elongate

1. V-T/V-I If you elongate something or if it elongates, you stretch it so that it becomes longer. (使)拉长; (使)延长[正式]

"Mom," she intoned, elongating the word. “妈妈，”她拖长了声音，缓缓地说道。

In this exercise, the muscles are elongating rather than contracting. 在这项运动中，肌肉是在延伸而不是在收缩。