EE stated he was helping set a breaker timber approx 10 1/2'L & 20"" dia. The timber had been cut to length & 2 co-workers lowered 1 end to the ground from the scoop & EE & another co-worker were holding other end of timber. W/out telling EE, other co-worker lowered the timber to rest on his knee, shifting the weight to EE who experienced pain in lower back.