This article is part of our Injury Analysis series.
The 49ers starting running back Jerick McKinnon made an early exit from practice on Sunday with what appeared to be a knee injury. McKinnon was seen grabbing the back of his right knee and would later undergo a MRI on the area. The medical imaging revealed a low-grade calf strain that will keep him sidelined for at least a week.
The results of the MRI may surprise people that don't normally associate the calf muscle complex with the knee. However, the calf is a two-joint muscle, meaning it's active in two different areas. Most people know the calf attaches to the heel via the Achilles' tendon, but they tend to overlook that it originates above the knee joint. The gastrocnemius and the soleus, the two muscles that comprise the calf, begin at the end of the femur and the tibia. As a result, the muscle group is able to play a role in knee flexion as well as plantarflexion of the ankle.
This positioning and dynamic role can prove detrimental when an injury occurs. A strained calf can cause limitations at both joints, a big setback for a position like running back that depends on explosive movement. Even worse, a strained calf can have a crossover effect on the muscles of the hamstrings. The aforementioned starting point of the calf sits in close proximity to the insertion site of the hamstrings. This location allows the two groups to work together to bend the knee. Unfortunately,