I'd say in this case the word sinew refers to the champion's vigor and strength, nothing to do with his opponent's flayed tendons and gristle.
The artist could very well have been trying to emphasize the point of race or tribe by portraying the victor with blond hair, but that would simply be an assumption on my part. Whatever the case, it's attention to small details that can bring an illustration to life ...
... always glad to share what little I know with you fine gentlemen.
