Playgrounds and gym equipment are subjected to large dynamic forces and must be designed to support these loads and protect users from unintentional hazards; if playgrounds equipment is not adequate to handle these loads, injuries could result. Falls  from equipment or due to surface imperfections are also a common source of injury. Adequate instructions and warnings  may be required if the assembly and proper use of toys or equipment are not obvious. There are also issues associated with safety, adequate supervision and maintenance of gyms and playgrounds to assure that they are used properly and do not become dangerous due to either misuse or normal wear and tear. Issues associated with playground accidents require an understanding of biomechanics, human factors, safety, dynamics and structural engineering.