Final answer:
The R-R interval is used to calculate heart rate because it represents the time between two consecutive R waves on an ECG tracing.
Step-by-step explanation:
The R-R interval is used to calculate heart rate in this practical because it represents the time between two consecutive R waves on an ECG tracing.
The R wave corresponds to the depolarization of the ventricles and is a clear and easily identifiable feature.
By measuring the time between consecutive R waves, we can determine the duration of one cardiac cycle and use it to calculate the heart rate.