Heart rate (HR) training is an easy way to ensure that your weekly training is structured and effective, but during 12 years that we have been coaching endurance athletes, we have gained a great deal of experience and insight which we'd like to share with you. Heart rate monitors can be great training tools, but there are some significant limitations.
Benefits of Heart Rate Training
1. HR monitors are relatively cheap to purchase and easy to use.
2. They can potentially provide an accurate gauge to how hard your body is working at 'aerobic' intensities (longer and steady training sessions). We use 5 zones for training with zone 4 being threshold and 5 above threshold. Heart rate training is reliable for zones 1-3 only.
Limitations of Heart Rate Training
1. Heart rate monitors become unreliable when training at intensities which equate to threshold zone or above (4 & 5 as above). If you are completing a high intensity interval session, you will struggle to reach your target heart rate. It doesn't mean you are not working at the correct intensity, it's just that your HR is slow to respond and rise.
This often leads to athletes pushing too hard, despite the intensity being 'correct'. They make the simple assumption that "HR is too low so I need to push harder" when in fact they are already at the right intensity.
If you are training close to threshold or above, it's generally better to ignore your heart rate monitor and use perceived effort or an alternate guide such as 'pace' for running (know your track lap times or speeds) or 'power' for cycling (use a cycle power meter or a turbo with power function).
2. Average heart rate is a pointless figure, time in zone is the key. If your training schedule requires you to exercise between 120-130 beats per minute (bpm), you should never be below the lower limit of above the upper limit. If you spend half your time at 150 bpm and the other half at 100 bpm, you will effectively spend no time within the 'zone' but your average will be 125 bpm.
Flat line your heart rate and maintain a 'constant' heart rate, slow down on uphill sections and go harder on downhill sections, this is a real skill. If your heart rate is constantly rising and dropping as you ride or run, the aerobic training quality is very poor for various reasons. If your heart rate tends to 'spike' or 'fluctuate' a lot in training or racing, this uses a lot of energy and you will become tired quickly.
3. If you are tired from training the previous day, or tired from work, family and other pressures, heart rate will always be lower than normal. This can often cause problems as athletes 'fight' to push the HR into correct zone. As a consequence, despite being tired, they train at an intensity which is too high. Your HR is a 'guide' only and sometimes it should be ignored, if you know it's wrong. If it feels like zone 4... then it's zone 4, even if the HR shows zone 3.
4. Heart rate will be higher in hot weather and lower in cold weather, hence during the winter, your heart rate may well be lower than summer.
5. Heart rate is also lower in the morning than during the evening, so a cold winter morning may lead to a heart rate significantly lower than a warm summer evening.
6. 'Cardiac drift' explains the fact that HR does not stay stable if you run or cycle at one pace. You may exercise for 40 minutes and your running speed may not change but HR will progressively rise throughout the workout. This is caused by by fatigue and also by a rise in body temperature (your heart beats a little faster to pump blood to the skin, helping you to cool down). The 'intensity' has not changed, so this drift in heart rate is allowed.
7. We mentioned earlier than fatigue can led to drop in heart rate. it is common for athletes doing long distance events such as Ironman triathlon or ultra running to see a progressive drop in heart rate during the race. This is a common 'fatigue response' and if you try to push harder and maintain your pre-set zone, this may result in you running / riding too hard.
8. Cycling on the turbo trainer will always produce a lower heart rate than cycling at a similar intensity on the road. For this reason any interval sessions conducted indoors on the turbo may need an adjustment of zones (drop them all 5 beats as a start point).
9. Cycling cadence and to some extent, running cadence have a real impact upon HR. If you push big gears with a slow cadence, heart rate will be lower compared to spinning faster with an easier gear (even if the power output / speed is the same). Running with a faster cadence, as opposed to longer and slower strides has the same response.
10. Tapering and event day adrenaline all lead to higher heart rates than normally seen for any given intensity. For this reason using heart rate (specifically in the early stages) as a race intensity guide can be misleading and needs to be accounted for. You may find that compared to training, your HR is 5-10 beats higher at the start of your event, compared to the same pace in your previous training sessions.