A femur, which is the largest bone in the leg and the body as a whole, can be fractured with approximately 4000 newtons of energy, according to an article in SciFighting. The tibia and fibula, both smaller bones, require significantly less force.
The human skeleton is surprisingly strong and, ounce-for-ounce, outperforms steel. Being thin and light, however, breaking a leg is relatively easy. A 2010 Wayne State University study calculated that a competitive mixed martial arts fighter can generate up to 5000 newtons in a single punch and 9000 newtons in a single kick, which is enough force to break a femur, a tibia, a fibula and just about every other part of the leg.