Running Science

by Owen Anderson

  There can be significant differences in kinematic variables even between similarly trained runners, but general movement sequences and force-application patterns are similar among runners. When the gait cycle is divided into stance (the period between first contact with the ground and toe-off) and swing (the period between toe-off and the next foot strike), certain patterns of movement, muscle action, and joint angle are uniform among runners. This can be illustrated by examining what happens to the thigh, knee, and ankle during stance and swing and linking the movements of these parts of the leg with muscular activity and force production (i.e., connecting kinematics with kinetics).1

  Thigh

  The key functions of the thigh during running are to stabilize the knee; cushion the impact of the foot colliding with the ground on each step, accomplished in part by changing the angle between the thigh and lower part of the leg; and produce forward propulsive force during knee and thigh extension, which occurs during every stance phase of gait.

  At the precise moment of foot strike at the beginning of stance for a heel-striking runner (an athlete who makes first contact with the ground with the heel rather than the mid- or forefoot), the thigh is ordinarily at about a 25-degree forward angle from an imaginary vertical line drawn straight down through the center of the body (figure 4.4). In other words, the thigh is not directly under the torso but is inclined forward by 25 degrees with respect to the imaginary line. For the heel-striker, the thigh reaches an angle of 0 degrees (positioned directly under the upper body) about half-way through stance and reaches about 35 degrees of extension (inclined behind the upper body with a 35-degree angle between thigh and midline) at toe-off (figure 4.5).

  Figure 4.4 The thigh of a heel-striking runner reaches about a 25-degree angle in front of the body as the foot strikes the ground.

  Figure 4.5 At toe-off the thigh usually reaches about 35 degrees of extension behind the body.

  These kinematics tend to change significantly in the runner who is a midfoot striker (one who makes first contact with the ground with the midfoot area rather than the heel or forefoot). At first contact with the ground, the angle of hip flexion tends to be significantly less than 25 degrees while the knee is flexed to a greater degree and the thigh reaches an angle of 0 degrees more quickly during stance (figure 4.6). Logic suggests that midfoot striking therefore produces less braking action with each step since the foot is not as far ahead of the body and the leg is less straight and thus should be more economical. Research also indicates that a reduced thigh angle helps minimize the duration of stance. About 95 percent of nonelite distance runners are heel strikers, but the prevalence is much lower among elite competitors. Taken together, these findings suggest that midfoot striking is superior to heel striking and that heel strikers can improve economy and performance by progressively shifting to midfoot landings. This idea will be discussed further in chapter 5.

  Figure 4.6 Midfoot-landing runners have a reduced thigh angle during initial impact with the ground, lessening the braking forces and duration of stance.

  The muscles that control the thigh during stance include the gluteus maximus, rectus femoris, hamstrings, vastus lateralis, and vastus medialis (figure 4.7). The activities of these muscles during stance are complex, and understanding these activities is necessary to create an optimal strength-training program for the thigh muscles. After toe-off, when the thigh begins moving forward so that contact with the ground can be made again, gluteus maximus activity is maximized (as determined by electromyographic analysis) compared with all other segments of the overall gait cycle. Human anatomy textbooks commonly state that the most important function of the gluteus maximus is to produce thigh extension, and yet during running the glutes are most active and forceful during thigh flexion when the thigh is in a forward angle from the hip. Clearly, the most important role of the glutes during running is to control thigh flexion, and exercises designed to strengthen the glutes for running should mimic this phase of the gait cycle. (See chapter 14 for more information.)

  Figure 4.7 The muscles of the hip and upper leg: (a) posterior view and (b) anterior view.

  Carrying out hip extensions on a machine in the gym does not strengthen the glutes very effectively for running because doing so produces concentric actions of the glute during hip extension. (In concentric actions, muscles produce force while shortening.) The key actions of the glutes during running are eccentric actions during hip flexion. (In eccentric actions, muscles lengthen as they exert force.) Eccentric, running-specific drills for the glutes and hamstrings are provided in chapter 14.

  The rectus femoris muscle, which is the middle of the quadriceps muscles, becomes moderately active just before foot strike and remains moderately active throughout the first two-thirds of stance: Its purported action, according to basic human anatomy texts, is to produce thigh flexion; however, since the thigh is gradually moving into extension over the course of stance, the key role of the rectus femoris during running stance is to act eccentrically to prevent the knee joint from collapsing. In other words, the rectus femoris stops the knee joint from flexing too much during stance. Running-specific exercises for the quadriceps are outlined in chapter 14.

  The vastus lateralis and vastus medialis muscles, which are the outer and inner parts of the quads, respectively, are activated shortly before foot strike and are extremely active over the first 67 percent of stance.2 This seems odd from a traditional perspective because the vastus lateralis and vastus medialis are supposed to assist with thigh flexion; in fact, these two muscles act most strenuously and in an eccentric way to control excessive rotational movements of the thigh as thigh extension develops during stance. The vastus lateralis and vastus medialis muscles also assist the rectus femoris with the job of preventing too much flexion at the knee during stance.

  The hamstrings, which are also thigh controllers, maximize their activity just before foot strike, toward the end of the swing phase of gait. They are eccentrically controlling forward motion of the thigh and preloading themselves to have increased stiffness at the moment of touch-down, when the foot first hits the ground. This enhanced stiffness allows the hamstrings to recoil effectively throughout stance and provide a great deal of propulsive force. A traditional view is that the hamstrings provide direct concentric force for hip extension shortly before and during toe-off, which would drive the body forward, but in actuality the hamstrings are completely inactive during the last third of stance. This doesn’t mean that the hamstrings aren’t doing any work; it’s just that the work can’t be measured by electromyography (EMG) since it is all based on the recoil of the hamstrings, which were stretched out during swing.

  During swing, the thigh gradually moves out of extension, reaches an angle of 0 degrees with no extension or flexion halfway through swing, and then maximizes its forward angle (flexion) about 70 percent of the way through swing, retreating just slightly before foot strike. The gluteus maximus waits until the very end of swing, just before foot strike, to become really active, which helps to stop flexion of the thigh. The rectus femoris is extremely forceful during the first 50 percent of swing because it is boosting the forward acceleration of the leg so that stride length can be maintained and the foot can find an appropriate landing point. The vastus lateralis and vastus medialis stay out of swing until the very end, when they preload the thigh, stiffening the knee joint for touch-down. The hamstrings wait until approximately the last 30 percent of swing, when they become active in order to control forward movement of the thigh and to stiffen the leg properly.2

  For coaches and runners, information about how the thigh muscles function during running is not esoteric or a late-night antidote for insomnia. Rather, understanding thigh operations leads to the creation of an optimal strength-training program that maximizes thigh-muscle function during quality training and competitive running. Traditional strengthening exercises for runners, including movements carried out on machines, non-gait-related drills for specific muscles, and even two-leg
activities can no longer be considered productive. These should be replaced by the running-specific movements described in chapter 14.

  Knee

  The functions of the knee during running are to minimize the shock forces associated with impact with the ground by increasing knee flexion at the initial moment of foot strike and to produce propulsive force by undergoing extension during stance. To understand how the knee works during running, it is important to examine the various angles adopted by the knee during gait. The angle of the knee during running is defined as the angle between the actual position of the shin and an imaginary line drawn to indicate the position the shin would occupy if the leg were perfectly straight. Thus, the knee angle is always positive during running because the knee cannot hyperextend during normal running gait, and the leg is never perfectly straight at the knee.

  At the moment of foot strike, the knee angle is somewhere between 0 and 25 degrees in the heel-striking runner and usually significantly greater in the midfoot striker. An increase in knee angle helps the heel striker position his or her foot more closely under the center of gravity of the body and is probably why impact forces tend to be smaller in midfoot-striking runners compared with heel strikers. With greater flexion, the knee can function more like a spring-like device instead of being part of a stiff pole extending from foot to hip.

  The knee angle increases for roughly the first third of stance as the knee flexes to absorb the shock of impact with the ground. At the end of this first third, the knee joint is maximally flexed for the stance phase of gait, and the knee gradually goes through extension as the rest of stance unfolds in preparation for a nearly straight-legged toe-off, with an angle of almost 0 degrees at the knee. The greater initial knee flexion associated with midfoot striking helps explain why the stance phase is shorter in duration for the runner who strikes midfoot; maximal knee flexion can be reached more quickly from the preflexed state, followed closely by knee extension.

  The muscles that govern the knee joint track these actions during stance. The rectus femoris muscle is active during the first part of stance to control the knee and prevent excessive flexion. The vastus lateralis and vastus medialis become extremely active about one-third of the way through stance as the knee joint begins to extend, presumably to control rotational movements of the knee that would block optimal force production at toe-off but also to straighten the knee as stance proceeds in preparation for toe-off. The hamstrings are active during the first two-thirds of stance, which would ordinarily be expected to increase knee angle throughout this part of stance. However, the hamstrings tend to be overruled by the rectus femoris and vastus muscles so that knee extension can proceed. In reality, the hamstrings are more occupied with the thigh and hip because they are engaged in the process of hip extension.

  The knee angle increases through the first 50 percent of swing, moving above 90 degrees (see figure 4.8) before retreating back to an angle of near 0 degrees again just before foot strike in the heel striker. The vastus lateralis, vastus medialis, and hamstring muscles are all relatively quiet as the knee angle increases; they don’t reach full activity until the final 30 to 40 percent of swing. It is only the rectus femoris that is truly active during the first half of swing, and it is obviously acting to increase forward swing of the thigh rather than to block knee flexion. The knee flexes during the first half of swing because there is nothing to stop it from doing so; it begins to extend during the second half of swing as the hamstrings and glutes kick into gear to slow down forward movement of the thigh, giving the lower part of the leg a chance to catch up.

  Figure 4.8 During the first half of the swing phase, the knee angle increases, optimally moving above 90 degrees.

  Ankle

  The functions of the ankle during running are to produce propulsive force during stance via plantar flexion of the ankle and to stabilize the leg and upper body by resisting excessive pronation, inward rolling of the ankle, and supination, the ankle’s outward movement. To understand how the ankle works during running, it is important to comprehend what is meant by ankle angle, which is defined as the angle between the bottom of the foot and an imaginary line that begins at the knee, runs down the shin and through the heel, and extends past the bottom of the foot at the heel (A zero-degree angle would have the foot pointing directly down from the shin). When the foot is flat on the ground as a runner stands in place, the knee is locked, and the leg is straight, and this angle is 90 degrees. Angles greater than 90 degrees signal ankle dorsiflexion with toes moving toward shin (figure 4.9a) while angles less than 90 degrees indicate ankle plantarflexion, with the toes moving away from the shin (figure 4.9b).

  Figure 4.9 The ankle angle increases with (a) dorsiflexion and decreases with (b) plantar flexion.

  Ankle angle at foot strike depends on whether a runner is a forefoot, midfoot or heel striker (see figure 4.4). For the heel striker, ankle angle tends to be greater than 90 degrees at first impact with the ground (figure 4.10a), while ankle angle is close to 90 degrees for the midfoot striker (figure 4.10b) and less than 90 degrees for the forefoot athlete. Ankle angle quickly increases as the foot passes through dorsiflexion during the first 60 percent of stance but then decreases steadily as the ankle moves toward toe-off. At toe-off, the ankle is plantar flexed, with the angle typically at 67 degrees. Over the course of swing, the ankle angle gradually increases, reaching close to 90 degrees again for the following foot strike.

  Figure 4.10 At foot strike, the ankle is (a) at an angle greater than 90 degrees for a heel striker and (b) at about 90 degrees for a midfoot striker.

  Two key muscles work in opposition to each other to control ankle angle (figure 4.11). The gastrocnemius muscle plantar-flexes the ankle and controls the rate and range of dorsiflexion, and the tibialis anterior muscle dorsiflexes the ankle and controls the rate and range of plantar flexion.

  Figure 4.11 The muscles of the lower leg: (a) posterior view and (b) anterior view.

  During the first part of stance, as the ankle dorsiflexes, the gastrocnemius muscle is active in its role as a dorsiflexion controller. As the ankle unwinds over the second part of stance and moves into plantar flexion, the gastrocnemius becomes inactive from an EMG standpoint. This doesn’t mean that the gastrocnemius is not working and producing force. In fact, this muscle is highly stretched during dorsiflexion, and its snapback motion provides much of the force for the plantar flexion that can produce a powerful toe-off. No nervous system stimulation is required for this snapback; it happens as naturally as the recoil of a stretched-out rubber band, without any need for additional energy input. Thus, the EMG recording for the gastrocnemius toward the end of stance shows no activity despite the considerable force being produced.

  Surprisingly, the tibialis anterior muscle mimics the gastrocnemius during the first third of stance as dorsiflexion is occurring. This is perhaps because one of the functions of the tibialis anterior muscle is to prevent rotational actions of the ankle, which could become a problem during the first part of stance. The tibialis anterior muscle becomes totally inactive during the plantar flexion phase of stance. It is as though it is saying, “Let it happen: Nothing should be blocking the strong forces that can be produced by the gastrocnemius to achieve toe-off and get the body flying forward.”

  During swing, the tibialis anterior is very active, especially during the middle stages. Apparently, the muscle is producing the forces required to get the ankle into neutral position for foot strike because the ankle is significantly plantar flexed at toe-off. The gastrocnemius stays out of swing until near the end when it becomes active in order to prevent too much dorsiflexion at the initial moment of foot strike—and when it stiffens itself properly for the inevitable dorsiflexion that follows the beginning of foot strike. If the gastrocnemius were not active at the end of swing, the ankle would flop into dorsiflexion like a limp noodle at the beginning of stance, and there would be less energy returned in order to move the ankle into plantar flexion and provide a powerful to
e-off.

  Upper Body

  The actions of the upper body during running involve the pelvic area, core muscles, and arms. The functions of the upper body during running are to provide an anchor point against which the legs can work with strength and economy, maintain proper posture in order to avoid a negative impact on stride length and rate, and balance forward actions of the legs so that the body does not move excessively in rotational directions.

  Upper body actions are coordinated with leg movements during running. When the left leg moves forward during swing, there is a natural tendency for the torso to rotate in a clockwise fashion. This tendency is counteracted during running by actions of the abdominal muscles and a forward swing of the right arm: These actions create forces directed in a counterclockwise motion that stabilize the body, conserve energy, and prevent excessive, washtub-like torso movements. As the right leg moves forward during swing, it initiates a counterclockwise upper body movement that is then counteracted by a simultaneous forward movement of the left arm and appropriate abdominal muscle actions.

  A strong, stabilizing abdominal core and appropriate arm actions that are synchronous with the opposite leg during swing should enhance running economy and improve running performances because oxygen-consuming muscular actions would not be needed to correct unproductive and energy-wasting movements. No study has ever directly linked upper-body mechanics and running performance, however. Some research has suggested that the effects of seemingly inappropriate upper-body movements on running-relevant physiological factors are small. In one study, running while holding the arms directly overhead instead of coordinating them with the legs (a major change in upper-body kinematics) harmed running economy by only 1 percent.2 This may be why Zatopek succeeded with ungraceful form. However, most athletes and coaches are aware that a 1 percent difference can be highly significant during competition; for example, it represents the difference between a 2:07 and a 2:05:44 marathon.