Just a random question, would a sharp object pierce your muscle more(stabbed with the same amount of force) when the muscle is flexed or when relaxed?
Well, if we look in very basic detail at how muscle contraction works within a myofibril of the sarcomere:
N.b. this isn't on a loop and only plays through seven times
The red lines represent actin filaments whilst the blue lines represent myosin filaments. During muscle contraction the filaments move over each other:
As shown in the above diagrams the "more contracted" a muscle is then the smaller the size of the H zone (the area where there are just myosin filaments).
So if we consider a penetrating impact that went through a plethora of identically aligned sarcomeres (obviously this is not the case in nature) then we could presume that (et ceteris paribus) the object would not penetrate as deeply into a contracted muscle as it will hit more tissue on its route than in a relaxed muscle.