The force you use when holding 1 kg in your hands is called muscle force. Here's why:

* Gravity: The 1 kg object is pulled towards the Earth by gravity. This creates a downward force.

* Muscle Contraction: To counteract gravity and hold the object, your muscles contract. This creates an upward force equal to the force of gravity on the object.

* Equilibrium: When you hold the object steady, the forces of gravity and muscle contraction are balanced. This is called equilibrium.

Therefore, you are using muscle force to counteract the force of gravity and hold the object.