It is typically assumed that moving the legs faster helps a person run faster. But did you know that research at one point actually called this assumption into question? Let's look at this study: "Faster top running speeds are achieved with greater ground forces not more rapid leg movements." First consider air time, the duration between foot contacts. The study found that faster runners do not spend less time in the air between foot contacts. In fact they might tend to use slightly more. This means the philosophy of getting to the next step quicker is not valid.
Next consider swing time. This is the time between two contacts of the same foot. It includes air time, contact time, air time. The study found that swing times "did not vary significantly" across the wide range of top speeds in the research subjects. This does not mean all the times were close to each other; it just means the data did not show correlation. However they did note that the top three finishers in the 1996 olympic 100m final did have shorter swing times than anyone in the study.
GROUND FORCES: the results showed that higher speeds are achieved by applying greater forces to the ground to get off the ground faster.
- Faster sprinting does require greater ground forces and shorter contact times. That's a fact.
- I am skeptical of the claims this study makes on swing time and speed of leg movement. If contact time is decisively lower in faster runners, shouldn't swing time also be lower since it includes contact time? If the ground is moving faster underneath the runner, don't the legs have to move faster to match that to some degree? I suspect a different group of subjects would show a clear connection between shorter swing times and higher speeds. Any time you take kinematic data across a random group of people, especially during a highly complex task like sprinting, it muddies the water, so to speak. Correlations may not be reliable in this type of scenario. Test two distinct groups such as sub-10.5 100m sprinters and basketball players and see what happens.
- How fast, slow, and average people compare is not the exact question we need to answer. What we need to know is, "Does swing time have to get shorter over time for an athlete to get faster?" I suspect the 100m champion did not have a swing time of 0.32 back when he was running 11.5 seconds.
- Regardless of swing time decreasing or not, as you get faster over time your legs will certainly have to move faster in the air. Keep reading...
Now consider this study: "Whip from the hip: thigh angular motion, ground contact mechanics, and running speed." The data shows that faster runners tend to use a larger thigh excursion angle, the range of motion from peak hip extension to peak flexion. Even if swing time stays the same, if the thigh moves through a greater range within that time, it has to be moving faster (greater angular velocity).
They also directly measured thigh angular velocity, and the data did indeed show a clear, strong connection between higher top speed and faster thigh movement. This study corrects the faulty conclusion from the last study.
Again this data shows correlation, not changes over time as speed increases. I believe the changes would vary depending on the individual. I have definitely seen young athletes use large thigh excursion angles during sprinting. In that case, perhaps swing time decreasing over time plays a bigger role. I began tracking swing time in one of the track athletes that I coach. Over the course of a few months her best flying 20yd sprint time decreased by 0.1 seconds. Along with that, swing time decreased from 0.40 to 0.36. We have to be cautious not to take the correlations across one group of research subjects and make assumptions about how an individual might change over time. Swing time may get shorter as an athlete gets faster. On the other hand, for young athletes that use a smaller range, maybe the thigh excursion angle getting bigger over time is more significant. Either way, as speed increases, the legs have to move faster in the air.
Applications
So what does this mean for training? One of the implications of faster leg movement is increased stress on the muscles that move the legs in the air. Let's look at some research on several different muscles.
Hamstrings
Consider the hamstrings. These muscles experience high stress during late swing phase when they stop the high speed forward movement of the shin and contribute to accelerating the the thigh down and back before contact. Let's look at some research on the effects of sprinting on the hamstrings. Study: "Hamstring Muscle Volume as an Indicator of Sprint Performance." This study found that sprinters had more thigh muscle volume than "active males" (not surprising). But the hamstrings especially were far more developed, and the data showed a correlation between hamstring muscle volume and 40m sprint time.
More research: "Hypertrophic muscle changes and sprint performance enhancement during a sprint-based training macrocycle in national-level sprinters." This study found that a period of sprint training following off season caused increase in the muscle volume of thigh muscles including the hamstrings. Again this gives us insight on the muscular demands of sprinting.
It's tempting to look at this data and say we can just sprint to develop the hamstrings. And we should consider that running/sprinting itself is a significant stimulus. But if we can use strength training to get the hamstrings accustomed to even greater stress than sprinting, this reduces the risk of injury and facilitates faster and more frequent speed training. Two key types of hamstring exercises are hip hinges and leg curls. I like to use each once per week alongside sprint work. For more thorough hamstring info, check out Hamstring Strength.
Hip Flexors
The legs moving faster in the air also puts greater stress on the muscles that flex the hip, which includes the psoas, iliacus, rectus femoris, some of the adductors, and a couple others. A handful of studies have examined psoas size and speed. Study: "Trunk and lower limb muscularity in sprinters: what are the specific muscles for superior sprint performance?" This research examined 10 different muscles and found that only the size of the psoas and glutes correlated with faster 100m times in sprinters. It's just a correlation and not a terribly strong one, but still it gives an indication that the psoas needs to be developed for speed.
Study: "Influence of the Psoas Major and Thigh Muscularity on 100m Times in Junior Sprinters." This research measured muscle cross sectional area and found a correlation between a larger psoas to quadricep size ratio and faster sprint times, more data in favor of hip flexor development. See video below for strength exercises.
Next consider the adductors (groin muscles). This group includes several different muscles with different attachment points. Along with performing adduction, these muscles have varying contributions to hip flexion and extension depending on the position of the thigh. With this role in mind, it is no surprise that research found significant hypertrophy in the adductors in response to sprint training. The same study that we looked at for the hamstrings found even greater percentage growth in the adductors. This clues us into the stress on these muscles during sprinting.
Adductor exercises alone may not be a big driver of improved performance, but just like with the hamstrings, if we can get these muscles adapted to a greater stress than sprinting, it allows us to train speed aggressively and not be held back. The adductors are trained to some degree by hip flexion and extension exercises (lunges, knee drives, etc). I like to get more thorough development from side lunges and some type of targeted adduction work (see video).
Finally consider the rectus femoris. This is the one quad muscle that acts as a hip flexor. The swing phase of sprinting places high stress on this muscle. In fact when people strain a quad during sprinting, the rectus femoris is the specific muscle that gets injured. As an athlete increases speed and leg swing gets faster, the stress on this muscle grows. With this in mind, we might expect research to find a connection. Study: "Thigh and Psoas Major Muscularity and Its Relation to Running Mechanics in Sprinters." This study compared sprinters to untrained men and found mostly greater muscle volumes in sprinters (no surprise). But they also took a bunch of measurements on the sprinters 35 meters into a sprint. In this group the only muscle volume that correlated with higher velocity was rectus femoris volume.
Due to it's anatomical uniqueness from the rest of the quads, the rectus femoris does not get good structural development from typical quad exercises and is easily neglected in a training program. To train it at long length we can do hip flexion with a bent knee or intentionally load the back leg in a split squat (see video).
Does training hip flexion strength increase speed?
We can say confidently that the various muscles that contribute to hip flexion experience greater stress and tend to increase in size as an athlete gets faster over time. There is correlation between those things. However we cannot necessarily imply causation here. This is a chicken or egg type situation. Does increasing hip flexion strength produce faster leg movements which produces higher speed? Or does something else increase speed, which brings with it faster leg movements, which produces muscle hypertrophy over time? There is one study that shows improvements in speed from a hip flexion strength training program: "Effects of Hip Flexor Training on Sprint, Shuttle Run, and Vertical Jump Performance." However this research has a massive limitation in that it was conducted on untrained individuals. This does not inform us on trained athletes who are already getting lots of hip flexor work from sprinting. Also of note, all the improvements in speed came within the first 10 yards of the sprint. And lastly, this is one of those studies where the results seem to be from a sports training fantasy world. The subjects did one hip flexion exercise twice per week and improved by around a quarter second over just 10 yards. That is wild.
Honestly I don't believe this question has an obvious answer. I really cannot promise that hip flexion strength training will make athletes faster. However I think it is more likely if an athlete has heavy legs, does not have much history of sprinting, or has not been sprinting lately. I also believe it's more likely to improve the sprint start rather than max velocity. I talk more about why this is a difficult question in the video below. Regardless of performance enhancement, training hip flexion strength is advisable to build up tissue resilience and facilitate faster, more frequent speed training as previously stated.