Heart rate monitors that allow users to monitor their level of activity during bouts of gym exercise are frequently inaccurate by as much as 20% a study has revealed.

Gym equipment, including treadmills, exercise bikes, cross trainers, steppers and elliptical trainers allow users to hold handles that then reads their heart rate and as a result calculates their exercise stress. Machines also display calories burnt and distances covered.

However, inaccurancy or miscalibration in these machines pose a danger to all users looking to improve or maintain fitness, especially those with heart conditions, the elderly and pregnant women.

A discrepency of 20% in heart rate readings can give users an inaccurate picture of how hard their hearts is working. For example, a treadmill that shows a 50 year old a heart rate reading of 110 beats per minute (bpm) suggests that s/he is working at approx 65% of their Age Predicted Maximum Heart Rate (APMHR). This should be a comfortable and reasonable level of exercise for someone of that age and allow them to improve their fitness at a comfortable rate.

However, a 20% level of inaccuracy may mean this same exerciser is actually working at 144 bpm, equalling 85% of APMHR - a level which could cause serious health issues for someone who is either inexperienced in exercise or someone who has a history of heart problems.

The most accurate way of measuring your heart rate is not by using the machine handles to measure heart rate - taking the pulse rate from your hands isn't an accurate way of measuring the load and stress of the heart anyway - is to wear a heart rate monitor strap that sits against your chest.

Also, many exercise machines over calculate the number of calories a user is burning during a given bout of exercise. Exaggertaing the number of calories burnt during exercise poses a danger as the user may feel like they have got more room for maneuver when it comes to eating a little more. In actual fact if their bout of exercise didn't burn as many calories as suggested by the machine, this may mean that eating a little extra may see the exerciser add weight rather than lower or maintain their weight.