Jumping is a natural exertion that occurs during a variety of human activities including playing sports, working, skateboarding, dancing, escaping from hazardous events, rescue activities, and many others. During jumping, the ankles in particular are expected to support the entire body weight of the jumper and that may lead to ankle injuries. Each year hundreds of patients are treated for ankle sprains/strains with ankle fractures as one of the most common injuries treated by orthopedists and podiatrists. The knee joint is also considered the most-often injured joint in the entire human body. Although the general anatomy of the lower extremities is fairly well understood, an understanding of the injury mechanism during these jumping tasks is not well understood. The aim of this study is to determine the reaction forces exerted on legs and joints due to vertical jumps, through musculoskeletal simulation and experimental studies to better understand the dynamic jump process and the injury mechanism. The joint reaction forces and moments exerted on the ankle, knee and hip joint during takeoff and extreme squat landing of a vertical jump were determined through the application of musculoskeletal simulation. It is concluded that during extreme squat landing of a vertical jump, joint reaction forces and moments were highest in proximal/distal and anteroposterior direction may cause most likely injury to the hip joint ligaments, ankle fracture and knee joint, respectively.