Final answer:
The Arnold study most commonly repeated the use of the oxygen saturation monitor or pulse oximeter to measure blood oxygen levels and determine metabolic rate.
Step-by-step explanation:
The most common repeat monitor in the Arnold study was the oxygen saturation monitor, commonly referred to as a pulse oximeter. This device is particularly known for its role in measuring the amount of oxygen in the blood, which can be used to determine a person's metabolic rate. The metabolic rate indicates the rate at which food energy is converted to another form, and these measurements can provide valuable information regarding a person's athletic conditioning and highlight potential medical issues.