If you go to any track meet or practice you’ll likely hear the sprint coach yelling KNEES UP to one or more of his athletes. And perhaps the most utilized drill in the entire sport is the high knee drill. So what’s behind this ubiquitous cue? Why do we want athletes to lift the knees when they run fast? Many recognize and observe the benefit in performance but few understand the actual mechanisms of benefit. Most understand that high knee recovery (synonymous with hip flexion) during the swing phase potentially increases stride length but if you don’t fully understand WHY you could miss some of the benefits or other related training off-shoots. So here are six benefits of knee lift for sprint performance:
- Increased Ground Reaction Force: Newton said that when one body exerts a force on another body, that second body will exert an equal and opposite force back on the first. In this case, we are the first force pushing down on the ground. As you flex your hip, the internal force necessary to do so moves down to the point of contact and increases ground reaction force. In turn, the ground applies an equal and opposite force back on us. As several studies over the past 15 years have shown, greater ground contact force (specifically, mass specific force) correlates very well with increased speed.
- Increases Pre-Stretch: During late swing phase, as the hip is approaching peak flexion values, a significant stretch is placed on the glutes and hamstrings on the swing leg side due to the positioning of the thigh relative to the trunk. When muscles are actively placed on stretch they are capable of a greater contraction due to the stretch reflex and improved actin-myosin interaction.
- Permits Greater Limb Acceleration: Ideally, ground contact is made with the foot relatively close to the plumb line of the hips with the thigh in a nearly perpendicular position with the ground. This is the position that allows for stability preservation while controlling the amount of breaking forces experienced by the athlete. If the athlete’s peak hip flexion positions the thigh parallel with the ground when they are in flight, this means the athlete has an opportunity to accelerate the thigh through almost 90 degrees prior to contact. I liken this to the benefit of withdrawing a hammer a greater distance prior to striking a nail. If you only pull the hammer back an inch from the nail, it will be very difficult to accelerate the hammer head enough to strike the nail with sufficient force. However, if you withdraw the hammer head back several inches from the nail, you can strike the nail with much greater force because now the hammer head has a greater range to accelerate.
- Preserves Posture: In late stance phase, the support leg moves in to a position of hip extension and in many athletes, hyperextension. Even amongst the world’s best sprinters, who toe off with minimal horizontal excursion of the hip relative to the foot there is still contralateral hip extension. With hip extension comes a tendency to move in to anterior pelvic tilt (see Young et al, 2004, and this article on ET). Excessive anterior tilt is commonly associated with poor ground contacts, hamstring injuries and a slew of other issues. So how do we control the inevitable? By flexing the contralateral hip! As one hip extends aggressively, the other should flex aggressively.
- Activates Crossed-Extensor Reflex: Lifting the knee on the swing leg side may increase motor output of the support leg via the cross-extensor reflex. When we actively flex the hip, the hip extensors on that same side relax and the the extensors on the opposite side are activated. Now obviously the extensors are already working hard during support, but this extra afferent boost may help an athlete put a little more force in to the ground.
- Improved Pelvic Oscillation: Although the commonly used cues knees up and high knees seem to imply the knees move linearly, anatomy dictates that the thigh rotates about the hip axis. As a result of this, the fact that we’re in single support, and the sacrum and illium forming a joint capable of movement, the swing leg side hip can and should move upward AND forward on every stride (unless of course you’re T-1000 and then you can run however you’d like). This oscillation of the pelvis should allow for greater elastic response from the soft tissue of the lumbo-pelvic-hip complex and increase stride length by a couple inches.