The acute increase in muscle thickness following resistance training is mediated by intracellular hydration changes driven by metabolic stress, as indicated by concurrent lactate accumulation and mechanical stimulus duration, though this does not establish a causal relationship with long-term hypertrophy.

When muscles work hard for a long time, they produce waste chemicals like lactate that build up inside the cells. This makes the inside of the cells more concentrated, so water rushes in from outside to balance it out. The extra water swells the muscle fibers, making the muscle look and feel thicker right after exercise.