Is linear momentum conserved in an isolated system always? Consider a isolated system of particles. If no external force acts on this system, the momentum of the system should remain constant.   
If the isolated system of particles represents living creature. And if we assume you (the reader) to represent that system. If you even shake your hand, you are changing the positions of the particle of your hand.   
I hope, we can move the particles of hand at any speed (till the limit allowed by laws), at least to cross the initial momentum of the system. Is this not possible? If it is possible, aren't we violating the principle of conservation of linear momentum?   
 A: If the (entire) system does not change if you displace it in space, then the total momentum (of all particles) will be conserved. 
Momentum is the generator of translation, which is a specific case of Noether's theorem. 
To relate it to your example of living creatures: imagine cat on very slippery ice and neglect air resistance and friction. If you slide the cat with some initial momentum, no matter how the cat moves and rotates, it will not be able to affect its center of mass as a function of time, i.e. its momentum is conserved.
A: Conservation of linear momentum, for a physical system whose particles are initially at rest in a given inertial reference frame is equivalent to the fact that the center of mass of the system remains fixed at its initial position. You see that this constraint is quite week for a system made of a large number of particles as a human body. Starting form a completely static situation, you can pass to move your hands or your legs or other parties of your body, very quickly preserving the position of the center of mass. 
A: Linear momentum is always conserved. If you swing your hand  there is always a change such that linear momentum is conserved. If you swung your hand in space there would be some motion. At least until the electrostatic forces in your body brought you to rest. On Earth, the friction between your body and the ground and the air drag wouldn't allow the resultant changes to be very visible.
