The difference in the strength of the muscle contraction needed to pick up a pencil versus lift a 30-pound weight is dependent on

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The strength of muscle contraction is dependent on both the size and the number of the motor units involved. A larger force can be generated by recruiting more motor units and by activating motor units with larger muscle fibers. The number of motor units recruited depends on the weight that needs to be lifted or the force that needs to be generated. In the case of lifting a 30-pound weight, more and larger motor units will be recruited compared to the act of picking up a pencil.

The Difference in the Strength of the Muscle Contraction Needed