Reaction time is the lapse of time between muscular movement and external stimuli

This statement is true. Reaction time is a skill-related fitness component that refers to the ability of the body to react to external stimuli and make quick decisions on what to do with those stimuli. A good example of this is when your hand accidentally touches the fire of a burning candle. Motor neurons send signals to the brain that trigger a response where you suddenly remove your hand from the source of pain. For exercise activities, reaction time is seen in athletes who are waiting for the gun start. 

Pe.dometer would be the most useful to you for monitoring heart rate.

God to do a cool down after working out.  hope this .      cheers!