What is the term for the ability of muscles to be slightly contracted at all times to be ready to act?

Answer:

The term for the ability of muscles to be slightly contracted at all times to be ready to act is Muscle tone.  

Explanation:

Regular partial contraction of the muscle or can be said as its resistance power to stretching during the resting phase. It helps in maintaining the posture of the body. The muscle tone occurs due to the hypotonia. This happens as the muscles depends upon the motor nerve signals which can be disrupted at the brain level, spinal cord or may be due to damage of muscle nerve. It can be measured with the stiffness which acts as a resistance to that of the passive movement.