Way back many months ago at (I think) one of the NTID conferences I've been lucky enough to go to, female sprinters were discussed.  One very common thing is that many of the ones that do very well in JW15 and JW17 often simply never go significantly faster once they get to JW19.

Why is this?

I have one thought about this, bear with my hypothesis, this is gut feeling not science :

When they're riding JW15 and JW17 the game is all about leg speed because they're restricted to tiny gears.  There's a certain amount of strength required (and you see this in the ones that do well out of the starting gate) but it's mainly a game of cadence.  This favours the girls who don't necessarily have a lot of strength but can spin like the clappers.

This is pretty obvious; girls aren't boys.  From a hormonal perspective, girls have roughly 10% of the testosterone that boys have.  Testosterone is the main hormone that drives muscle growth (amongst other things).  As such, it's really hard, without cheating, for girls to pack on significant amounts of muscle.  They can certainly grow stronger and put on some muscle, but unless they resort to training with the aid of the needle, they never get big and thus, strong enough to push bigger gears at high cadences.   The only female sprinters that ever looked like Sean Eadie were cheating (eg Tammy Thomas and Annalisa Cucinotta).  Combine this with old-school training methods that has them out riding lots of road miles, which blunts any muscle growth stimulii that they may get from sprint training and you get a kid that can spin, but will really struggle to push bigger gears and thus, go any faster when they're old enough to be able to push bigger gears.  We see this with some of the girls I work with, they're amazing as JW15's and JW17's but come JW19 the game changes, and it changes a lot.  The stronger girls start to take over and the super-spinners become less dominant.

Why do boys do ok in spite of mixing in lots of road riding?  They're awash with anabolic hormones in their late teens and for them it's not too late to undo the damage done to their fast twitch by endurance training.  But for the girls, their opportunity, I think, comes a lot earlier and is lost if it's burnt up by too much endurance training.

So, if that's true, or at least on the right track, what do we do to get the girls strong without cheating?

The time when they're growing the most is early to mid puberty.  This is when they have the most of the other growth hormone, HGH.  This is when they need to be in the gym getting as seriously strong as you can possibly make them, and doing high power and high torque efforts on the bike and NOT DOING ANYTHING CATABOLIC.  This means STAY AWAY FROM LONG ROAD RIDES!

Conventional wisdom says keep the kids out of the gym, I say nuts to that and I'm not alone.  I'm in favour of getting, in particular, the girls, in the gym as early as possible to get strong so when they're old enough to push big gears, they're strong enough to do it.  Keep them doing short, sharp efforts.  Anna Meares started as a kid racing BMX.  Short and sharp, high power, high cadences and high torque. Shanaze Reade and Willy Kanis are more elite track sprinters who started (and still do) race BMX.   You can add the required endurance work later, and that's endurance for dealing with the needs of a track sprinter, which is not the same thing as the endurance needs of an enduro cyclist and should be trained differently.  You may pay for this in the short term with them being a bit heavier as JW15's and JW17's because to put on muscle they need an anabolic diet (calorific surplus high in protein and low in the foods enduros live on, ie: simple carbs), but getting the girls strong AND able to push high cadences is, I think, the key to getting them fast in the long term.