1. Placement
When putting on your HR strap, you want to make sure the logo on the front is facing up and the HR electrode sensors (located to the right and left of the head unit w/logo) are directly on the skin. You don't want to ever put the HR monitor on over clothes. The HR monitor chest strap should go right across your xiphoid process in your sternum (see below), making sure the sensors are across the bottom part of your pectoralis (chest) muscle.
The HR monitor reads electrical signals from your heart through muscle tissue. For women, the HR strap should usually be right under the elastic band of a sports bra.That being said, if you are extremely petite and have trouble keeping the sensors flush against the skin, you can rotate the strap to the back to be placed over the lungs. Sometimes shifting the strap a little up or down, or to the left or right will work as well, if you're having issues picking up the signal.
2. Signal Conduction
When putting on your HR monitor, you may have some trouble picking up the signal if a dry chest strap is placed on dry skin. After a few minutes of exercise, you should have a little bit of sweat forming, which will increase your HR signal for picking up by a monitoring device (Smart Phone/Watch) but most of us don't want to wait for data and want HR info as soon as you start your workout.
You can do one of several options for increasing the signal strength for immediate pairing to your phone/watch.
Electrode Gel - this is the preferred method for Cardiac and Sports Lab testing (VO2/Blood Lactate analysis). You can usually find a bottle of gel online or at a running store for $3-5 and just a drop on each sensor will make HR signal pick up instant. Also, this gel can be a little cold, so feel free to place near a heating vent or on a sunny windowsill to warm up before placing directly on the skin. Example: Spectra 360 Electrode Conductivity Gelo
Water - Just a few droplets of water from a water bottle or sink onto each sensor will work almost as well as gel and is free.
The Licking Method - If you don't have any water or gel handy, you can lick the sensors of your monitors. This is somewhat gross, but Wahooligans are pretty hardcore.
3. Synthetic/Dry Fit Clothing
If you train in cotton shirts, this shouldn't be an issue, but some synthetic/dry fit type training clothing can build up static electricity and cause interference with HR signal conduction and is usually only an issue when the strap/skin is dry. Adding gel or water will alleviate this problem immediately.
4. Batteries
Most HR straps these days are operated by a coin cell battery and can be replaced by using a coin to open the back of the monitor. Usually you should be able to get about a year of training out of each battery, but if you start seeing erratic numbers or have trouble pairing to your device and you've already tried adding moisture/gel, this should be the next thing you check.
5. Resting Heart Rate
Your resting heart rate is an excellent indicator of your cardiovascular fitness level. As your cardiovascular system adapts to exercise and your fitness level increases, your resting heart rate will begin to drop. This is, in part, due to your heart enlarging as a response to training. A larger, stronger heart will pump more oxygenated blood with every beat. Which means it will need to beat less often to get the amount of oxygen needed by muscle cells for exercise. The best way to measure your resting heart rate is to put your heart rate monitor on first thing in the morning just after waking up, but before getting out of bed. Relax for a few minutes (~5min) in a horizontal position and note the lowest reading. The normal range for a healthy cardiovascular system is a resting HR of 60-90bpm.
Athletes will usually see resting HR values below 60 and elite endurance athletes can see resting heart rates as low as 35-40bpm. Once you have established a normal baseline resting HR value for yourself, and you notice one morning that your resting heart rate is five to eight beats higher than normal, you should avoid any high intensity training that day and stick to an easy run or ride. If it is more than 8-10 beats higher than baseline then take the day off of any cardiovascular training
An elevated resting heart rate is usually strong indicator that you may be over trained or at least haven't recovered from previous training sessions. An elevated resting HR could also mean that you are coming down with an illness or your body is fighting off an infection. Regardless of the cause, your body is telling you that you need rest.
6. Dehydration
If you are dehydrated, the amount of blood circulating through your body decreases. Your heart will try to compensate by beating faster, increasing your heart rate giving you a different result than normal.
7. Heart Rate Variability
Using your heart rate for training is a great option for biofeedback, as opposed to using something absolute like power, which is the same number, every day, regardless of environmental conditions or how you're feeling that day. HR can vary based on many factors and will fluctuate sometimes. The temperature/weather, the terrain you train on, the time of day and a number of other factors will affect your heart rate. Use it as a tool to give you feedback about what your heart and cardiovascular system is doing.