Mechanism of action and functions of caffeine
Pharmacologically, caffeine is an adenosine receptor antagonist. Adenosine's role is to inhibit the activity of the nervous system so it doesn't work at top speed all the time, which would be incompatible with maintaining body homeostasis. Caffeine occupies a place on many adenosine receptors, which makes it impossible for adenosine to attach to them and produce its own specific effect. As a result, the nervous system is stimulated, which is known by all people who have drunk strong coffee at least once.
When it comes to training, everyone wants their performance to be as high as possible. The main studies of caffeine on muscle strengthening were made in reference to people working out at the gym. The results were unambiguous - people who took caffeine before a workout had a greater ability to lift the maximum weight in a given exercise. Although it's also important to note that the CNS stimulation and training mindset itself definitely had to have an effect on this. It is well known that it is better to exercise stimulated than muddled....
Summary
However, be careful - as with everything in life, one should keep moderation. Too much caffeine can be toxic. Cold sweats, accelerated heart rate, psychomotor agitation are some of the side effects of excessive caffeine use.
The optimal stimulant dose obviously depends on a person's body weight. It is generally said that between 3-6 mg of caffeine per kilogram of body weight should produce beneficial and noticeable effects. See Crossthelimits