Final answer:
The EMG activity is related to grip strength by the number of myofibers in the muscle that receive action potentials from the controlling neuron. When picking up small objects, only a few myofibers respond, but when picking up large objects, all myofibers participate, generating close to maximum force.
Step-by-step explanation:
The EMG activity will be related to the grip strength by the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber. When picking up a small object like a pencil, only a few neurons in the biceps are signaled by the motor cortex and only a few myofibers respond. However, when picking up a large object like a piano, all neurons in the biceps are signaled and every myofiber participates, resulting in close to the maximum force the muscle can produce.