The Polar OH1 armband optical heart rate sensor has in mind those that won't or can't properly use wrist heart rate monitor or electrical chest straps which many female athletes report being very uncomfortable to wear because chest straps either get tucked up under their sports bra or slide down.
Both males and females can definitely testify to the experience of a chest strap slipping down to the waistline becoming a belt!

In the end, most of us just suck it up and get used to the chest strap shenanigans as the accuracy of chest straps is second to none. Chest straps measure electrical signals of the heart like ECG medical devices that's why chest strap tracking devices tend to be more accurate.
With that said, alternative to chest straps like the mentioned Polar OH 1 are getting more accurate with their green LED light technology which measures blood pulsations instead of electrical signals of the heart like in the case of chest straps.
After using an armband optical heart rate sensor like the Polar OH 1, you may never go back to using chest straps. Fitting the band to the arm is definitely much easier than fitting the chest strap and it's more comfortable too.
The Polar OH 1 weighs only 12 grams and consists of an elastic armband and a standalone Bluetooth Low energy optical sensor that works very much like a chest strap but instead of wearing it on the chest you wear it on your upper or lower arm.
The built-in memory in the Polar OH1 optical sensor is large enough to save up to 200 hours of heart rate training.
What's neat about the Polar OH1 is that it offers two ways for handling vital data. You can either pair the OH1 to a smartphone to view data live or you can store the data in the internal memory of the OH1 itself to view later so you don't have to be tethered to a smartphone.
The back of the Polar OH1 has four metal contacts which are designed to make contact with the metal pins inside the OH1 charging cradle that comes included with the OH 1 optical sensor.
The OH 1 has no LCD screen so the way you communicate with it is via light signals. When you see the light blinking red three times it means you haven't turned it on. To turn on the OH1 optical sensor, press the button until the light comes on and wait for the green light to blink rapidly which tells you your heart rate has been detected by the OH1.
Once the OH1 sensor is powered on, you press the button again to switch on recording mode (the green light blinks twice). To stop the recording after your session is finished, hold down the button until the light switches off.
The stored data will be then synced automatically to your phone via bluetooth when you press the button again on the OH1 and your smartphone is within bluetooth range.
The Polar OH1 is also 30 meters waterproof like the Polar H10 chest strap that comes with Polar's wrist watches. If you plan on doing mostly swimming though, it's better to go for the H10 chest strap as it uses 5kHz frequency to transmit data so your heart rate data measurement will be up much more accurate under water.
Other than that, the Polar OH 1's software can also updated over the air (OTA) and its compatible with 3rd party apps as well as Polar's own apps like Polar Beat and FlowSync.
The FlowSync app can be installed on your computer or smartphone. The smartphone version of the FlowSync is downloaded via the Google Play Store or Apple Store.
The FlowSync desktop version is downloaded via the Internet browser of your computer by going to https://flow.polar.com/start and downloading either the Windows or Mac version for your computer.