While at roughly 33% stamina left in the bar, i was unable to sprint or jump anymore. I had a mountain backpack, field vest and a KA rifle. Everything weighing in at approximately 31 kg. When i tossed off my backpack i was able to sprint, and when i took the backpack on again i was able to sprint, although my character slid across the ground. After some seconds my sprint showed the proper animation (although lagging), but moving around the same speed as when you jog.
Had several characters try it out, although the weird laggy running described above never happened again. All of them loses the ability to use sprint and jumps when roughly below 33% stamina.