TITLE: Limits to sustainable muscle performance: interaction between glycolysis and oxidative phosphorylation
five key ideas
1. The sustainable level of power output is determined by the muscle itself, the ability to maintain ATP supply, and the by-products of glycolysis that may limit oxidative phosphorylation.
2. When power output is above the sustainable level, the muscle gets tired due to the build-up of by-products from creating energy.
3. Sustainable power output relies on the ability of mitochondria to supply ATP, but this is not the only factor.
4. Glycolysis plays a crucial role in supplying ATP during exercise, especially during sustained activity.
5. The higher than expected use of glycolysis indicates limits to oxidative phosphorylation in muscles.
TERMINOLOGY
Adenosine diphosphate (ADP): a molecule that is converted to ATP during energy metabolism.
Adenosine triphosphate (ATP): a molecule that stores energy and is used by cells as a source of energy.
Aerobic capacity: the maximum amount of oxygen that the body can use during exercise.
Creatine kinase (CK) equilibrium: a chemical reaction that involves the breakdown of PCr and the synthesis of ATP.
Creatine phosphate (PCr): a molecule that is stored in muscles and used to generate ATP during exercise.
Glycolysis: a process that breaks down glucose to produce energy in the form of ATP.
Glycogenolysis: a process that breaks down glycogen into glucose to produce energy in the form of ATP.
H+: a positively charged ion that is produced during the breakdown of creatine phosphate and glycogenolysis.
Inorganic phosphate (Pi): a molecule that is produced from the breakdown of creatine phosphate during exercise.
Lactate: a by-product of glycolysis that can accumulate in muscles during exercise and contribute to fatigue.
Lactate production: the process of generating lactate from glucose through glycolysis.
Metabolic steady state: a state where the production and consumption of energy in the body are balanced.
Mitochondria: organelles that produce ATP through oxidative phosphorylation and play a critical role in energy metabolism.
Non-invasive 31P magnetic resonance: a technique that uses magnetic resonance imaging to measure the levels of ATP and other metabolites in muscle tissue.
Non-oxygen ATP supply: ATP generated through glycolysis or other anaerobic pathways that do not require oxygen.
Oxidative phosphorylation: a process that uses oxygen and the electron transport chain to generate ATP in mitochondria.
Substrate-level phosphorylation: a process that generates ATP by transferring a phosphate group from a substrate to ADP.
ABSTRACT
This article proposes a way in which our muscles can perform at a sustainable level. We suggest that this is determined by a few factors: the muscle itself, the ability to maintain ATP (a type of energy) supply, and the by-products of glycolysis (a process in the body that creates energy) that may stop the body from creating more energy. We believe that it is not one single factor that stops us from performing at a higher level, but rather the way that the body's processes work together. We looked at how this works in human muscles and rattlesnake muscles by measuring their energy sources during contractions. We found that glycolysis creates between 20% to 40% of the ATP supply during contractions. This high level of glycolysis causes an accumulation of H+ (a type of acid) that stops the signal for oxidative phosphorylation (a process that also creates energy) from increasing. This means that oxidative ATP (created by oxidative phosphorylation) is limited to below the capacity of the body to create it. Both glycolysis and oxidative phosphorylation play an important role in determining the highest steady-state ATP synthesis flux and the sustainable level of work that can be done by exercising muscle.