Why your sprinter's back leg never fully extends out of the blocks
The rear leg is supposed to finish straight at clearance, and when it does not, the fix is almost never in the leg itself.
Watch the block start back frame by frame and the rear leg is still bent when the front foot leaves the block. It looks like a strength problem. Coaches send the athlete to squat more. The leg often stays bent.
What full extension is actually buying
The rear leg's job in the first push is to stay in contact with the block long enough to straighten completely, applying force for the whole range of the leg rather than a fraction of it. A leg that leaves the block half-bent has cut its own push short. It is not that the athlete lacks the strength to straighten the leg. It is that something ended the push before the leg had the chance.
Where it actually comes from
- Too much weight already on the front leg in the set position, so the rear leg has almost nothing left to push against.
- Hips set too high, which points the first push up rather than back and shortens the range the rear leg can work through.
- An athlete who pulls off the block rather than pushing through it, usually because they are trying to get the front leg moving fast rather than finishing the drive behind them.
- Blocks angled for a taller or shorter athlete than the one currently using them.
What the phone can tell you, and where it stops
The block start event in the Film Room reports drive-phase trunk lean and whether the athlete stands up too early, and whether the hips climb instead of driving out. Both of those get a clear warning and a named drill when they are off. The back leg is different. It reports the knee's range of motion at clearance as information, not a pass or fail, and tells you to step frame by frame to the instant the front foot leaves the block and look at whether the back leg is straight.
That is a deliberate choice, not a gap somebody forgot to fill. Leg length, block spacing and hip height all change what a fully extended rear leg looks like in degrees, so there is no single angle that is right for every athlete. A finding that cannot honestly separate a real fault from a body shape does not get a threshold, and it does not get a drill of its own.
What to do about it anyway
The standing remedies for a rushed drive phase are the same ones that fix a hip that climbs early: wall drives held for a three count, so the shin-and-trunk line the drive needs is something the athlete can actually feel; falling starts into ten metres, which make popping upright impossible because there is nothing to pop up onto; and sled or partner-resisted pushes, which keep the hips down without the athlete having to think about it. All three ask the same question of the rear leg that the clearance number does, which is whether it gets to finish the job before the athlete stands up.
A side-on camera sees one leg push against the block at a time and cannot tell you whether the two legs are pushing evenly, which needs a view from behind. And the extension itself happens in well under a tenth of a second, so a coach's eye at full speed is guessing. Slow the clip down and look, do not trust the impression from real time.
Time your next meet with the phones you already have
Timing, the Film Room and the speed gates are free forever. Publishing results to your own page starts at $59 a month.
Related
Why your sprinter's first step out of the blocks goes nowhere
A short, flat first step usually means the front leg never got the chance to push, not that the athlete is slow off the gun.
Read →How far forward should a sprinter lean?
A lot out of the blocks, very little at top speed, and never by bending at the waist, which is the version that gets taught by accident.
Read →Which foot goes in front in the starting blocks?
The popular rules of thumb contradict each other, which is why the only answer worth trusting is a timed one.
Read →Try it: Sprint timer app: 40-yard dash and flying sprints with one phone