On May 9, 1997, the National Institute for Occupational Safety and Health (NIOSH) received a request for a health hazard evaluation (HHE) from the U.S. Department of the Interior. The request specified cases of muskuloskeletal disorders of the upper and lower extremities and back among workers who build and maintain the back country trails of Yosemite National Park. Some of the work tasks specified as particularly hazardous were "drilling", "hammering", "shaping granite rocks", "lifting rocks and timbers", and "backpacking on rough terrain with heavy loads." During August 25-28, 1997, NIOSH visited the Buena Vista Lakes camp of Yosemite National Park, a site selected by the management of the Park's trail maintenance department. The visit included an opening conference, an evaluation of the trail maintenance projects being performed at the time, interviews with each of the workers, videotape and still photos of the various work tasks, and measurement of manual forces required to perform some of the work tasks.