Can anyone guarantee an increase in fitness?

According to the trainer, based on training experience and research, it is possible to predict the body’s reactions to a certain point. This is what training is all about.
What makes sport beautiful and timeless?
The beauty of the body. Its unpredictability, enormous complexity, level of intricacy.
The ability to rebel and reward.
The multitude of factors that affect the final well-being, strength, or endurance.
This is where training errors and failures come from.
Despite the tools a person has, the body is still a beautiful mystery.