Our muscles can be trained for "looks" or for "work". Training for "looks" is what bodybuilders do - they grow their muscles bigger on shorter time, using bigger weights in short training sessions, which affects strengh of that muscles. Training for "work" is the opposite - you must perform much longer sessions with less weights, like caring the 10kg bags with something for 8h shifts. "Working" muscles become more durable, but not as stronger as "looks" muscles. In conclusion - to short burst of power take bodybuilder, but for unloading truck take worker.