What are the concentric and eccentric contractions involved in returning a tennis serve?

This is a very complex question.  The answer depends on if you are looking to focus on a specific area of the body, or conduct a full body mechanical analysis. A very basic answer is that muscles will both concentrically and eccentrically contract during this movement based on the phase of the movement (i.e. Wind-up, strike, follow-through), which side of the body is producing the most movement, which type of stroke return (forehand versus backhand versus volley), one-handed versus two-handed return, and possibly singles versus doubles.  Below are some examples of what I have just described:

Upper Body (Forehand Return -- Strike)

1. 
   Concentric: Anterior Deltoid, Coracobrachialis, Pectoralis Major and Minor, Subclavius, Biceps Brachii, Flexor Digitorum Profundus and Superficialis, Flexor Pollicis Longus and Brevis, Opponen Pollicis


2. 
   Eccentric: Triceps Brachii, Posterior Deltoid, Levator Scapulae, Lateral Aspect of Upper Trapezius, Proximal Aspect of Latissimus Dorsi

I hope this answer provides you a basic foundation on analyzing a service return.  Please feel free to contact me if you need additional help.