Due to worker injuries in industries such as nursing and construction, we have developed a tunable and concealable worker-wear assistance suit that consists of elastic belts to prevent arm muscle fatigue and injury in workers whose arms repeatedly perform motions upon their shoulders. According to the layout and the material of the belts, the suits may assist the user's arm without any electrical resources. To verify the feasibility and functionality of the layout and the structure of our proposed suits, simulations and experiments were employed. The simulation was conducted based on the average body dimensions of young Japanese people, and its results showed that the proposed suits may effectively assist the user's arm muscles. By measuring the muscles activities of six subjects wearing these suits and lifting a load of 10 kg, practically and concretely verifying the functionality of the suits, the results showed a 45% improvement ratio for their arm muscles. The proposed assistance suit can not only directly support and assist the elbow and shoulder j oint but also indirectly prevent lower back injury.