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Relative strength of the legs at various knee angles was determined in the standing position by three different test procedures. Man's weakest knee angle was found to be 60 degrees. X-ray studies of the legs and feet during vertical loading failed to reveal any bending of the femur or tibia or compression of cartilages in the knee or ankle. There was a slight lateral bending of the fibula and the tarsal and metatarsal bones were displaced downward. Human voluntary tolerances to vertical impact were determined while (1) standing with knees locked, (2) standing with knees bending, (3) squatting, and (4) seated in a rigid chair. In addition, various energy-dissipating materials and devices were evaluated for protection against vertical impact. This exploratory study of man's tolerance to vertical deceleration was divided into three phases: (1) Determination of relative strength of the legs at various angles of knee flexure. (2) Study of deformation of joints and long bones of the legs by double exposure X-rays of subjects static loaded with weights. (3) Determination of man's voluntary tolerance to vertical impact forces in drop tests; and evaluation of various energy-dissipating materials for protection against vertical impact. (Author)