Running Science

by Owen Anderson

  Hamilton argued that significant hip extension is a key element of good form and suggested that a runner should deliberately alter the way in which he or she runs, focusing on the muscles around the buttocks so that they push backward with significant amounts of force with each step. “Rather than reaching out with the leg in an effort to get maximal forward distance, a runner should think about pushing back as forcefully as possible with each step,” said Hamilton. “Runners should use the buttocks and hamstrings to do so, very much the way a sprinter pushes out hard from a set of starting blocks.”7

  It is possible that greater hip extension during the end stage of stance can be developed in response to specific training strategies, including high-speed running, hill training, speed bounding, and the deliberate mental focus on hip extension Hamilton recommends. This possibility has yet to be demonstrated in scientific research, however, primarily because no one has studied it. It is clear that enhanced dynamic flexibility of the quadriceps muscles would be required in order to permit augmented hip extension since overly tight quads would resist backward movements of the legs. Systematic stretching of the quads and drills that increase the quadriceps’ dynamic flexibility (for example, the explosive one-leg squats with lateral hops described in the speed progression section of chapter 28) could be very productive from a hip-extension standpoint.

  Knee Flexion During the Swing Phase

  Hamilton found that increased knee flexion during the swing phase of gait was another form factor that correlated with higher maximal speed. (This was also one of the eight aspects of gait recommended by Messier and Cirillo in the Wake Forest study discussed earlier.) With her video analysis, Hamilton demonstrated that the fastest runners had their knees highly flexed during the swing phase of gait so that their feet were significantly angled toward their buttocks (see figure 5.1). The slowest runners held their knees less flexed during swing so that the lower part of the leg was essentially at a right angle with the thigh.

  Figure 5.1 Runner with highly flexed knee and foot angled toward buttocks during the swing phase.

  Keeping the knee less flexed and the foot down at the level of the knee during swing—rather than having the foot perched near the buttocks—as the leg is brought forward to prepare for the next contact with the ground converts the leg into an extra-long lever with a heavy foot dangling at its end. When the knee is highly flexed, the lever is no longer than the distance from the hip to the knee; when the knee is less flexed, the lever length extends to the foot. Less flexion of the knee is a negative during the swing phase of running because longer levers are harder to move compared with short levers. Longer levers require more force and thus more oxygen to move. In addition, the foot represents a significant weight, and having a large weight at the end of a long lever makes it very difficult to accelerate forward. This is why the heavier person on a teeter-totter tends to remain planted on the ground unless he or she moves toward the middle.

  In running, as the leg moves forward during swing, it is best to have the knee highly flexed and the foot tucked up by the buttocks, in effect cutting the leg lever almost in half by making the knee—rather than the foot—the endpoint of the limb. It is unclear whether runners can increase knee flexion during swing simply by practicing this form adjustment. Once again, upgrades in the quadriceps’ dynamic flexibility should have the positive effect of making the knee joint more permissive during swing and allow a greater range of flexion.

  Excessive Knee Flexion During Stance

  Hamilton discovered that while knee flexion was very good during swing, excessive knee flexion was a negative factor during stance when the foot was on the ground. The problem with too much knee flexion during stance is that extra time must then be taken to straighten the knee out again just before push-off. “The greater the flexion of the knee during stance, the greater the amount of time spent in stance,” Hamilton noted.7 This makes it more difficult for a runner to reach his or her true speed potential since too great a time is being spent with the feet attached to fixed points on terra firma.

  Decreases in knee flexion during stance that lead to reductions in contact time can have significant effects on performance. An experienced female road racer who requires 32 minutes to complete a 10K and takes 190 steps per minute during the race will need 32 × 190 = 6,080 steps to get from the starting line to the finish. If she lands with stiffer legs (i.e., with less flexion at the knees) and thus saves just 5 milliseconds per step—and there is no loss in stride length associated with the stiffer landings—her race time will improve by 6,080 × .005 = 30.4 seconds, an excellent performance upswing.

  Research has shown that increased leg stiffness is also associated with enhanced running economy (see chapter 8 for more on running economy). In one study in which knee stiffness decreased and knee flexion increased during the stance phase of running, the oxygen cost of movement increased by almost 50 percent.8 Scientific research has revealed that explosive strength training increases leg stiffness, probably to some extent by limiting knee flexion during stance. Endurance runners are wise to use explosive strength training (see chapter 28) since it appears to optimize various aspects of form, including shortening the stance phase of gait, increasing leg stiffness, upgrading dynamic flexibility of the quads, and promoting hip-extension range and power.

  Vertical Oscillation

  Additional components of good form have been substantiated in other inquiries. Research has shown that elite runners have less vertical change in their centers of mass during running; that is, the center of mass moves upward to a smaller extent during the jumps from one foot to the other for elite runners than for middle-of-the-pack competitors. The investigations have also revealed that a reduction in vertical movement during running tends to enhance economy.9 A focus on pushing or bouncing forward with each step during running, instead of pushing or bouncing upward, should be helpful in reducing vertical oscillation of the center of mass. Leaning forward from the ankles slightly during stance instead of adopting a completely upright posture during gait should also help decrease vertical oscillation (see figure 5.2).

  Figure 5.2 Runners can enhance economy by focusing on moving forward instead of upward and decreasing the vertical movement of the center of mass.

  Straight Leg at Toe-Off

  Having an almost straight leg at toe-off (similar to Hamilton’s hip-extension conclusion) has been linked with upgraded running economy when compared with maintaining greater flexion at the knee at the end of stance9 (see figure 5.3). It appears to be important to eliminate knee flexion almost totally at the moment of toe-off but then rapidly flex the knee for the subsequent swing stage of the gait cycle.

  Figure 5.3 Runner using a nearly straight leg during toe-off.

  Research carried out by Williams and Cavanagh has demonstrated that heightened plantar flexion of the ankle at toe-off and increased rapidity of plantar flexion are also associated with enhanced running economy.10 The optimal anatomical position, or form, for toe-off seems to be a straight leg with fully pointed toes. No scientific research has explored the question of how to train to advance the rapidity and extent of plantar flexion at toe-off, but logic suggests that high-speed running and explosive drills would have the greatest effect on these variables.

  Economical Arm Swing

  Many runners, believing that arm movements help propel them forward, use rather expansive arm swings while running. Scientific research convincingly shows that this is not a good strategy. Faster, more economical runners actually tend to have less arm movement than slower, less-economical competitors.11 Quick, little arm movements carried out in synchrony with the swings of the legs (i.e., right arm swings forward as left leg moves ahead, and vice-versa) appear to be the ones that produce the most economical running.12 This is one form change runners should be able to make consciously without much effort. Swinging the arms across the front of the body is energy consuming and unlikely to be linked with optimal economy. The arms should swing forward and backwa
rd.

  Excessive movements of the upper body have been linked with poor economy. As both the speed and amount of rotation of the shoulders and hips around the center axis of the body increase during running, economy is harmed.12 Such washtub motions are controlled by the body’s core muscles, the muscles of the torso that attach to the pelvic girdle and spine, bringing into focus the potential importance of acquiring great, running-specific core strength. Optimal routines for increasing core strength are described in chapter 13.

  Foot-Strike Pattern

  Another form factor—foot-strike pattern—may have a very strong impact on running economy, performance, and the incidence of injury. Although foot-strike pattern is an essential element of form, it is often ignored by runners or simply assumed to be okay. Foot-strike technique has a strong impact on the duration of stance phase of gait, stride rate, and the work performed by various muscles of the feet, ankles, and legs during running. Although research in this area is still rudimentary, foot-strike pattern is likely to have an effect on running economy, competitive performance, and the likelihood of overuse injuries.

  The two most commonly used foot-strike techniques are the rear-foot strike or striking pattern (RFS), in which the heel of the running shoe (or foot for the unshod runner) is the first structure to make impact with the ground during gait, and the midfoot strike or striking pattern (MFS), in which the middle portion of the running shoe’s sole (or sole of the foot) makes initial contact or at least makes contact simultaneously with the heel. RFS is more popular than MFS: In a recent, elite-level half marathon, about 75 percent of participants were using the RFS pattern at the 15K point of the race, 24 percent were using MFS, and 1 percent were using the forefoot strike or striking pattern (FFS), in which the front portion of the foot hits the ground first.13

  Fast Runners and MFS

  While experienced runners tend to favor RFS over MFS by at least a three-to-one margin, motion analysis of Olympic Games competitors has suggested that Olympic medalists are more likely to employ MFS. In addition, video analysis of world-champion and world-record-holding runners, including Paul Tergat, Haile Gebrselassie, and Paula Radcliffe, has indicated that such competitors employ MFS and occasionally FFS, but not RFS, while training and competing.

  Research has also shown that the frequency of the MFS pattern increases with competitive ability; in a study carried out with elite and near-elite runners, 36 percent of the male, top-50 finishers employed MFS in a race as contrasted with just 20 percent of runners finishing in places 51 through 200 in the same competition.13 In this investigation, MFS was used by 3 of the first 7 female finishers (43 percent) but by only 4 of the other 28 (14 percent) slower women in the race.

  Compared with RFS, performance may be enhanced with MFS because ground-contact time is shorter by about 17 milliseconds at a running velocity of approximately 5 to 5.5 meters (16-18 ft) per second and by approximately 10 milliseconds at slower speeds. As a consequence of the more abridged contact time, stride rate is also higher for MFS at any specific speed. Decreases in ground-contact time and increases in stride rate have been linked with enhanced running economy and faster 5K performances.14 In addition, a key difference between the top competitors and the slower performers in an elite road race is the shorter average ground-contact time of the faster finishers. Such findings suggest that MFS may be the superior foot-strike pattern from a competitive standpoint.

  Comparing MFS and RFS

  Although the world’s best endurance runners prefer MFS over RFS, the effects of either on running economy have yet to be completely determined. One study has detected enhanced economy with RFS compared with MFS. Research has also revealed that runners tend to adopt the most economical running style possible for their individual anatomical and physiological characteristics. At least 75 percent of experienced runners favor RFS over MFS, suggesting that rear-foot striking may be the most economical pattern. However, another strong possibility is that highly cushioned, big-heeled modern running shoes tend to push runners toward an RFS pattern even though it is suboptimal; this will be discussed further in chapter 6.

  Although RFS is the most popular way to hit the ground during running, it is important to note that it is highly likely the world’s best endurance runners have optimized most aspects of their running mechanics, and that such optimization has elevated them to their superelite status. Given that running economy is such a strong predictor of performance, the ubiquity of MFS among superelite runners suggests that MFS may produce highly economical running. Compared with MFS, it is also clear that RFS leads to a more extended leg at foot strike and a longer time of maximal knee flexion during the support phase of gait. This suggests that RFS produces a longer period of muscle activation per running step, which might increase the oxygen cost of running and lead to poorer economy with the RFS pattern.

  Interestingly, runners who have trained barefooted for their entire running careers almost always employ MFS rather than RFS, and runners who shift from shod to unshod running tend to change from RFS to MFS as part of this changeover. Compared with shod locomotion, barefoot running is more economical, is linked with a higher stride rate, and limits impact forces traveling up the legs. These positive aspects of barefoot running may be strongly tied to the nearly universal use of the MFS pattern. (See chapter 6 for more about barefoot running.)

  There is very little scientific information available to assess whether the MFS and RFS patterns have different effects on the likelihood of running injury. Ground-reaction force (GRF) is thought to be an important predictor of running injury: Runners with higher vertical maximal forces tend to experience greater lower-extremity pain, and elite runners with elevated GRFs tend to have an increased risk of stress fracture. The extent of motion around the ankle and knee joints during gait is also believed to be a predictive factor for injury.

  It is clear that MFS and RFS produce different GRF patterns. Runners who employ RFS generally demonstrate a pronounced, initial spike in GRF during the first few moments of stance, which is usually absent when MFS is the ground-contact strategy. In contrast, average peak-to-peak amplitude for mediolateral GRF can often be three times greater in MFS runners than in RFS competitors.

  Research reveals that MFS and RFS are associated with different patterns of muscular work, force production, and power absorption in various parts of the leg during running. Compared with RFS, MFS has been linked with higher peak power absorption and eccentric work at the ankle during gait. It is possible that these effects may lead to overworking the lower-leg muscle groups and increase the risk of Achilles tendon injury for MFS runners. An alternative possibility is that the use of MFS will progressively lead to dramatic improvements in ankle strength compared with using the RFS pattern. Anecdotal evidence suggests that runners who attempt to convert from RFS to MFS experience significant muscle fatigue and sometimes severe delayed-onset muscle soreness in their calves, although these factors may merely be the result of a change in running style and thus motor-recruitment patterns rather than a reflection of the negative characteristics of MFS. Runners changing from RFS to MFS should do so gradually to permit the neuromuscular system to adjust optimally to the new gait pattern.

  Although RFS may reduce the forces placed on the ankle during running, research suggests that it tends to increase the power absorption and total work performed at the knee as compared with MFS. Thus, it is possible that RFS could be connected with a higher rate of knee injury.

  Accelerometers attached to the skin or embedded in the tibia have been used to gauge loads placed on the lower extremities during running. Accelerometer studies demonstrate that peak acceleration measures are greater at slower stride rates, and thus longer stride lengths, for any specific running speed. Such data suggest that the loading rate for impact forces would be lessened by a shift from RFS to MFS since the latter is strongly linked with higher stride rates and shorter strides.

  It is clear that the differing effects of MFS and RFS on ground-reactio
n forces, rotational forces, and muscle and tendon strain in various parts of the leg during running are not yet completely understood. However, future research will probably find that MFS

  enhances economy because it is not associated with the braking effect present with RFS,

  promotes performance because of shorter foot-strike times and thus higher stride rates, and

  lessens the likelihood of injury because of its lower level of strain placed on the knee as compared with RFS.

  Shin Angle

  A critically important—but almost completely ignored—element of running form is shin angle at impact with the ground. Shin angle can be defined as the angle the shin makes with the ground as the foot makes first contact and initiates the stance phase of gait (see figure 5.4). By definition, if the shin is perfectly perpendicular to the ground, the shin angle is 0 or neutral. If the shin is inclined forward from an imaginary line drawn perpendicular to the ground at the impact point, then the shin angle is positive. If the shin is inclined backward from the imaginary line, then the shin angle is negative.

  Figure 5.4 (a) Positive and (b) negative shin angles.

  Shin angle is tremendously consequential during endurance running because horizontal braking force increases as shin angle becomes more positive.15 A runner whose shin angle advances in a positive direction must use more force, energy, and oxygen per step to overcome the braking force that is automatically created compared with another runner traveling at the same speed whose shin angle is less positive.