Acute adaptations or responses of skeletal muscle to resistance training occur during or shortly after a bout of exercise. The immediate response is the accumulation of fatigue-producing metabolites in the muscle. Chronic adaptations, on the other hand, are changes that manifest long term as a result of resistance exercise. Chronic adaptations are a result of consistent acute changes in skeletal muscle and persist for longer periods of time, as well.
Acute Acid Buildup
One acute or immediate response resistance training has on skeletal muscle is the accumulation of fatigue-producing metabolites. Inorganic phosphate, ammonia and hydrogen ions (H+) are all byproducts of metabolism. H+, for example, lowers the pH of the muscle when present, making the environment more acidic, commonly known as lactic acid. This is manifested during training as a burning sensation leading to muscular fatigue.
Acute Energy Depletion
Energy depletion is another response to resistance training. Creatine phosphate (CP) and glycogen are fuel substrates that help provide immediate energy to working muscle. Essentially, adenosine triphosphate (ATP) is the immediate source of energy for muscular contraction. However, because it can only be stored in small quantities, energy depletion occurs rapidly. Therefore, ATP needs to be restored early and often.The CP compound is broken down into separate creatine and phosphate molecules and the phosphate is used to create more ATP for muscular contraction. But, with continued training, levels of CP and glycogen decrease. Glycogen replenishment could take several hours up to several days.
Muscular hypertrophy, or growth, is the primary chronic adaptation to resistance training. Hypertrophy is the increase in the cross-sectional area of the muscle fibers. Similar to the lattice work found in lawn or pool chairs, the cross-sectioning of the fibers in skeletal muscle increases their force and power capabilities. Resistance training increases protein synthesis, or building, within the muscle fibers. Long-term training results in increased capacity for creatine phosphate in the muscle, thus reducing or delaying the onset of fatigue during a session of exercise.
Muscle Fiber Types
Skeletal muscle can be broken down into two types: slow-twitch, or Type I, and fast-twitch, or Type II, muscle fibers. Basically, slow-twitch muscle fibers are involved in activity lasting over a period of time, while fast-twitch fibers are integral for rapid and powerful movements of short durations. With consistent resistance training, Type I fibers remain the same while the percentage of Type II increases. However, these newly formed fibers are classified as Type IIa because they take on some of the characteristics of Type I fibers.