Describe the primary cellular mechanism by which creatine monohydrate directly enhances exercise performance and contributes to increased muscle mass.

Creatine monohydrate primarily enhances exercise performance by increasing the availability of phosphocreatine within muscle cells, which is crucial for rapid energy regeneration. Muscle contraction directly uses adenosine triphosphate (ATP) as its immediate energy source. However, the body's intramuscular ATP stores are very limited and deplete within seconds during high-intensity, short-duration activities. To quickly replenish ATP, muscle cells utilize phosphocreatine (PCr), a high-energy phosphate compound. When ATP is broken down for energy, it loses a phosphate group, forming adenosine diphosphate (ADP). The enzyme creatine kinase facilitates the transfer of a phosphate group from PCr to ADP, rapidly regenerating ATP. By supplementing with creatine monohydrate, the total phosphocreatine stores within the muscle are elevated. This increased PCr availability allows for a faster and more sustained rate of ATP regeneration during intense, anaerobic efforts, such as weightlifting, sprinting, or repeated high-power output activities. This cellular mechanism enables individuals to perform more repetitions, lift heavier loads, or maintain higher power output for a slightly longer duration before fatigue sets in, directly enhancing exercise performance. This enhanced capacity for high-intensity work indirectly contributes to increased muscle mass by allowing for a greater training stimulus. The ability to perform more total work volume, which includes more repetitions, sets, or heavier loads, over time is a primary driver of muscle hypertrophy, the increase in muscle cell size. Furthermore, creatine directly contributes to increased muscle mass through cellular volumization. As creatine is transported into muscle cells, it is an osmotically active substance, meaning it draws water into the cell. This influx of water causes the muscle cell to swell, a phenomenon known as cell volumization. This cellular swelling is recognized by the muscle cell as an anabolic signal, which directly stimulates muscle protein synthesis, the process of building new muscle proteins, and concurrently inhibits protein degradation, the breakdown of existing muscle proteins. Additionally, creatine may directly influence the expression of genes associated with muscle growth and enhance the activity of satellite cells, which are stem cells vital for muscle repair and regeneration after exercise-induced damage.