Anatomy of running: QUADS & HIP FLEXORS

The quadriceps are a group of muscles located at the front of the thigh, spanning from its origin at the front of the hip and forming the patellar tendon to attach below the knee cap at the tibia. The vastus lateralis, vastus intermedius, and vastus medialis are located from the outer part of the thigh to the innermost part of this group. The fourth quadricep muscle is the rectus femoris, which acts on both the hip and knee joint as a hip flexor and knee extensor.

The rectus femoris works in concert with three other hip flexor muscles, namely the sartorius, the iliacus and the psoas (collectively the iliopsoas) muscles. During running, hip flexion begins as elastic energy is stored during the initial swing or "push-off" phase. The stored energy accompanied by the hip flexor muscle group contracting helps drive the leg forward through swing phase.

The quadriceps and knee extensor group has essential functions both in the stance phase and the swing phase of the gait cycle in running. During the initial contact portion of the stance phase, the quadriceps act eccentrically to control knee flexion as the knee rapidly bends under load. The lower limb absorbs 2-4x body weight during this phase, so stabilization from the quadriceps are crucial. Throughout mid-stance phase the quadriceps remain active, but reduce activity during terminal stance until the last 20% of the swing phase. During this terminal swing phase, the quadriceps act concentrically to extend the knee in preparation for foot strike. This action occurs simultaneously as the gluteal muscles extend the hip and absorbs impact as the runner lands.

Adequate quadricep strength is important for speed or force production, endurance, and injury prevention. Stronger quads can increase push-off power, which can help runners boost their speed. Improving muscular endurance of the quads can help maintain form and reduce excessive knee flexion during stance phase, which can improve endurance and reduce any breakdown in form over the course of longer runs. It has been shown that inadequate quad strength is a high risk factor for patellofemoral pain in runners, and therefore maintaining strength here can reduce incidence of runner's knee.

Considering that running is a single leg sport -- a series of single-legged forward hops -- unilateral quad and hip flexor strengthening may carry the most specificity. Exercises such as rear-foot elevated "Bulgarian" split squats, eccentric step downs or single leg squats are effective for building unilateral stability and strength. These exercises may be progressed with added resistance or weight.