关于过去完成时的问题

Eventually, the couple found that their bodies turned for the better,with re-found strong muscles that they thought had lost forever.

老师，这边用had lost,用过去完成时表达如何理解？是相对于身体好转这个动作之前吗？

Expert 的剖析有一个错误：

that they thought had lost forever：是定语从句修饰muscles，这不假。

但，lose 是及物动词“失去”，关系代词that 在从句中作had lost 的宾语，不是主语。主语是they。

thought 是“插入成分”，因为其主语和定语从句的主语一致，所以可以把thought 去掉，变成如此就更容易理解：

with re-found strong muscles that theyhad lost forever.