Typically there is a lag in heart rate response meaning once you start working at an effort your heart rate takes some time to match that effort.  If the repetitions you are doing are short then your heart rate may not come up to the specified target range in time.  Do not panic, this is ok!  On repetitions that are short the primary objective is to stimulate a nervous system response (train your nervous system and muscular system to move at that speed)

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