We all know that god for general medicine is Dhanvantri, but I want to know whether there is any god / deity particularly associated with muscles and muscle powers.
Lord Hanuman is associated with muscles and muscle powers/strength.
From Hanuman Chalisa:
बुद्धिहीन तनु जानिके सुमिरौ पवन कुमार । बल बुद्धि विद्या देहु मोहि हरहु कलेस बिकार् ॥
O Hanuman, O Lord, as I am ignorant and lack of wisdom, I pray to you to take away all my woes and worries and shower on me wisdom, strength and knowledge.
रामदूत अतुलित बलधामा । अञ्जनि पुत्र पवनसुत नामा ॥ 2 ॥
Victory to you, Lord Rama’s messenger, who embodies strength beyond comparison and who is also known as Pavanputra and Anjaniputra.
महावीर विक्रम बजरंगी। कुमति निवार सुमति के संगी॥ 3 ॥
You are the great hero, you are endowed with valour, your body is as strong as Indra's Vajra. You are the destroyer of vile intellect, and you are the companion of one whose intellect is pure.