Tendon, ligament (5)
I believe the answer is:
sinew
I believe this clue is a double definition.
'tendon' is the first definition.
(synonyms)
'ligament' is the second definition.
(thesaurus)
(Other definitions for sinew that I've seen before include "power source" , "Force" , "Tendon - ligament" , "source of physical strength" , "It indicates strength" .)