THE SCIENCE
Why stride is measured against leg length.
Taller kids take longer steps. That’s not technique, it’s just legs. Here’s how Skill Grader makes sure every student is judged on what they do with the body they’ve got.
Timothy Starkey
PE teacher and founder of Huddl · 4 min read · October 2026
Line up any Year 4 class for a sprint and you’ll see a big spread of heights on the start line. Some students are a full head taller than the kid next to them. When the whistle goes, the taller students almost always cover more ground with every step. That isn’t because they’re better runners. Mostly, it’s because their legs are longer.
This was one of the first problems we had to solve when we built the running assessment in Skill Grader. If we graded stride in metres, the tallest kids in every class would collect the best marks. The smallest kids would be told to take bigger steps when they were already doing exactly what their bodies should be doing. That’s not fair, and it isn’t useful feedback either.
Bigger bodies take bigger steps
Step length grows with leg length. A longer leg swings through a bigger arc, so each step covers more distance even when the movement pattern is identical. People who study movement have known this for a long time, which is why they rarely compare raw distances between people of different sizes.
In 1996, biomechanist At Hof published a short paper in Gait & Posture called Scaling gait data to body size. His suggestion was simple. Divide measurements like step length by leg length, measured from the hip joint to the floor. What you’re left with is a number with no units, and it lets you compare a small child with a tall adult on fair terms. Looking at walking data, he found that once the numbers were scaled this way, children from about three and a half onwards looked much the same as adults.
What Skill Grader actually measures
That idea sits behind the first running metric in Skill Grader, which we call stride length ratio. Instead of asking how far each step travels, it asks how far each step travels compared with the student’s own leg length.
Here’s an example with tidy numbers. Mia’s legs measure 68 cm and her steps cover 95 cm. Jack’s legs measure 60 cm and his steps cover 84 cm. On a tape measure, Mia looks 11 cm better. As a ratio, they’re identical. Each step is 1.4 times the length of their leg, so both students are using their legs equally well and both get the same result for this metric.
ON A TAPE MEASURE
Step length in metres
Mia
0.95 m
Jack
0.84 m
!
Mia looks 11 cm ahead, but most of that gap is height. Jack would be marked down for being shorter.
AGAINST LEG LENGTH
Stride length ratio
Mia
1.4
0.95 m step ÷ 68 cm leg
Jack
1.4
0.84 m step ÷ 60 cm leg
✓
Same ratio, same result. Both students are judged on how they run, not how tall they are.
Same technique, different heights. Example numbers, kept simple to show the idea. They aren’t Skill Grader benchmarks.
That’s the fairness bit. The ratio rewards what a student does with the body they have, not the body itself.
It works from any distance, too
There’s a second reason we use a ratio, and it’s a bit more technical. Skill Grader works from video. In a video, distance is counted in pixels rather than centimetres, and the number of pixels changes with how far you’re standing from the student. Film up close and their legs take up a lot of the screen. Step back to the other side of the track and the same legs might be half the size.
If we measured stride in pixels, the result would change depending on where you stood. Because step length and leg length are both measured from the same footage, the camera distance cancels out when you divide one by the other. A step that’s 1.4 times the length of the leg is still 1.4 times the length of the leg, whether you film from two metres away or ten.
Filmed up close
Leg 290 px
Step 247 px
Ratio 0.85
Filmed from further back
Leg 145 px
Step 124 px
Ratio 0.85
The same moment filmed at two distances. The blue line traces the leg from hip to knee to ankle. The yellow line is the distance between the feet. Step back and every pixel count halves, but the ratio stays exactly where it was.
What it means for your class
For teachers, the practical upside is simple. The tallest kid in the class doesn’t get a head start, and the smallest kid doesn’t get marked down for being small. It also means the feedback makes sense. If a student’s ratio is low, the conversation is about how they use their legs, like driving the knee forward and pushing back through the ground. It isn’t just “take bigger steps”, which is hard for a young runner to act on anyway.
Stride length ratio is one of seven things Skill Grader looks at in a sprint, alongside torso lean, cadence, foot strike, elbow angle, knee drive and stride symmetry. Each one is benchmarked against what’s expected for the student’s year band, so a Year 1 student is never held to a Year 6 standard.
Fair assessment in PE starts with measuring the right thing. A tape measure tells you how long a step was. A ratio tells you how well a student is using the legs they’ve got, and that’s the part we can actually teach.
SOURCES
Hof, A. L. (1996). Scaling gait data to body size. Gait & Posture, 4(3), 222–223.
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