This GIF was floating around the internet,
so we [SpotMeBro] asked Stefan Gehrig, a PhD in muscle health and disease, what he thought was happening:
It looks like this guy has some sort of edema, either localized to the upper arm area, or more generalized (which is impossible to see). Edema is an accumulation of fluid, in this case likely in the layer underneath the skin rather than within the muscle itself. This is known as ‘pitting’ edema, because when pressure is applied to a small area by the fingers, the imprint of the fingers remains clearly visible. The fluid has been forced out of the tissue by the gripping pressure. Once he contracts his arm a couple of times, the fluid returns/redistributes. It is possible that this swelling has occurred as a result of the extra blood flow and minor damage sustained during a heavy biceps session. If this is the case, the ‘pitting’ will likely disappear in the minutes-hours afterwards, as the excess fluid is drained by the lymphatic system.
However, there are many more possible causes, ranging anywhere from compartment syndrome to viral infection to kidney disease. Sever dehydration is also a distinct possibility. Given the significant ramifications of some of these conditions, it would be prudent to consult a doctor and rule out any of these more serious symptoms if this characteristic ‘pitting’ remains for more than a day.