Did you ever take chemistry in school? If so, you may remember something known as the rate limiting step. What this means is that when there’s a chemical reaction the speed of the reaction may be limited by a particular element. Increasing the concentration of this element allows the entire reaction to happen more quickly.
It’s important to know the rate limiting or determining step because then we know where our efforts will generate a better result.
Let’s look a different example to see how this works.
Imagine a busy restaurant that measures each day’s success by the number of tables turned over. There will be a maitre-d, a hostess, a server, sous-chefs, head-chefs, bussers and dishwashers. If the dishwasher doesn’t show up for work eventually the plates pile up and there will no clean plates to set a new table. Everything backs up at this point. And it doesn’t matter if everyone else does a little bit more because the dishes are still piling up, not getting washed and tables are not getting turned over.
We can know appreciate that when we’re looking to achieve a goal there may be something that limits the outcome.
With sprinting for example, we know there are contributions from the lower body joints i.e. the ankle, knee and hip. When walking there is a contribution from each of these joints. As we progress from walking to sprinting is the increased role of these joints proportional? Or are certain joints prioritized?
A recent study looked to answer this question.
Researchers looked at the involvement of the ankle, knee and hips joints of 7 test subjects. The subjects moved at a variety of speeds from 1.5 m/s (walking) to 9 m/s (sprinting) to answer the question of whether the contribution of these joints increased proportionately or if there was more emphasis on certain joints depending on whether the speed was closer to walking or sprinting.
The researchers looked at force production and work done by the test subjects moving on level ground. The inclination did not change during the study. The results were broken down into the stance phase and swing phase. As well both of the phases were sub-classified based on whether energy was being generated or absorbed.
The stance phase would describe the time the lower limb is contacting the ground. The swing phase would describe the time the lower limb is in the air. Sprinting is therefore considered both an open and closed chain activity.
The second aspect looked at was whether energy was being generated or absorbed. As we return to the ground after bounding energy would be absorbed. Once on the ground and preparing to leave the ground energy would be generated.
What did they find?
Christopher Beardlsey of Strength & Conditioning Research did a great job summarizing the data. And I’ll give credit to him and explain what is represented in the tables above.
The top tables show stance phase power generated and absorbed. The hip is red, the knee is green and the ankle is blue.
Within each phase there is a table for slow running on the left and sprinting to the right. This pairing of slow running and sprinting is repeated for each of the four groupings of tables.
When looking at the stance phase power generated we see the ankle is the primary joint involved. When we move from slow running to sprinting the contribution of the hip increases substantially.
To the right the next two tables show the stance phase power absorbed for slow running and sprinting. In this case we see less involvement of the knee and more from the hip as we contact the ground.
The bottom right two tables represent the power generated during the swing phase. With both slow running and sprinting we see a very large contribution from the hip i.e. lots of red. As we move from slow running to sprinting the contribution from the hip is even greater. Visually this represented by a predominance of red colour.
The last two tables on the bottom right look at the power absorbed during the swing phase. Here we see lots of green indicating the knee is the primary joint during this phase of the gait cycle in terms of power absorbed.
What does this all mean?
Well first of all, the contribution of the hips is greater at higher speeds for both the stance and swing leg. As we move from walking to running to sprinting we need more contribution from our hips. And this makes sense. If we imagine someone walking along there is a nominal amount of separation between the knees. Running requires more separation and sprinting even more separation between the knees.
The other observation is the ankle is more involved during the stance phase compared to the swing phase.
Lastly, we see the contribution of the knee decreases with increasing speed. The knee is most important when is absorbs power during the swing phase.
How does this help me?
If your goal is to improve sprinting ability you need to have strong and mobile hips. If you’re coming off an injury, say a sprained ankle, recognize that you may struggle in producing power in the stance phase. Producing power during this phase would involve plantar flexion and pushing the big toe into the ground. You should also be aware that the knee joint plays a minimal role in power generation during the swing phase but a substantial role in force absorption during the swing phase.
Going forward check your ankle, knee and hip mobility and the strength of the muscles associated with these joints. And if you have a goal to get faster put more emphasis on hip mobility and hip dominant exercises.