The issues raised by Klas in his comments on my recent post on Usain Bolt’s sprinting style have led me to wonder just what it is that determines peak sprinting speed and what a runner might do to increase sprinting speed.
The key relevant scientific study is the investigation of 33 physically active adults (aged between 18 and 36) of varying sprinting ability, published by Peter Weyand and colleagues from Harvard University in Journal of Applied Physiology (J Appl Physiol, 89: 1991–1999, 2000). They measured characteristics such as cadence, time on stance, swing time and ground reaction force observed across a range of speeds up each individual’s top sprinting speed. The range of top speeds extended from 6.2 metre/sec up to 11.1 m/sec. They observed that the faster sprinters exerted a stronger push on the ground while on stance and concluded ‘runners reach faster top speeds not by repositioning their limbs more rapidly in the air, but by applying greater support forces to the ground’.
I agree with their conclusion, but closer inspection of their data leads me to a slight modification that might have important implications for how a runner should train to increase speed.
Limb repositioning time
First let us consider the time taken to reposition the swinging leg from its position behind the centre of gravity (COG) at lift-off from stance, to a position a little ahead of the COG at foot-fall. This is the swing time. It embraces two airborne intervals and a period of stance on the other leg. Perhaps surprisingly, the swing time at top speed varies very little between runners of markedly different sprinting ability. The average swing time of the 33 runners was 0.38 seconds with only weak evidence that faster runners have a shorter swing time. For comparison, the average swing time of the three medal winners in the male 100m at the 1996 Olympics was 0.33 sec. However, there is little evidence of a consistent trend across the range of sprinting ability. For example, the slowest of the 33 individuals studies by Weyand had a swing time of 0.34 sec despite running only a little faster than half the speed of the fastest runners.
Although faster runners spend less time on stance, because their speed is greater, the foot gets left further behind during stance. Typically, a slow runner has to move the foot forward by about 85 cm relative to the COG during the swing, while the fastest runners have to move the foot forwards by about 105 cm. Thus, the faster runners do swing their foot forwards a little faster. For an elite sprinter it is worthwhile expending some effort on improving swing dynamics, for example by flexing the knee to create a short lever arm at mid-swing. However, this is only fine tuning – perhaps it might make the difference between a gold medal and fourth place, but it is not likely to produce the magnitude of improvement that might encourage a recreational distance runner to choose to become a sprinter instead.
It is interesting to wonder why swing time at top speed varies so little between elite sprinters and non-athletes. It appears that most of the gain a faster sprinter derives from increased ability to reposition the foot rapidly relative to the COG is required to compensate for the modest increase in the range of the swing required at higher speed. It appears to be impossible to get swing time appreciably below a third of a second. Although the swinging leg is not merely a passive pendulum it is hard to drive it much faster than its natural swinging rate
Time on stance
The strongest predictor of top sprinting speed is ability to get off stance rapidly. In Weyand’s study, the slowest sprinters spent 0.135 sec on stance while the fastest spent about 0.09 sec on stance. Furthermore, there was a very consistent trend for decreasing time on stance to predict faster top speed, across the full range of sprinting ability. The correlation between stance time and top speed was 0.76.
Shorter time on stance is associated with stronger push against the ground. The average vertical ground reaction force (vGRF) during stance increased from 1.9 times body weight to 2.4 times body weight, although the relationship was not quite so consistent across the range of top speeds. The correlation between average push and top speed was 0.62. Thus the average vGRF while on stance was not quite such a reliable predictor of top speed as stance time.
It is of interest to note that because stance time decreases as strength of push increases, the impulse delivered (product of force by time for which the force acts) varies relatively little between the slower sprinters and the fastest. The vertical impulse was 2.49 newton-sec at a top speed of 6.2 m/sec and 2.25 newton-sec at a top speed of 11.1 m/sec. As the vertical impulse determines how much upward momentum is imparted to the body, it determines how high the COG is elevated between mid-stance and mid-flight. .The peak elevation of the COG was marginally lower in the fastest spinters. The precise gain in elevation from a given impulse depends on the shape of the relationship between force and time while on stance. . For a forefoot runer it is approximaltey sinusoidal and in this case, the range of vertical oscillation of the COG was 5 cm at 6.2 m/sec and 4.3 cm at 11.1 m/sec.
These observations indicate that if one wants to sprint faster, one should aim to increase push and decrease time on stance. Although these two variables are related, in fact the decrease in time on stance is a stronger predictor of peak speed than the magnitude of the push. This is not surprising because decreased time on stance directly reduces braking, which leads not only to increased fuel efficiency, as discussed in my post on 16th January, but also to more efficient utilization of peak power.
It is necessary to have strong leg muscles to get off stance quickly, so it is worthwhile training so as to increase leg strength. As eccentric contraction is required, plyometrics are potentially helpful. However, the fact that the ability to get off stance quickly is the strongest predictor of top speed, suggests that one requires not only adequate strength but also good coordination of the muscles so as to capture impact energy as elastic energy and then release that energy in a smoothly coordinated way. This conclusion is similar to that reached on the basis of considering the style of Usain Bolt. If I want to increase my sprint speed I should focus not only on increasing my strength, but also my coordination.
I suspect that genes and development during infancy play a large part in determining how quickly a person can get off stance. Nonetheless, the fact that top speed decreases with age demonstrates that top speed is not fixed, and suggests that a training program aimed at producing changes opposite to those produced by aging might produce an increase in sprinting speed.
How might I increase my coordination? Plyometrics are likely to increase coordination in addition to increasing strength, though they are risky, and should be performed in moderation. A more direct focus on coordination might be worthwhile. Coordination depends on proprioception (the ability to sense where ones limbs are) and the ability to send messages from the central nervous system to the muscles with the appropriate precise timing. I believe that drills such as ‘change of stance’ are likely to be an effective way to achieve this