When people new to weight training exercise, their back and arm muscles show signs of getting tired over time, no matter how they warm up.

When people who don't usually exercise do seated row exercises, their back and arm muscles work harder as they get tired, showing the body recruits more muscle fibers to keep going.

When beginners do intense leg press workouts to the point of exhaustion, their muscles get increasingly tired, and this shows up as changes in muscle activity signals — some muscles work harder, others change how they fire, and these shifts tell us fatigue is building up.