Here are some ways if using heart rate to monitor training stress and recovery, from one of my old article: https://www.t-nation.com/training/top-10-tips-to-grow-bigger-stronger
A) Heart Rate Measures
A1. Morning (resting) heart rate: This is one of the simplest yet most straightforward ways of knowing if you're performing an excessive amount of high intensity training. Here's how to use this tool:
• You must take this measure first thing in the morning upon waking to avoid fluctuations due to daily activities.
• You must always take your measurement in the same position. If you start by taking your pulse lying down, then all of your measures must also be taken in this position because body position influences resting heart rate. Your RHR is 6-8 beats per minute higher sitting compared to lying down, and 2-4 BPM higher standing compared to sitting. So there can be up to a 12 BPM difference between taking your HR standing up or lying down.
• Measure your heart rate by placing your fingers on the carotid artery. Avoid applying too much pressure as this artificially reduces heart rate by activating the baroreceptors.
• Measure for a full 60 seconds, not 15 seconds multiplied by 4 (this quadruples the margin of error).
• If your RHR is elevated by 3-5 BPM above baseline, you have a slight overexcitation of the sympathetic nervous system, which might indicate incomplete recovery from high intensity work.
• If your RHR is elevated by more than 6 BPM above baseline, there's a significant neural overstrain due to excessive high intensity work.
• If there's a drastic reduction in RHR compared to your baseline, this indicates CNS inhibition which is a sign of type II (addisonic) overtraining. This rarely occurs with strength athletes though; it's typically due to an excessive volume of endurance work.
A2. Heart rate after cold immersion: This is another effective way of clueing us in on the status of our nervous system (specifically its excitability). It consists of comparing two different heart rates one after the other. You take the first one sitting down (relax for 1-2 minutes before measuring your pulse). Afterwards you dip your right hand up to the wrist into very cold water and keep it there for 45 seconds. You then immediately take the second measurement. Finally, calculate the difference between both.
A sympathetic nervous system with normal function should lead to an increase in 4-8 BPM under cold conditions. If the increase is above 10 BPM, it indicates sympathetic overactivity which might mean a potential basedowic overtraining state (too much high intensity work). If the increase is lower than 4 BPM this can indicate sympathetic inhibition, which could mean a potential addisonic overtraining state (too much volume).
A3. Post-workout heart rate: After a training session, heart rate should gradually go down toward baseline level. One hour after a workout, a 10-20% elevation compared to resting heart rate (RHR) is desired.
If the heart rate is more than 20% above the RHR, it indicates that the workload was excessively stressful on the body and nervous system. If the heart rate is elevated by less than 10% above the RHR, it means that the workload for the session was below the capacity of the body to tolerate training and that future workloads can be higher if maximum results are desired