The ballerinas around the wall are stretching their legs and toes so their muscles don't stiffen.
Literal
Wall's around ballerinas [topic-は] muscles [subject-が] stiffen [so-that-not-ように] legs [and-と] toes [object-を] stretching are.
A complex sentence with a purpose clause: 筋肉が硬直しないように ('so that muscles don't stiffen') motivates the action 脚と足先を伸ばしている ('stretching legs and toes'). バレリーナ達 uses the plural suffix 達. 壁のまわり places the dancers physically around/along the wall — a ballet studio scene.