Bioenergetic Pathways and Energy System Contributions

Overview of Bioenergetic Pathways

• The human body relies on three primary bioenergetic pathways to supply adenosine triphosphate ($\text{ATP}$) for skeletal muscle contraction:

  • Phosphagen ($\text{ATP-PCr}$) system

  • Glycolytic (lactic acid) system

  • Aerobic (oxidative) system

• These bioenergetic systems differ across five fundamental parameters:

  • Speed of $\text{ATP}$ production

  • Total energy capacity

  • Primary substrate utilization

  • Oxygen dependence

  • Intensity and duration of physical activity supported

• Energetic Continuity and Interaction:

  • The bioenergetic pathways operate continuously and interact simultaneously rather than functioning in a strict sequential sequence.

  • The relative contribution of each system shifts dynamically along a continuum based on exercise intensity and duration.

Phosphagen (ATP-PCr) Energy System

• Bioenergetic Classification and Speed:

  • Represents the fastest anaerobic energy pathway in human physiology.

• Substrate Requirements and Storage:

  • Relies on intramuscularly stored adenosine triphosphate ($\text{ATP}$) and phosphocreatine ($\text{PCr}$), which is also designated as creatine phosphate.

• Enzymatic Mechanism of Action:

  • When $\text{ATP}$ is hydrolyzed to adenosine diphosphate ($\text{ADP}$) and inorganic phosphate ($\text{P}_i$) to power muscle contraction ($\text{ATP} \rightarrow \text{ADP} + \text{P}_i$), the enzyme creatine kinase rapidly transfers a high-energy phosphate group from $\text{PCr}$ to $\text{ADP}$ to resynthesize $\text{ATP}$.

• Performance Characteristics and Limitations:

  • Extremely rapid rate of $\text{ATP}$ regeneration.

  • Very limited total capacity, sustaining maximal effort for roughly $5\text{--}15\,\text{s}$.

  • Requires no oxygen ($\text{O}_2$ dependence: none) and produces no lactate.

• Primary Physical Applications:

  • Dominates short, maximal, all-out physical efforts.

  • Specific examples include a maximal vertical jump, a 1-repetition maximum ($1\text{RM}$) heavy lift, or the initial few seconds of a high-intensity sprint.

Glycolytic (Lactic Acid / Anaerobic Glycolysis) System

• Bioenergetic Classification and Pathway:

  • An anaerobic pathway that breaks down glucose or glycogen to pyruvate via glycolysis.

• Enzymatic Mechanism Under Low Oxygen:

  • When oxygen supply is insufficient relative to metabolic demand, pyruvate is converted into lactate and hydrogen ions ($\text{H}^+$).

  • The process regenerates a modest amount of $\text{ATP}$.

• Performance Characteristics and Limitations:

  • Faster rate of $\text{ATP}$ production than the aerobic system, but slower than the phosphagen system.

  • Energy Provision Window: Sustains high-intensity efforts lasting roughly $15\,\text{s}$ to $2\text{--}3\,\text{min}$.

  • Produces lactate and associated metabolic acidosis ($\text{H}^+$ accumulation), which contributes directly to fatigue.

• Primary Physical Applications:

  • Dominant pathway during high-intensity efforts such as a $400\,\text{m}$ sprint, high-intensity interval training (HIIT) efforts, or repeated sets of heavy resistance training.

Aerobic (Oxidative) System

• Bioenergetic Classification and Pathway:

  • The oxygen-dependent energy pathway.

• Substrate Requirements and Cellular Site:

  • Fully oxidizes carbohydrates, fats, and to a lesser extent, proteins.

  • Substrates are processed inside the mitochondria via the Krebs cycle and electron transport chain to produce large quantities of $\text{ATP}$.

• Performance Characteristics and Limitations:

  • Possesses the slowest rate of $\text{ATP}$ production among the three pathways.

  • Exhibits by far the greatest total energy capacity.

  • Highly efficient and sustainable as long as oxygen delivery and cellular utilization match metabolic demand.

• Primary Physical Applications:

  • Predominates during prolonged, lower- to moderate-intensity continuous physical activities lasting generally longer than $\sim 2\text{--}3\,\text{min}$.

  • Supports sustained endurance activities including distance running, cycling, swimming, and team-sport play conducted at submaximal intensities.

Dynamics of Energy System Contributions in Sprint Events ($\le 15\,\text{s}$)

• Simultaneous Activation:

  • All three bioenergetic pathways are activated simultaneously at exercise onset rather than operating sequentially (Archacki et al., 2024; Hargreaves & Spriet, 2020).

  • The phosphagen system dominates brief maximal efforts, providing $87\%$ of total energy during a $3\text{-second}$ cycling sprint and declining to $50\%$ by $12\,\text{s}$ (Dunst et al., 2025).

• $100\text{-Meter}$ Sprint Running Bioenergetics:

  • Phosphagen ($\text{ATP-PCr}$) system: Accounts for approximately $61\text{--}70\%$ of total energy expenditure (Park et al., 2021; Saleh & Aga, 2025).

  • Glycolytic system: Contributes $\sim 23\%$ of total energy expenditure (Park et al., 2021; Saleh & Aga, 2025).

  • Oxidative system: Contributes as low as $9\%$ of total energy expenditure (Park et al., 2021; Saleh & Aga, 2025).

• $15\text{-Second}$ All-Out Sprint Dynamics in Speed-Power Athletes:

  • Phosphagen system contribution: $45\%$ (Archacki et al., 2024).

  • Glycolytic system contribution: $48\%$ (Archacki et al., 2024).

  • Aerobic system contribution: $7\%$ (Archacki et al., 2024).

• Peak Glycolytic Power Timing:

  • Glycolytic power reaches its peak at approximately $6\,\text{s}$ into an all-out sprint, coinciding directly with the transition from acceleration to deceleration (Briand et al., 2025).

Dynamics of Energy System Contributions in Middle-Distance Events ($60\,\text{s}\text{--}4\,\text{min}$)

• System Transition Dynamics:

  • As maximal effort duration extends beyond $30\text{--}60\,\text{s}$, the glycolytic pathway becomes the primary overall energy contributor.

  • During $60\text{-second}$ cycling sprints, the glycolytic pathway supplies $42\%$ of total energy expenditure (Dunst et al., 2025).

• Bioenergetic Crossover Threshold:

  • A maximal effort lasting approximately $75\,\text{s}$ derives equal energy ($50\%$ each) from aerobic and anaerobic systems, representing the exact crossover point (Tortu et al., 2024).

• $400\text{-Meter}$ Track Running:

  • Total anaerobic sources ($\text{ATP-PCr}$ plus glycolysis): Provide $62\text{--}63\%$ of total energy expenditure (Hill, 1999; Saleh & Aga, 2025).

  • Glycolytic component specifically: Contributes roughly $30\%$ (Hill, 1999; Saleh & Aga, 2025).

  • Substantial phosphagen depletion is confirmed by Excess Post-exercise Oxygen Consumption ($\text{EPOC}$) derived measurements (Hill, 1999; Saleh & Aga, 2025).

• $800\text{-Meter}$ Track Running:

  • Anaerobic energy contribution drops to $33\text{--}39\%$ (Hill, 1999).

• $1500\text{-Meter}$ Track Running:

  • Anaerobic energy contribution falls further to $17\text{--}20\%$, illustrating the progressive shift toward oxidative metabolism as event duration increases (Hill, 1999).

Dynamics of Energy System Contributions in Endurance Events (>4\,\text{min})

• Bioenergetic Proportions Across Specific Distance Modalities:

  • $3000\text{-Meter}$ Run:

  • Phosphagen ($\text{ATP-PCr}$) contribution: $\sim 6\%$ (measured via $\text{EPOC}$) (Saleh & Aga, 2025).

  • Glycolytic contribution: $\sim 6\%$ (Saleh & Aga, 2025).

  • Oxidative contribution: $\sim 68\%$ (Saleh & Aga, 2025).

  • Continuous Efforts Lasting >6\,\text{min}:

  • Phosphagen ($\text{ATP-PCr}$) contribution: <10\% (Rios et al., 2026).

  • Glycolytic contribution: $3\text{--}5\%$ (Rios et al., 2026).

  • Oxidative contribution: $75\text{--}88\%$ (Rios et al., 2026).

  • Marathon Running ($\sim 2\text{--}2.5\,\text{h}$):

  • Phosphagen ($\text{ATP-PCr}$) contribution: Minimal (Hargreaves & Spriet, 2020).

  • Glycolytic contribution: Minimal (Hargreaves & Spriet, 2020).

  • Oxidative contribution: Near-total carbohydrate oxidation (Hargreaves & Spriet, 2020).

• Substrate Selection and Metabolic Efficiency:

  • Oxidative phosphorylation dominates events lasting beyond $1\,\text{min}$, with intramuscular glycogen serving as the primary fuel source (Hargreaves & Spriet, 2020).

  • At exercise intensities approaching $80\text{--}100\%\,\text{VO}_2\text{max}$, fuel selection shifts heavily toward carbohydrate because aerobic $\text{ATP}$ production from carbohydrate is approximately $7\%$ more efficient than from fat (Hargreaves & Spriet, 2020).

• Intermittent High-Intensity Activities:

  • In intermittent high-intensity efforts with short recoveries—such as ice hockey shifts—the oxidative system supplies $63\%$ of total energy despite the sprint-based nature of the activity (Gabrys et al., 2026).

Modulating Factors Influencing Bioenergetic Contributions

• Athlete Specialization:

  • Athlete specialization shifts proportional bioenergetic contributions even when performing identical physical tasks (Archacki et al., 2024).

  • During a $15\text{-second}$ sprint:

  • Endurance-trained athletes derive $47\%$ of energy from phosphagen, compared to $35\%$ in team-sport athletes (Archacki et al., 2024).

  • Endurance-trained athletes achieve a $12\%$ aerobic contribution, exceeding the $7\text{--}8\%$ aerobic contribution observed in speed-power athletes (Archacki et al., 2024).

• Repeated Sprint Protocols and Rest Dynamics:

  • In repeated sprint protocols, glycolytic contribution drops precipitously from $\sim 36\%$ during the first 5 sprints to below $7\%$ by sprints 11–15 (Ulupınar et al., 2024).

  • Oxidative contribution rises to $33\%$ across repeated sprints, surpassing glycolytic contribution in later repetitions (Ulupınar et al., 2024).

  • Interval Duration Dynamics: Longer sprint distances and shorter rest intervals increase glycolytic contribution, whereas shorter sprints with longer rest intervals elevate the relative aerobic share (Ulupınar et al., 2021).

• Sex-Based Energetic Comparison:

  • Sex does not significantly alter the relative proportions of energy system contribution during sprint exercise (Archacki et al., 2024).

  • Absolute energy expenditure differs significantly between male and female athletes during matching tasks (Archacki et al., 2024).