Hi it's Wasim from Curious Doc. We're going to continue with this theme of
analysing the biomechanics in sports except today we're going to talk about sprinting.
I also wanted to make this because it seems like there's so much misinformation out there.
Almost everyone has a different opinion about leg movement,
arm swing and foot positioning etc. And almost no one backs this up with scientific reasoning.
Let's first look at the acceleration phase, the first 10 or so meters of the race where
the sprinter is gaining speed. Here the goal is to lean the entire body forward and maximise length
of time the foot is planted into the ground. If you look at the foot pushing off the ground, you
can see the force vector is being applied in this direction. When we break down the force vector
into horizontal and vertical components, you can see that some of the force is being used to propel
you forwards and some of the force is vertical, keeping your body off the ground. Obviously,
sprinting is all about moving horizontally as quickly as possible, so the aim is to maximise
this horizontal component which is done by leaning the body forwards. As you can see, the torso and
shin should be parallel to each other, forming an acute angle with the ground. In this case it's
about 45 degrees with the ground but the more acute the angle, the more horizontal force you
produce and the faster you'll accelerate. However, you have to be careful because if you lean too
much, there won't be enough vertical force keeping your body up which means that you'll start to
stumble. Your legs also won't be able to move underneath your body fast enough to catch your
falling body. One trick to counteract a stumble is to drag the trailing foot just above the ground
instead of bringing it up high like you're used to. This way the foot has to travel a lot less
distance and can reposition underneath your center of mass before you stumble. So in summary, the aim
of the acceleration phase to create an aggressive forward lean to maximise horizontal force
production whilst ensuring there's just enough vertical force to keep your body from stumbling.
Once you've reaching maximum velocity, you transition into the maintenance phase,
where the biomechanics of the body changes completely. The stride length
increases and the contact time between the foot and ground decreases. The body also needs to
straighten out and become more upright. A lot of new runners will assume that the body lean
will help in the maintenance phase like it does in the acceleration phase but unfortunately,
since you're no longer accelerating, it only works against you and slows you down. A 5-10
degree lean is okay but any more and the range of motion of the legs will be severely compromised
and as we'll see soon, the center of mass moves to a suboptimal position over the legs.
Let's talk about the phases of running and the goals of each phase by concentrating on
one leg at a time. When the foot is on the ground, it's called the drive phase. At the beginning of
the drive phase, when the foot hits the ground, the goal is to step directly underneath or just
in front of your center of mass. If the foot steps too far in front of the center of mass,
we can see a force is applied in this direction. And as we know, according to Newton's 3rd law,
there's an equal and opposite force in this direction which is called the ground reaction
force. When the foot is in front of the center of mass, this ground reaction force is in the
opposite direction to the runner, acting like a braking force and causing the runner to slow down.
It's a common problem for all new runners and is often compensating by activating the hamstring
muscles to pull on ground with the foot. This is a really inefficient way of running and
tires you out quickly. However, if you focus on landing the foot underneath your center of mass,
a lot of the force is in the horizontal direction. If we look at a video of Usain Bolt, you can see
that his leg starts to move backwards while it's still in the air. And the foot only hits
the ground whilst it's underneath his hips. Also note how his knee is above his foot at the point
of contact so he's pushing with his legs instead of pulling. This is another reason why we need to
have an upright torso while in the maintenance phase of running. If you lean too far forward,
your foot will tend to land behind the center of mass. As a result, you're missing out on a
big part of the drive phase and won't be able to transfer energy into the ground as efficiently.
After initial contact with the ground, we enter the middle part of the drive phase. The aim here
is to plantarflex the ankle with the calf muscles, extend at the hip with the glute and hamstring
muscles and keep the torso upright using the core. This is often referred to as triple extension
and the whole point of it is to use the entire range of motion of the legs whilst keeping the
body upright. However, the term triple extension is controversial because people often refer to
the knees being fully extended. If you watch any elite runner, you'll notice the knee is
always in a tiny bit of flexion throughout the entire range of motion. At no point,
do the knees fully lock out. And this is for good reason, because locking out the knees in full
extension creates a biomechanical DISadvantage to your hamstring muscles. The hamstring originates
at the pelvis and inserts on the tibia (and fibula) just below the knee. In engineering,
this is called a class 3 lever, with a pivot at the knee joint, the effort at the insertion
of the hamstring and the load being applied to the distal tibia where the foot is. Now if you imagine
your knee is fully locked out, you can see that no matter how much you pull with the hamstring,
the knee won't bend. This is why the knees are always slightly bent in running or rock climbers
always have a slight bend in their arms when doing an isometric pullup. Next, we have the recovery
phase where the leg is in the air. This is an often overlooked part of running but extremely
important. The recovery of the leg determines the speed and momentum of the other leg's drive phase.
So the aim is to recover the leg as quickly as possible so it's ready to hit the ground running
for the drive phase, pun intended. To optimise the recovery phase the knee should be almost
fully flexed and the ankle should be dorsiflexed. This decreases the moment of inertia produced by
the leg so that less effort is required to swing the leg forwards. You can think of the moment of
inertia as the torque required to rotate an object from a standstill, so if you imagine holding a
hammer at arms length, it's much harder to rotate at the shoulder compared to when the hammer is
closer to the body. It's also a similar concept when ice skaters spin really fast when they bring
their arms and legs in. And the same thing happens in running, the knee and ankle is brought closer
to the hips so that the moment of inertia is smaller and the leg can recover quicker.
And the faster the recovery, the more faster and forceful the drive phase of the other leg will be.
So in summary, for the maintenance phase of running, you want to land
with your foot underneath your center of mass to minimise the braking force.
Then you want to push off the ground using the calf muscles to plantarflex the ankle,
the glutes and hamstrings to extend at the hip and the core to keep the torso upright.
And finally when your foot leave the ground, you want to flex at the knee and dorsiflex at
the ankle to minimise the moment of inertia of the leg and speed up the recovery phase.
As you can see the whole motion is rhythmical and smooth.
If you look at the position of the hip, it doesn't bounce up and down
but instead seems to float above the ground. This shows that a lot of the energy is
going into horizontal motion and isn't being wasted in doing mini-jumps with each stride.
Sprinting biomechanics is one of the most complex things I've learnt
and there's a lot of things that I didn't have time to cover in this video
such as how to push off the blocks at the start or how to move the arms correctly. So if you enjoyed
this video and would like to see part 2, consider subscribing with notifications on. Thank you.