Increasing the number of active motor units within a skeletal muscle is called

Motor unit recruitment.

Explanation:

Motor unit recruitment may be defined as the  addition of motor units to increase the contraction strength in a muscle. Motor unit consists of muscle fibers and motor neuron.

The single motor unit activates the weak muscle contraction. The increase in the number of active motor units within a skeletal muscle increases the muscle contraction power.  

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lysosome is this part of the cell is surrounded by a very thick outer membrane to protect the rest of the cell from its strong enzymes.