In reference to exercise, intensity is measured by heart rate. Any body movement involves an increased heart rate. Heart rate also is affected by internal factors such as hormones in response to stress or fear. Classifying workouts as leg day, arm day, chest, back, whatever, is archaic. Organizing workouts based on heart rate is the most lucid way to train. There is no mystery. You schedule your most intense workouts early in the week, and the lesser intense workouts later in the week. Exercises utilizing larger muscle groups (squats, deadlifts, lunges) are generally more intense than those utilizing smaller muscle groups (bench press, biceps curl, and shoulder raise). Other factors which influence intensity are multi joint vs single joint exercises, open chain vs closed chain, time under tension, tempo, load, rest time in between sets, temperature & humidity, and drugs.
Exercising should be in everyone’s daily routine. Over training is not as prevalent as you think. Shitty programming is. Dad was telling me about a co worker that was lamenting on soreness and his inability to recover from workouts. He told my Dad he lifts 4 days on 1 day off. Dad suggested he do 2 days on 1 day off. Great suggestion, but not necessary. Using a basic heart rate monitor and target heart rate he could rearrange his workouts based off heart rate. For example say the guy in question is 45 years old. If he organizes his workouts so that his Monday workout keeps him at or below 154 bpm, then 149 bpm on Tuesday, 144 on Wednesday, 138 on Thursday, Friday off, this fellow can now continue working hard every week in the gym without burning out and without taking more days off.
Heart rate training is universal. It works for pilates, olympic lifting, yoga, body building, barre, strength training, running, rowing, cycling, etc. The endurance community understands this very well. The insights are much clearer once you stop “feeling” your way through workouts. It is truly the best measure for any workout.