July 2012

Researchers at The Pennsylvania State University have identified structural char­ac­teristics that distinguish the foot and ankle mechanics of trained sprinters from nonsprinters. Are similar underlying vari­ables responsible for the reductions in mobility that affect older adults?

By Stephen J. Piazza, PhD

Track coaches often say that great sprinters are born and not made and coaches will readily try to convert a mediocre sprinter into a great distance runner. They are less optimistic, however, about the chances for accomplishing the reverse, as the common perception among coaches is that genes determine elite sprinters’ ability. Success in competitive sprinting depends on the ability to accel­erate quickly at the start of a race and then maintain top speed with strides that are both long and rapid. Do muscle, joint, and bone structures in the lower extremity differ between those who possess these abilities and those who do not? The answer to this question about athletes with excellent mobility may have important implica­tions for understanding the mechanics of mobility impair­ments associated with aging and neurological disorders.

Musculoskeletal structure of sprinters

Understanding the relationship between structure and function in the limbs of sprinters requires consideration of sprinting energetics, especially during the critical acceleration period at the start of a race. When experts talk about distance runners rather than sprinters it is often with reference to “running economy.”1 A runner’s ability to maintain a high speed over a long distance depends on his or her doing so with minimal metabolic energy expenditure. Sprinters, however, have no motivation for “economizing” to maintain speed. A successful sprinter should spend energy as fast as possible, as long as that energy is spent effectively in accelerating the sprinter toward the finish or in preventing slowing down after top speed has been reached.2,3 A sprinter who is not moving when the starting gun is fired must increase the kinetic energy associated with the forward velocity of the body’s center of mass in a short time. This rapid acceleration requires that muscles shorten quickly while generating force; when a force acts through a distance in this manner we say that work is done by the force. Muscle force produces the forward-directed ground reaction force that acts upon the feet and gives the body kinetic energy associated with forward velocity.

Recent research suggests that the muscles and tendons of elite sprinters are specialized in ways that should facilitate muscle work. As might be expected, sprinters possess muscles that are thicker than those of athletes not engaged in sprinting4 and muscle thickness seems to correlate with sprinting ability.5The plantar flexor muscles of sprinters have also been shown to have longer fascicles (bundles of muscle fibers) than those of distance runners.6 Although these longer fascicles do not necessarily imply longer optimal muscle fiber lengths, they may be associated with the addition of sarcomeres, the basic units of muscle force generation that make up muscle fibers. In animal models such adaptations have been shown to occur in response to training.7-9 More sarcomeres in series are able to produce greater shortening of whole muscle fibers before force production drops off, so the longer fascicles are likely indicative of muscles that have greater capacity for doing work. Similarly, sprinters’ muscles seem to have smaller pennation angles (i.e., fibers are more in line with the tendons),4,6 which would imply greater muscle fiber shortening and greater muscle work. The Achilles tendons and plantar flexor aponeuroses of sprinters have been shown to be stiffer than those of both distance runners or nonrunners;10 one potential effect of such an adaptation would be to promote muscle fiber shortening (again, enabling those fibers to do positive work) rather than energy storage in the tendon.

It should be noted that the correlations between muscle-tendon structure and performance described in these studies do not imply clear causal relationships between structure and function. Such correlations are not always found; one recent investigation found almost no structural differences between very good sprinters and great ones.11

Measurements of leverage

Recent work in our laboratory has been directed toward under­stand­ing how ankle joint function and foot structure contribute to locomotor performance. Specifically, we have been concerned with the leverage of the plantar flexor muscles and its relation to the length of the forefoot and toes. It is not enough to study plantar flexor tendon leverage without reference to the forefoot because the forefoot is the output lever arm that determines the demands placed on the plantar flexors during pushoff. Consider, for example, what it feels like to try walking while wearing skis. The ski tips that extend far in front of your ankles put your plantar flexor muscles at a terrible mechanical disadvantage, resulting in a lack of pushoff power that causes you to take short, shuffling steps.

Another helpful analogy is the wheelbarrow. A wheelbarrow, like the foot, is a second-class lever in which the upward effort applied by the hands (corresponding to the Achilles tendon force) acts farther away from the fulcrum at the wheel (the metatar­sophalangeal joint and the toes) than the downward load in the bin of the wheelbarrow (the joint reaction force at the ankle) (Figure 1, top). A wheelbarrow is a useful tool in the garden because of the strong mechanical advantage it affords the user. This advantage occurs because the load is normally situated close to the fulcrum while the hands apply upward forces to the handles at a location that is much farther back (Figure 1, middle).

Our first study comparing sprinters to nonsprinters yielded some unexpected findings. In that study, we used ultrasound imaging of the Achilles tendon and gastrocnemius coupled with foot length measurements made using bony landmarks such as the head of the first metatarsal and the lateral malleolus.12 We computed the Achilles tendon lever arm from measurements of excursion of the tendon with respect to ankle joint rotation made while subjects were seated with knees extended.13,14 Prior to the study we expected that, if anything, sprinters would benefit from large tendon lever arms that would afford them excellent leverage to achieve powerful pushoffs, so we were surprised to find that the opposite was true. Our sprinters had smaller Achilles tendon lever arms than the height-matched nonsprinters in our control group, giving them worse leverage for pushoff. Sprinters also were found to have longer toes than nonsprinters, but the two groups’ feet were no different in length. The feet of sprinters appeared similar to a badly designed wheelbarrow (Figure 1, bottom) in which the load was placed closer to the handles. This arrangement would require greater Achilles tendon force to produce the same joint moment and thus appeared to handicap the sprinters in terms of mechanical advantage.

A simple computer simulation of a muscle-driven sprint pushoff helped us to understand this apparent contradiction. Part of the answer lay in the force-velocity property of muscle, from which we understand that the faster a muscle shortens, the less force the muscle is able to generate.15 When the Achilles tendon has a short lever arm (as was the case with our sprinters) the plantar flexor muscle fibers do not need to shorten as much or as rapidly during pushoff, enabling them to generate more force.16,17 Leverage is compromised by these shorter tendon lever arms, but the losses are more than made up for by gains in muscle force.

Another part of the answer had to do with the time of contact between the foot and the ground. Having a long forefoot relative to the total foot length permits the toes to stay in contact with the ground for a longer period of time (think of how difficult it would be to raise your toes off the ground while running in skis). Later in a race it becomes important to minimize contact time that has the potential to slow the sprinter down, but in the first few steps of the race, ground reaction forces are the only means available to accelerate the body forward. Extending, even by a very short time, the period in late stance before the plantar flexor muscle fibers become too short to generate force allows for greater propulsive impulse and greater acceleration. Experiments have shown that better performing sprinters exhibit significantly longer ground contact times at the very start of a race than lower performing sprinters,18 suggesting an advantage for increased contact time similar to the one described above.

In that first study, we used ultrasound to estimate Achilles tendon lever arms from the excursion of the tendon with respect to ankle rotation. When this tendon excursion method is employed in vivo it is subject to an artifact associated with tendon stretch and relaxation that occurs due to variation in tendon force during movement. Because not all of the tendon excursion measured can be attributed to joint rotation, this method tends to underestimate Achilles tendon moment arms.

We performed a second study using magnetic resonance imaging (MRI) of the foot and ankle to avoid the potential for this artifact and to permit direct measurement of the lengths of forefoot bones.20 Using MRI we could locate the center of rotation between the tibia and the talus and measure the Achilles tendon lever arm as the shortest distance between this point and the Achilles tendon (Figure 2). Our findings in this MRI study essentially corresponded to those from the earlier ultrsound study; we again found that sprinters had significantly shorter Achilles tendon lever arms than height-matched nonsprinters, but the differences were not as great as in our first study. We also found that the first metatarsals and first phalanges were significantly longer in sprinters, though there was no difference in overall foot length between the two groups. Because we used MRI in this study, we had the opportunity to investigate whether sprinters’ shorter Achilles tendon moment arms were due to short calcanei. We found this not to be the case; we could attribute the differences in Achilles tendon lever arm entirely to variation in the location of the center of tibiotalar rotation within a reference frame fixed to the tibia (Figure 3). In other words, sprinters had short Achilles tendon lever arms not because their Achilles insertions were more anterior but because their centers of ankle rotation were more posterior.

Ankle leverage in the elderly

Our research group has also made measurements of foot and ankle structure to determine how it influences locomotor function in a different population concerned with speed: older adults.Mobility is a well-established determinant of quality of life among the elderlyand gait speed is a strong predictor of the risk of disability in such populations.Gait speed is known to correlate with ankle plantar flexor strength measured either during isometric or isokinetic tests.We wanted to determine whether gait speed would correlate with plantar flexor muscle architecture or Achilles tendon lever arm. We found no correlations between walking gait speed and plantar flexor fascicle length, pennation angle, or muscle belly thickness in a sample of 20 healthy elderly menWe did, however, note that, among our slower-walking participants (those who walked slower than 1.4 m/s, as identified by cluster analysis), there was a strong positive correlation (R= 0.69) between gait speed during a six-minute walk trial and Achilles tendon moment arm (Figure 4). This correlation was absent in the faster participants.

The positive correlation was not surprising for these elderly individuals because we did not expect force-velocity behavior to be a determinant of function, as appeared to be the case with the sprinters in our previous studies. Looking for other differences between the fast and slow subgroups, we found that the slower participants were significantly older and had significantly greater body mass and body mass index than faster individuals. Future research should further explore the relationship between gait speed and plantar flexor moment arm in older adults with limited mobility. Additionally, a prospective study investigating whether a large moment arm is protective against losses in gait speed due to age-related muscle mass reductions, especially in those who have the constraint of greater body mass, may yield useful information.

If muscle moment arms and joint structure are important determinants of mobility in older adults or other populations, we will want to understand how some people come by favorable joint properties that elude others. The lengths of bones are highly heritable, but their shapes are not necessarily determined by genetics. It may be that the loads applied to bones determine joint structure, especially those loads applied before skeletal maturity is reached. Bony deformities secondary to neurological disorders have the potential to disrupt healthy locomotion by altering skeletal lever arms,and osteotomies are routinely performed in children with cerebral palsy with the goal of normalizing those lever arms.It would be interesting to determine whether surgical alteration of muscle moment arms in neurologically normal populations might be a viable means of improving mobility in the mobility-impaired elderly, or possibly even for preserving mobility among middle-aged adults at risk for loss of mobility because of their joint mechanics.

Stephen J. Piazza, PhD, is an associate professor of kinesiology at The Pennsylvania State University in University Park, PA.