Basic Strength Training Guidelines
True strength training is very taxing on the nervous system, but is far less taxing on the musculoskeletal system than some other forms of resistance training. Common recommendations are to train a muscle group for strength 2x per week. This will certainly provide enough stimulus for adaptations and ensure proper recovery.
This is not a bad system, but if strength is the sole goal, THEORETICALLY strength training-even on the same muscle group- can be done much more frequently. Think of Olympic weightlifters- they use a high % of their 1RM very frequently, sometimes squat, clean and jerk, and snatch multiple times per week (same primary muscles). This is because there is a relatively small amount of structural (muscular) damage. Neural adaptations happen very quickly, even yes, over night. Of course frequency is related to volume and intensity and all three need to be balanced to optimize strength gains. Under most circumstances, strength training requires HIGH intensity. Now we can either have high volume or high frequency, but choosing both would quickly lead to negative consequences such as injury or overtraining.
Each athlete is different, but a few hard sets of strength training per day-even on the same muscle group - can send a sufficient signal without causing excessive structural damage. My recommendation is not to training the same muscles every day, but it is not as ridiculous as many experts are trained to believe.
In general, I recommend picking 2-3 main exercises per block/cycle that you would like to get stronger at, this works perfect for powerlifting or Olympic weightlifting. Each exercise can be trained AT LEAST 2x per week and up to 4x per week. Each session, perform at least 1 very hard set between 1-6 reps that will be sufficiently difficult to stimulate growth. It is very reasonable to train both upper and lower body strength on the same day. We all respond differently to different stimulus, so find what works for you, be safe, and have fun with your training!