As your muscles contract during activity, more blood is returned to the heart. which variable would be affected and what would be the outcome of this action?

Heart rate. From this site, the two channels of the autonomic nervous system originate the sympathetic and parasympathetic components. The sympathetic components increase heart rate by releasing the neural hormone catecholamines - epinephrine and norepinephrine. These hormones are cardio accelerators. Acceleration of the heart rate is called tachycardia.