Surface electrodes on the body can capture the heart's electrical activity, creating an electrocardiogram (ECG or EKG).

A. Skeletal muscle fibers are activated by somatic motor neurons, while cardiac cells generate action potentials via pacemaker activity: Skeletal muscle fibers are innervated by somatic motor neurons, and each action potential originates from an external neural stimulus at the neuromuscular junction. Cardiac contractile cells, in contrast, can depolarize spontaneously due to pacemaker cells in the sinoatrial node, generating intrinsic action potentials that propagate through gap junctions without direct neural input.

B. Cardiac contractile cells require anaerobic metabolism, whereas skeletal fibers depend solely on aerobic metabolism: Both cardiac and skeletal muscle fibers primarily rely on aerobic metabolism to meet energy demands. Cardiac muscle has a high density of mitochondria for continuous aerobic ATP production, whereas skeletal muscle can use both aerobic and anaerobic pathways depending on activity intensity.

C. Cardiac cells require direct stimulation from motor neurons, while skeletal fibers generate their own action potentials: Cardiac contractile cells do not require direct neural stimulation; they depolarize via pacemaker activity and conduct impulses through the myocardium. Skeletal fibers, on the other hand, rely entirely on motor neuron input to initiate contraction and cannot generate spontaneous action potentials.

D. Skeletal muscle fibers have a long refractory period, unlike cardiac cells: The refractory period of cardiac contractile cells is much longer than that of skeletal muscle fibers. This prolonged refractory period prevents tetanic contractions in the heart, allowing sufficient time for filling between beats.

A. A 40-year-old person would have an expected maximum HR of 160 bpm: Maximum heart rate is estimated using the formula 220 minus age. For a 40-year-old, the expected maximum HR would be approximately 180 bpm, not 160 bpm. This formula provides a general guideline for exercise and stress testing.

B. Resting HR decreases from infancy to young adulthood: Resting heart rate is higher in infants (typically 120–160 bpm) due to higher metabolic demands and smaller stroke volume. As the cardiovascular system matures and stroke volume increases, resting HR gradually decreases through childhood into young adulthood, stabilizing around 60–100 bpm in healthy adults.

C. Maximum HR is independent of age and remains around 220 bpm throughout life: Maximum heart rate declines with age due to physiological changes in the sinoatrial node and cardiac responsiveness to sympathetic stimulation. It is not constant at 220 bpm; instead, it decreases roughly 1 bpm per year after adolescence.

D. Newborns typically have a resting HR of around 75 bpm: Newborns normally have a resting HR between 120–160 bpm due to their high metabolic rate and limited stroke volume. A HR of 75 bpm in a newborn would be abnormally low and may indicate bradycardia or cardiac compromise.