растяжка (о мышцах) sport.

[Acidfellow]

[Tamerlane]

Не только hip:

Maintaining and increasing hip and leg flexibility and strength helps to prevent injuries during athletic and non-athletic movement.

Apr 29, 2010 | By Sarah Smenyak
http://www.livestrong.com/article/113772-exercises-hip-leg-flexibility/

По suppleness:

We usually call Shotokai Karate a supple style of karate. But we never really specify what we mean by "suppleness" and so everybody develop their own view on the topic.
Mostly when we speak of suppleness we think of something I call "acrobatic suppleness". Meaning we call people supple who are able to lift their legs high up in the air and who can do the splits.

http://shotokaieurope.org/en/kdse-shotokai/what-does-qsupplenessq-mean-in-shotokai-karate.html

[Tamerlane]

по еxtension:

extension - the ability to raise the working leg high in the air; "the dancer was praised for her uncanny extension";
"good extension comes from a combination of training and native ability"

http://www.thefreedictionary.com/extension

[Мысль]

Just as variant :

Lengthening of muscles in the Legs

[vinni_puh]

Мысль,

типа так что ли? :))
http://rus.rucosm.com/index3.htm

[Bobcat]

Комментаторы (американцы/канадцы/британцы) в фигурном катании во время спиралей (элемент типа ласточки, только свободная нога должна быть чем выше, тем лучше) все время используют stretch (сущ.): great stretch / not a great stretch. Насколько это разговорно и характерно только для этого вида спорта не в курсе ))