We propose a design of a back assistance exoskeleton for workers powered by pneumatic artificial muscle and validated the efficiency of the system using sEMG data based evaluation index (sEMG peak, Integrated EMG, RMS values). The selection of commercial artificial muscle for instantaneous compression force (similar to 6000N) with developed power transmission structure of minimized level of friction could effectively transmitted the assistance power. The level of air pressure from the compressor were determined from the human feedback experiment, ranges from 0.65Mpa similar to 0.75Mpa. The sEMG based indices on the hamstring and erector spinae muscles represented the decreased level of 20% to 30% in muscle activation when lifting the loads. We found the feasibility of using lumbar support hardware in weight lifting, also the validity of evaluation index based on sEMG processing technique.