The scenario you propose, where all humans engage in bodybuilding for their entire lives without exception, would create a strong selective pressure favoring individuals with genetic predispositions for increased muscle mass and strength. However, estimating the exact number of generations it would take for humans to evolve as much more muscular creatures is challenging due to several factors:
Genetic diversity: The starting genetic diversity of the human population would significantly impact the rate of change. If there is already a considerable amount of genetic variation for muscle mass, some individuals may have a head start in terms of advantageous traits.
Rate of reproduction: Evolution relies on the transmission of genetic traits from one generation to the next through reproduction. The rate at which individuals reproduce and pass on their genes would affect the speed of evolutionary change.
Mutation rate: New genetic variations can arise through mutations. The rate of beneficial mutations related to muscle growth would influence the speed of adaptation.
Gene flow: If there is migration or gene flow between populations with different levels of muscle mass, it could slow down the evolution towards more muscular creatures.
Pleiotropy: Genes often have multiple effects on an organism's traits. Enhancing muscle mass through selection may have unintended consequences on other aspects of health and survival, which could influence the course of evolution.
Genetic-environmental interactions: The interaction between genetic factors and the bodybuilding environment would play a significant role. Some individuals may respond better to bodybuilding training due to their genetic makeup.
Given the complexity of these factors, it's challenging to provide a precise estimate. However, it's essential to recognize that evolution is a gradual and continuous process that operates over vast time scales. Significant changes in the human body, even under strong selective pressures, would likely require thousands to tens of thousands of generations.
Moreover, it's worth noting that in real-world scenarios, such an extreme and uniform selective pressure is unlikely to occur. Evolution is driven by a combination of many factors, including natural selection, sexual selection, and genetic drift, which act on various traits and aspects of human biology.