Stretch-Shortening Cycle
Runners may be surprised by the biomechanical information presented in this chapter so far because it seems to defy conventional wisdom about how the muscles produce the forces necessary to sustain running. The hamstrings, for example, are supposed to contract and provide the force necessary for hip extension while the foot is on the ground, yet biomechanical analysis reveals that the hamstrings are actually most active during swing when the leg is moving forward and the thigh is flexed.
One basic problem is that runners are routinely taught that there are three basic types of muscle activity:
Isometric actions—a muscle exerts force without lengthening or shortening
Concentric contractions—a muscle produces force, shortens, and moves a body part
Eccentric actions—a muscle creates force but elongates instead of shortening3
It is natural to expect that the leg muscles use these actions to produce the propulsive and braking forces required to run on firm ground. However, as the Finnish exercise physiologist Paavo Komi has pointed out, the fundamental aspect of muscle activity during running is quite different.4 Movement and propulsion during running are produced by what is called the stretch-shortening cycle (SSC), in which a leg muscle is first stretched (preactivated) and then recoils, producing with each snapback the propulsive force that moves the body forward. This has extremely important consequences for strength training for running and for pure running training.
A close inspection of hamstring actions during running illustrates the operation of the SSC. As mentioned, a popular conception among runners and coaches is that the hamstrings move a runner’s body ahead by contracting while the foot is on the ground, thereby extending the hip and pushing the upper body forward.
EMG measures the electrical activity of muscles, the degree to which the muscles are using energy to generate force. In the case of the hamstrings, EMG recordings show that these key muscles are actually only moderately active during the first two-thirds of stance, the portion of the overall gait cycle when a foot is on the ground, when they are supposedly working to create propulsive forces. Surprisingly, the hamstrings are totally inactive during the last third of stance, when they are supposedly driving the body forward.5 The hamstrings produce the propulsive force required to move the body forward by being stretched, or preactivated, during swing and then recoiling elastically and snapping back like a rubber band during stance; this recoil extends the hip and pushes the body forward. Thus, it is the operation of the SSC in the hamstrings, not active concentric contractions, that produces propulsion.
From an EMG standpoint, the hamstrings are actually most active toward the end of swing, just before their associated foot hits the ground. This is because the hamstrings are producing braking force for the swinging leg, helping to keep the leg under control and preparing for the impending collision with the ground. The hamstrings are also preactivating themselves, becoming stiffer so that the resulting recoil after ground impact will be a forceful, economical action. If the hamstrings are too loose, they will act like limp pieces of spaghetti when the foot hits the ground, producing sloppy, slow, and low-power recoil.
An understanding of the SSC is essential for designing optimal strength training for runners. A key goal of such strength training is to optimize the amount of propulsive force generated by the leg muscles during running. However, gains in strength are always specific to the movements used during strength training. Working the hamstrings in a manner that does not replicate the SSC will not maximize gains in running-specific strength; in fact, it might not lead to any increases at all.5
For example, many runners rely on traditional fundamental exercises such as hamstring or thigh curls in their attempts to boost hamstring strength,6, 7 even though such exertions have little resemblance to the stretch-shortening cycle. During hamstring curls, the hamstrings are stimulated by the nervous system and expend energy to contract as they are exerting force, the exact opposite of what happens during running when the hamstrings shorten and generate force by recoiling elastically. When hamstring curls are performed, the essential swing phase of hamstring action is totally missing, and thus the hamstrings’ key running-related action—exerting braking force and stiffening at the very end of swing—is not replicated or strengthened. There is no moment of high activation at the end of swing when hamstring curls are conducted. A much better hamstring-strengthening exercise for runners, one that includes hamstring activation at the end of swing and elastic recoil, would be bicycle leg swings. This exercise and other drills that emphasize the SSC of muscle activity are presented in chapter 14.
During running, the hamstrings are not alone in their preference for SSC activity, as opposed to isolated concentric, eccentric, or isometric actions. The all-important gastrocnemius muscle in the calf is automatically stretched during running whenever the foot hits the ground. This ground impact forces the ankle joint into dorsiflexion, a joint-angle change that stretches the gastrocnemius significantly.
A traditional view is that the gastrocnemius is fully active during stance, creating the force necessary to plantar-flex the ankle, lift the heel, and move the foot into toe-off position so that the body can soar forward through the air before landing on the opposite foot. EMG analysis reveals that the gastrocnemius is actually barely active during the last half of the stance phase and not active at all over the final 25 percent of stance. Like the hamstrings, the gastrocnemius uses elastic recoil energy to provide propulsive force rather than an energy-consuming concentric contraction. Also like the hamstrings, the gastrocnemius becomes preactivated just before the foot makes contact with the ground in order to control impending dorsiflexion and provide a stiffer, stronger spring that increases force production during subsequent gastrocnemius snapback.5
Conventional exercises employed by runners to enhance gastrocnemius strength fail to simulate the SSC. One popular exercise, heel raises, uses concentric contractions of the gastrocnemius throughout the full range of plantar flexion, quite different from the pattern established by the gastrocnemius during the stance phase of running. Heel raises help to build general strength in the gastrocnemius, but fail to properly preload the muscle in a manner specific to running. A far better and much more specific—but seldom-used—gastrocnemius strengthener would be an exercise called falls to earth with forward hops. This exercise—and others like it that rely on the SSC—is described fully in chapter 14.
Conclusion
For the runner and coach, the study of biomechanics is extremely important. It leads to a comprehension of how the human body actually works during running and thus a firm grasp of how to strengthen running-relevant movements. The specificity of training principle, developed after years of research, reveals that motions are strengthened optimally through the use of training movements that effectively mimic those motions. Thus, drills and exercises bearing a small to moderate resemblance to the actual movements of running gait will have a much smaller impact on running capacity than those routines that are highly running specific and in line with our understanding of running biomechanics.
Chapter 5
Refinement in Running Form
The science of running form is still in its infancy, but there are a number of studies that have linked adjustments in running form with positive changes in running economy (the oxygen-consumption rate associated with a specific running velocity), competitive performances, or the risks of various running-related injuries. Endurance runners should nonetheless be wary about running-form transformations advocated by so-called running-form experts, prescribed by coaches, or proposed in magazine articles; often there are very little or no scientific data available to support these recommendations in these forums. Runners interested in changing form should rely on the results of well-controlled scientific research rather than on fad or unsupported speculation.
Research on Running Form
In an investigation of form carried out at Wake Forest University, res
earchers Stephen Messier and K.J. Cirillo worked with 11 runners over a 5-week period.1 Three times per week, the Wake Forest scientists discussed their conceptions of proper running form with the runners and displayed videotapes of athletes with allegedly excellent form as the subjects ran on laboratory treadmills. As a result of the instruction, the 11 runners gradually transformed their gait characteristics in the following ways:
They increased stride length slightly so that more ground was covered with each step during running.
They decreased the amount of time spent in the support phase of running so that the feet didn’t rest lazily on the ground.
They lifted the heel more forcefully at the end of the contact phase of stance in order to provide a more vigorous push-off.
They flexed the knee to a greater extent during leg swing to increase the speed of swing.
They flexed the knee to a greater degree at impact in order to facilitate shock absorption.
They kept the trunk slightly flexed forward as they ran.
They held their forearms at 90-degree angles to the upper portions of their arms as the arms moved back and forth.
They landed on the heel and rolled forward to the front of the foot with each step.
Note that some of these advocated changes are controversial. For example, it is not clear that an expansion of stride length (1) would be advantageous for all runners; it might actually create greater braking force per step because the foot would be farther in front of the center of mass and thus harm running economy. It is also not certain that the trunk should be flexed during running (6); while the entire body should probably be inclined forward, trunk flexion per se could prestretch the lower back and gluteus muscles and decrease stride length. Heel striking (8) is the recommendation that arouses the most suspicion: Compared with midfoot striking, heel striking has been linked with longer ground-contact times and higher impact forces that are undesirable running gait features.
Messier and Cirillo suspected that the changes in form would improve running economy in the 11 runners after the 5 weeks of training. Many coaches of endurance runners would agree that most of the preceding changes are desirable. However, the 11 athletes were not even slightly more economical with oxygen at the end of the study. Even though the runners looked more graceful as they ran, their average rate of oxygen consumption for any chosen speed was the same as it had been prior to the form makeovers. Assessment of economy is a very logical way to determine whether form makeovers are actually valuable. Running economy is a valid predictor of endurance performance. It is the oxygen cost of running and is expressed as milliliters of oxygen used per kilogram of body weight per minute.
One goal of form transformations is to make runners more economical, another is to decrease the risk of injury, and a third is to enhance performance. The recommended enhancements in form also failed to make running feel any easier for the 11 runners in the Wake Forest study. When the subjects ran at a typical training intensity, they looked better than before, but to them the workout felt as tough as it had always been.2
The study can be criticized for its use of controversial transformations and also for its brevity: There is no specific reason to believe that a 5-week period is long enough for running economy to improve in response to a change in running form; economy enhancement might take longer. Running economy probably depends to some extent on muscle length, dynamic mobility of joints, muscle and tendon flexibility and stiffness, and other factors that could respond to training over longer periods of time.
Messier and Cirillo’s research also raises the possibility that each runner may adopt a style of running that, although it might look awkward or unconventional, actually produces the best economy for that individual and his or her unique anatomical and neuromuscular characteristics; in such cases, a shift to a smoother or more acceptable style could actually harm running capacity. Supporting this notion, research reveals that economy deteriorates and oxygen cost increases for most runners when they increase or decrease their stride rate even slightly.3 Runners tend to be most economical when they run with their freely chosen stride lengths, even when they appear to observers to be overstriding or understriding.
In their laboratory at Penn State University, biomechanics researchers Peter Cavanagh and Keith Williams studied stride length and economy in 10 fit runners as they moved along at a 7-minute-per-mile pace.4 Cavanagh and Williams found that most of the runners naturally moved along with very close to their most economical stride lengths. On average, the 10 runners were merely an inch away from their optimal stride lengths, the ones that produced the lowest oxygen cost during running, and that small deviation away from the optimum boosted oxygen-consumption rate by a paltry 0.4 percent.
Athletes in other sports also appear to use their most economical stride lengths. Research carried out with race walkers reveals that the oxygen cost of race walking increases when the athletes increase or decrease their stride lengths by just 5 percent of leg length.5 However, it is important to note that the conclusion that runners naturally adopt the best stride length is not without a troubling caveat: The initial, immediate downturns in economy associated with stride-length change might be due simply to the nervous system’s unfamiliarity with the new stride length, thus resulting in a diminishment of control by the nervous system and a greater degree of uncoordinated, uneconomical movement. With gradual nervous system adaptation, however, the new stride length might actually become effective over time—even better than the freely chosen original style.
In addition, it is undoubtedly true that some runners use suboptimal stride lengths and should undergo a stride transformation. One athlete in Cavanagh and Williams’ investigation tended to overstride by about 2.5 inches per stride, an expansion that would have tacked an extra 3 minutes to his marathon time and forced him to use an extra 10 liters of oxygen over the course of the race compared with the oxygen use associated with his most economical stride length.2 Despite what experts may say, it can be difficult to detect such form problems and oxygen issues merely by watching an athlete run; laboratory testing with oxygen-measuring equipment is required for definitive diagnosis.
Kinematics of Improved Form
Research concerning the kinematics of running has provided strong clues about ways in which endurance runners might improve form. Nancy Hamilton at the University of Northern Iowa has approached the form problem from a unique angle by linking changes in running form to the process of aging.6 The impetus for her research came from her observation that running performances and running form change significantly and simultaneously as runners get older. Traditionally, most investigators have linked age-related performance falloffs with declines in cardiac power and losses in muscle mass and strength, but Hamilton wondered whether alterations in form might also account for an important part of the performance downturn (note that drop-offs in muscular strength would probably alter form, so the factors may be linked). Hamilton set out to identify those aspects of form that must be preserved in order to sustain fast velocities and run economically at quality speeds.
At the 1989 World Veterans’ Games in Eugene and the National Championships in San Diego, California, Hamilton spent many hours filming 162 competitive runners (83 males and 79 females) as they engaged in competition. She then digitized the performances into a computer and devoted hundreds of hours to analyzing the runners’ kinematics. Fast runners were compared with slow ones, and senior runners were compared with younger individuals. Hamilton was unable to measure running economy, but she did identify three aspects of running form that were linked to higher maximal running speeds and superior performances: (1) greater hip extension at push-off, (2) increased knee flexion during forward swing, and (3) avoidance of excessive knee flexion during stance. These as well as some factors based on other research are discussed in detail in the sections that follow.
Hip Extension at Push-Off
Hamilton demonstrated that maximal running speed declined by about 40 percent betw
een the ages of 35 and 90, and range of motion at the hip fell by 38 percent. This decrease in range of motion at the hip was tightly connected to the direct cause of the falloff in maximal running velocity: a drop in stride length. While stride rate during top-speed running for runners in their eighties was only 4 to 5 percent lower than that of runners in their thirties, stride length declined by nearly 40 percent for older runners.
The fundamental factor associated with the decline in stride length was a loss of range of motion at the hip during running. Hamilton found that the key to optimal range of motion at the hip during high-speed running was the conservation of hip mobility in the kick or drive phase of running: When the foot becomes a lever for toe-off, the gluteal and hamstring muscles recoil to propel the leg backward, and the quads also activate themselves to straighten the leg for the backward push. The basic motion during the drive phase is hip extension—the backward movement of the leg at the hip. A decline in hip extension means that less force is applied to the ground with each step, and thus stride length must shorten (with less force exerted, the body flies a shorter distance forward per step). What was lost with aging was hip extension and thus stride length and speed, not hip flexion.
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