Faster Further Longer: How we run faster and what this means for clinical practice
Whether we’re trying to set a park run personal best, intercept a football pass or win the Olympic 100m final; increasing running speed is something many of us will try to attain. However, in doing so the biomechanical demands placed upon the body change, and whether we are capable of meeting these demands can influence our performance and/or our injury risk. Therefore, understanding how we increase running speed and the demands this places on the human body can help us, both as runners, and as clinicians to guide safe return to run and optimise our performance.
How do we increase running speed?
To increase running speed we tend to adopt a combination of two strategies, either increasing stride length and/or increasing stride rate (1, 2). When increasing running speed from relatively low speeds, the initial mechanism appears to come from increasing stride length. Then once at higher speeds, there appears to be a subsequent increase in stride rate.
In a paper published by Schache et al (1), based on the work of Tim Dorn and colleagues (3), increasing running speed from 3.48 meters per second to 5 meters per second (such as when increasing from 50 min to a 32 min 10km pace) was achieved through a 30% increase in stride length. This is compared to only a 11% increase in stride rate. As speed increases further, stride length and rate appear to increase at a similar rate, until maximal stride length is attained. Where higher speeds appear to be attained via a switch in strategy towards increasing stride rate (figure 1).
Figure 1:Changes to stride length (top bars measured in meters) & stride rate (bottom bars measured in Hz) as speed increases. Cited from Schache et al (1).
Importantly, both strategies impart different loads on the musculoskeletal system and therefore will have different implications to consider. So, lets explore these implications a little more….
Increasing stride length
To increase stride length we need to project our centre of mass upwards and forwards through the air, increasing the ground covered during the flight phase. The mechanism by which we achieve this comes from our ability to apply large forces into the ground to overcome gravity (1-4). Several studies have shown that a runner’s ability to apply large mass specific forces into the ground, over short ground contact times, influences the maximal velocity attainable. The work of Peter Weyand and Ken Clark has demonstrated that both elite sprinters and non-sprinters increase running speed by generating large mass specific ground reaction forces across short contact times (4, 5), with elite level sprinters doing this to greater effect. This is similar to mechanisms observed in middle- and long-distance runners (6, 7).
The ability to generate large ground reaction force appears to come from a coupling pattern between the hip, knee and ankle (8-11). Looking at joint forces across running speeds and the time in which they occur, it would appear that generating force, in part depends on our ability to first swing the thigh into the ground, and then absorb and return energy within the ankle joint. A simple analogy I’ve heard before is to think of the hip as the hammer and the ankle as a nail, the harder you swing the hammer the more force applied to the nail. And the stronger the nail, the more force is transferred to the ground to send us back up into the air.
Interestingly, based on several modelling studies the largest contributor to vertical ground reaction force and increasing stride length, initially comes from the soleus muscle. When running at speeds up to around 7m/s, the muscle forces required to swing the hip are relatively small when compared to the ankle. Hamstring muscle forces range from 2.1x body weight at 3.5m/s to around 4.6x body weight at 7 m/s. In contrast, soleus muscle forces range from a massive 6.7x body weight at 3.5m/s reaching up to 8.7x body weight at 7m/s (figure 2) (3). In fact, the soleus is reported to be the main contributor to vertical support during stance (contributing up to 67% of the VGRF) and contribute up to 77% of the upward acceleration of our mass during late stance (9, 10). Therefore, even when jogging, we need a pretty strong Soleus!
Figure 2: Muscle forces as running speed increased cited from Dorn et al (3).
Impact on the lower limb
Muscle forces this large will have a clear impact upon lower limb structures. In a recently published paper from our research group, we looked at the effect of running speed on Achilles tendon forces (12). Looking at speeds ranging from 8 minute/mile to 5 minute/mile we found significant increases in Achilles tendon forces with even small increases in speed. Increasing from 6.4x BW at 3.2m/s (8min/mile), to 7.7x BW at 5.6m/s (just under 5min/mile).
Figure 3: Achilles tendon forces as running speed increases
Similarly, the work of Brent Edwards (13) highlights that subtle increases in running speed, creates large increases in tibial bone stress. Importantly, Brent’s work highlighted that these increases in load, can result in a significant reduction in the number of loading cycles before tissue failure occurs. Meaning if we run faster, don’t assume we can attain the same volume.
Increasing Stride Rate
Now, above 7m/s the force generated by the lower limb tends to plateau (for complex reasons we could discuss another day). Our strategy for increasing speed then switches to one where we increase our stride frequency/ cadence. Rapidly swinging our legs backwards and forwards. Recent work from Ken Clark (11) (with the cool title “Whip from the Hip”) found as running speed increased, there was a significant increase in frequency and amplitude of thigh angular acceleration across the entire gait cycle, the contact phase and the leg retraction phase. Which strongly correlated to maximal top speed and was greater in those who could attain faster top running speed. Effectively meaning that faster runners drive the thigh both down, into the ground, and back upward, faster than slower runners.
But once again, this is going to influence the demands on our musculoskeletal system. Swinging our legs through the air more quickly requires greater muscle activity and muscle force from the hip flexors and hip extensors (3, 14). In fact, returning to the work of Tim Dorn, at speeds of 9m/s both hamstring and iliopsoas muscle forces reach a whopping 9x BW! (figure 4). Now I don’t think it takes much imagination to think of the stress this creates!
Figure 4: Effects of running speed on stride frequency/rate and the associated muscle forces
Implications for clinical practise
To this point it appears that increasing stride length is the main mechanism of increasing speed at slower running speeds, followed by an increase in stride frequency. Both strategies imparting different biomechanical loads on the body, which as a clinician I feel it’s important for us to consider within our rehabilitation and return to run process. So here are some of my thoughts:
1. Soleus needs to be able to tolerate large forces, just to start jogging:
This is no doubt why soleus injuries can be challenging to manage, with many people trying to run too early. Just because they may be symptom free walking, doesn’t mean the muscle is ready to tolerate those running forces.
2. Are we conditioning tissues appropriately in the rehab process?
The soleus experiences internal muscle forces of over 6.5x BW over contact times of less than 0.2 seconds. A quick clinical go to for calf injuries is often the body weight calf raise, however in my opinion, this is simply not going to prepare the lower limb for running! And returning to run without adequate conditioning, is basically rolling the re-injury dice and hoping for the best.
3. Do tissues have sufficient structural capacity tolerate load:
I think this is particularly important for tendon and bone injuries. The high loads encountered means the structure needs to be able tolerate this load. Following injury we need to acknowledge the need for graded load exposure to gradually restore these tissue properties.
4. To condition ourselves to run, we need to run!
It’s easy to see that we are going to struggle to recreate these muscle, tendon and bone forces solely in the gym or clinic room. Therefore, the only way to condition the body to run, is to run! But for me, if we are attempting to return an injured runner to the track or road, there’s an important consideration….
5. We need to get better at filling in the gaps:
I think it’s important to consider how we can gradually progress our way up the loading ladder so that return to run is a calculated and considered part of the process and not just a gamble. We need to think about how we can progress load, the speed of loading, the number of cycles we are exposed to. These are all factors that could help us to progressively bridge the gap between the clinic room and the road.
6. Training load management:
Finally, I think its important for us to acknowledge how subtle changes in running speed has a large impact on tissue loads. Just because we can tolerate 30 minutes jogging doesn’t mean that we can run 30 minutes hard. Remember, rehab doesn’t end in the clinic room.
Finally, although we’ve looked at the gross biomechanical requirements of increasing speed, it’s important to acknowledge that individuals may vary in the strategies they use to increase running speed. This may be due to different physical qualities they possess or the varied demands across running sports/ events. If an athlete lacks adequate force production at the lower limb, they may adopt a strategy of increasing step frequency much earlier than an athlete with greater force capabilities. Alternatively, athletes may find a strategy that best works for their physiology. This is particularly pertinent when working with the endurance runner, where there is a need to optimise running economy through balancing the energy cost of generating force with that of swinging the limbs in the air. With experience it is likely that many endurance runners will self-select the method of increasing and maintaining their maximal running speed which best optimises their running economy and physical capabilities. So, let’s not lump everyone into a one-size-fits-all approach, but instead consider the individual approach to increasing speed, and think about what this might mean for clinical practise.
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