. Adaption in metabolic performance through supporting physiological pathways can bring the result of the same effort/work with more efficiency (I.e., lesser metabolic cost). This due to an increase in muscle capillary density and volume, several enzymes of oxidative metabolism, and or monocarboxylate transporters (I.e., catalyzing lactate). Gharbi and colleagues in 2008 explored this by having a control, LSD, and HIIT group. The HIIT group completed intervals 90%-100% of Maximal Aerobic Speed for 2min and 1min of inter-set rest for four weeks. Compared to the other groupings there was a significant improvement in lactate exchange/removal (I.e., shuttling/utilization) for the HIIT group although interestingly enough the continuous training group had the same total volume.

Example: Anaerobic Threshold Intervals (5 x 2min @ RPE 6-7 w/ 1min active and 1min passive recovery inter-set)

HR scroll from individual w/ Polar H10

Addressing the VO2max AKA "The Ceiling"

VO2max, or maximal oxygen consumption, is a measure of the maximum amount of oxygen that an individual can utilize during exercise. It is considered the gold standard measurement of cardiovascular health and performance via aerobic capacity.

As previously mentioned, humans rely on oxygen for survival, and during exercise our muscles progressively continue to demand oxygen to produce energy. VO2max represents the maximum rate at which the body can take in (via inspiration), transport, and utilize. It’s typically expressed relatively as milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). Influenced by various factors including genetics, age, gender, fitness level, and training status. It can serve as a benchmark not only for performance, but vitality as well.

Illustrating the need for intent and purpose-driven programming, Helgerud and colleagues (2007) compared the effects of LSD, threshold intervals, and high-intensity intervals on VO2max, Stroke Volume, Lactate Threshold, and running economy. The 15/15 (15s of running at 90%-95% HRmax and 15s of 70% HRmax of recovery) method and 4x4 (4 sets of 4min at 90%-95% HRmax) method were found to be superior by influencing significant change in participant VO2max by ~4-5 mL/kg/min and improving Stroke Volume by ~10%. Essentially if you want to raise the ceiling, you need to push the peddle down and move that needle on the speedometer.

Example: Norwegian 4x4 Protocol (4 x 4min @ RPE 8+ w/ 3min recovery inter-set)

HR scroll from individual w/ Polar H10 on Strava

Addressing the Anaerobic Speed/Power Reserve

One’s Anaerobic Speed/Power Reserve is a concept used to describe the relationship between an athlete's aerobic capacity (represented by their velocity at VO2max) and their maximal sprint speed and/or power output. This represents the athlete or professional’s ability to generate high speed/power output during short high intensity efforts, beyond what their aerobic capacity alone can sustain- more specifically looking at Fast Glycolysis and the ATP-PCr System.

A large Anaerobic Speed/Power Reserve indicates that an athlete has the capability to produce high speed and/or power outputs that could also be longer or more repeatable than their counterpart(s). Training methods are dependent on the counterpart(s) and demands the stage has set. However, the building of an Anaerobic Speed/Power Reserve calls for the interplay between systems. This is where lactate utilization, shuttling/tolerance, and accumulation all intertwine and display the true relationships between the three processes aforementioned. A high basement yet low ceiling is limiting, conversely a low basement and high ceiling can be as well. Sport, event, profession, or objective specificity should be factored into this decision-making process via the methods below in tandem with qualitative and quantitative analysis of the demands.

Sandford and colleagues (2021) compared and contrasted Anaerobic Speed/Power Reserve to Critical Speed/Power and Maximal Sprint Speed/Power. Highlighted were not only the framework, but profiling potential that can begin to factor in an individual’s fiber typology, locomotor strategies, and other physiological mechanisms that may or may not be predominant. Overlayed with other monitoring strategies and targeted outcomes, this could be used as a programming tool as one may respond to LSD, interval make-ups, and repeat sprints differently. At times being a call for attention, while at others an indication to avoid exposure.

Dalamitros and colleagues (2015) demonstrate this concept with practicality in 12 swimmers by using Critical Swim Speed and Anerobic Distance Covered to determine a reserve that can be used to determine training-induced changes. With the positive correlation it could also be used to profile athletes and as a programming tool due to a maximal front crawl being used for both the 50m and 400m timings, and Maximal Aerobic Speed being estimated and used for pacing for 6 intervals x 100m with 30s rest inter-set. You can begin to see how the addition of monitoring strategies in conjunction with this prescription could attack the principle of individualization.

Example: 10 x 100m Tempos @ RPE 8-9 w/ 2min rest inter-set

HR scroll from individual w/ Polar H10 on Strava

Conclusion

The pathways involved in bioenergetics are interconnected and work together to produce muscular contractions and ultimately movement. Training confluences the metabolic interplay that is managed when task/environmental demands are placed on an organism.

For example, during high-intensity training the ATP-PCr system and anaerobic glycolysis work simultaneously to produce ATP to support rapid muscle contractions. Creating a buffer for the organism to have time to adapt to producing sustained effort if possible. Glycolysis produces ATP with lactate as a byproduct resulting in a drop in pH, coupled with calcium ion accumulation, muscle excitability is reduced and can now not transmit and generate to meet force production requirements, leading to a decrease in performance (I.e., fatigue). However, when intensity decays and oxygen availability rises, the Citric Acid Cycle is the process by which pyruvate is converted to ATP in the presence of oxygen within mitochondrion. The Electron Transport Chain is then a critical final stage of aerobic respiration with the largest return on investment.

Training for vitality, sports, specific professions or objectives, will all have specific demands. Through general and specific preparation it is important to understand what underpins the demands that need to be met, how beneficial it is to have a speedometer allowing a reserve (I.e., lesser metabolic cost), and how to train to meet these accordingly. The purpose of this article is to allow you the reader to take action on all three with a basic understanding of “the why” behind it.

Alexander J. Morgan, MSc., CSCS, RSCC, CEP

References

1. Casado, A., Foster, C., Bakken, M., & Tjelta, LI. (2023). Lactate-guided threshold interval training within a high-volume low-intensity approach represent the "next step" in the evolution of distance running training? Journal of Environmental Research and Public Health, 20, 3729.

2. Gharbi, A., Chamari, K., Kallel, A., Ahmaidi, S., Tabka, Z., & Abdelkarim, Z. (2008). Lactate kinetics after intermittent and continuous exercise training. Journal of Sport Science and Medicine, 7 (2), 279-285.

3. Helgerud, J., Hoydal, K., Wang, E., Karlsen, T., Berg, P., Bjerkaas, M., Simonsen, T., Helgesen, C., Hjorth, N., Bach, R., & Hoff, J. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and Science in Sport and Exercise, 39 (4), 665-671.

4. Sandford, GN., Laursen, PB., & Buchheit, M. (2021). Anaerobic speed/power reserve and sport performance: scientific basis, current applications, and future directions. Sports Medicine, 51 (10), 2017-2028.

5. Dalamitros, A., Fernandes, R., Argyris, T., Vasiliki, M., Dimitrios, L., & Spiridon, K. (2015). Speed reserve related to critical speed and anaerobic distance capacity in swimming? Journal of Strength and Conditioning Research, 29 (7), 1830-1836.

6. Thom, G., Kavaliauska, M., & Babraj, J. (2020). Changes in lactate kinetics underpin soccer performance adaptations to cycle-based sprint interval training. European Journal of Sport Science, 20 (4), 486-494.

7. Kemp, G. (2005). Lactate accumulation, proton buffering, and pH change in ischemically exercising muscle. American Journal of Physiology- Regulatory, Integrative, and Comparative Physiology, 289, 895-901.

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