Grug move arm. When grug arm move straight bottom muscle get hard. When grug move arm back top muscle hard. But when grug squat top leg muscle is hard and bottom leg muscle is hard at same time? Grug know, arm muscle only move with elbow, one joint. But when grug squat knee joint and hip joint are used, two joint. Grug can only pick up small rock when leg are straight and grug can only squat with small rock when body is straight. To pick up big rock Grug need to bend leg and body same time. Grug get very tired when pick up big rock and grug no feel tired when pick up small rock. So grug will have better time picking up big rock. Maybe grug get bigger leg and bigger back if grug mostly pick up big rock instead of only small rock.