Loading rate

When your foot contacts the ground, the ground pushes back up against it with an equal amount of force. This is ground reaction force. We usually think first about "how much force was applied," but loading rate looks at "how quickly that force was applied."

It is a bit like the difference between gently placing a ball in your palm and catching the same ball after it is tossed to you. The weight is the same, but the feeling at the moment you receive it is completely different. Even if two landings have the same peak force, loading rate is high when the force rises steeply and low when it rises gradually.

Loading rate is usually measured using a force plate. A force plate is a device that records ground reaction force over time while the foot is in contact with it. Plotting this record produces a curve showing the rise in force just after landing. The slope of that curve is the loading rate.

Studies often distinguish between two types of slope.

• Average loading rate — The average slope across the initial period when force begins to rise.
• Instantaneous loading rate — The slope at the moment when the curve rises most steeply.

It is usually expressed in newtons per second (N/s), or normalized to body weight in body weights per second (BW/s). Normalizing to body weight makes it easier to compare people of different weights.

When comparing heel-first and forefoot-first landings, studies report a tendency for heel-first landings to concentrate the initial impact into a shorter time, resulting in a higher loading rate. This is why loading rate often appears in studies comparing foot strike patterns.

However, loading rate is not determined by foot strike pattern alone. It also varies with shoe cushioning, surface hardness, running speed, fatigue, and familiarity with the movement. Even for the same person, values differ between running on asphalt and running on a dirt path. If fatigue changes the way they land, the values change too.

You may rarely hear the term loading rate, but you can feel the concept in situations like these.

• Your knees feel different when you stomp down stairs versus when you step down lightly.
• When you run on a hard floor in shoes with thin cushioning, you feel a pronounced, sharp tapping impact through the soles of your feet.
• When you are tired and your feet land heavily, your landings become louder. This sound is a sign that force is being applied quickly.

When running coaches or exercise professionals say "land lightly" or "try shortening your stride a little," these are ultimately strategies for making the application of force more gradual. Studies have reported a tendency for landing impact to decrease when step rate is increased slightly and stride length is reduced.

• Listen to your landings. Simply checking whether your footsteps are quiet and soft as you run can help you gauge how force is being applied.
• Do not change everything at once. When changing your foot strike pattern or stride length, make gradual changes over several weeks to give your calves and Achilles tendons time to adapt to the new demands.
• Consider both your shoes and the surface. Cushioning, shoe wear, and the hardness of your running surface affect loading rate.
• Try slowing down when you are tired. Fatigue can make your landings harsher, so it is better to lower the intensity on days when you are not feeling your best.