If this is the case then what do you have to do to get to 188? Are you nervous about the workout or dealing with a lot of stress?
There is a method out there that is designed to effectively build your aerobic base. There are some mixed feelings about the efficacy of this but when I was running I felt like it really helped me.
The MAF Method
To find your maximum aerobic training heart rate, there are two important steps.
1. Subtract your age from 180.
2. Modify this number by selecting among the following categories the one that best matches your fitness and health profile:
a) If you have or are recovering from a major illness (heart disease, any operation or hospital stay, etc.) or are on any regular medication, subtract an additional 10.
b) If you are injured, have regressed in training or competition, get more than two colds or bouts of flu per year, have allergies or asthma, or if you have been inconsistent or are just getting back into training, subtract an additional 5.
c) If you have been training consistently (at least four times weekly) for up to two years without any of the problems in (a) and (b), keep the number (180–age) the same.
d) If you have been training for more than two years without any of the problems in (a) and (b), and have made progress in competition without injury, add 5.
For example, if you are 30 years old and fit into category (b), you get the following: 180–30=150. Then 150–5=145 beats per minute (bpm).
In this example, 145 must be the highest heart rate for all training. This allows you to most efficiently build an aerobic base.
You would then do rides up to your HR determined in the formula above. Not average HR, maximum HR. Over time you will see your ability increase to ride at much higher wattage for the same HR.
Once you have a more solid foundation of base, you should be able to get on the trainer and do 150 watts or perhaps even a much greater wattage for that same 120bpm.