there are few sources that could help such as

The effects of progressive relaxation training on the disruptive behavior of a boy with autism

Article The effects of progressive relaxation training on disruptive...

In addition, theoretically, relaxation is the opposit to stress level...so if you used stress measures,,,then low score could reflext high relaxation.

1. decreasing heart rate

2. decrease standard deviation of pulse rate

Measurement of cortisol levels and changes in heart rate and blood pressure

Need take different between acute and chronic stress. The change of hormones level more typical of acute, change of autonomic nerve system typical of chronic stress. Data also suggested the measurements depend on the nature of the investigation.

Dear Dr. Ivan,

Thank you for your answer.

We are already using the HRV measures as indicator of relaxation along with galvanic skin resistance. We anted to know if there are any validated subjective tools to assess relaxation (such as questionnaires).

Thanks again,

Apar