We come to the end of our introduction on human movements. I believe all sport training, whether professional or recreational, physique training, or training simply for health and longevity should be based in human movements.
Human movements require multi-joint activity, meaning that motion (flexion/extension) is occurring in more than one joint. Squats involve the hip and knee joints; push-ups involve the shoulder and elbow joints, and so on.
Simply put, humans don't move in isolated steps, so we should NOT train our bodies in isolation with single-joint exercises (e.g., bicep curls, tricep extensions, leg extensions, etc.).
However, there are exceptions. Competitive bodybuilding, rehab, or corrective exercises are circumstances where single-joint training can supplement a regimen based in human movements.
Make it a great day!