How can they be both strapless and wrist-worn?

Thank you for seeking clarification. I meant chest-strapless and ONLY wrist-worn. I apologize for the confusion.

Here are some assessments of wrist-worn monitors:

Article Accuracy of Heart Rate Watches: Implications for Weight Management

Article Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart R...

Article Clinical Evaluation of the Measurement Performance of the Ph...

Here are some additional assessments of the heart rate monitor and other features of the Philips HealthWatch:

http://www.trustedreviews.com/reviews/philips-health-watch

https://www.goodhousekeeping.co.uk/institute/product-reviews/activity-trackers/philips-health-watch

https://www.pcmag.com/review/351666/philips-health-watch

The HealthWatch is licensed as a medical device:

https://www.accessdata.fda.gov/scrIpts/cdrh/cfdocs/cfRL/rl.cfm?lid=488751&lpcd=ISD

More athletically-minded users have reasons for dissatisfaction with it, but its heart rate monitor is very accurate, it has 27/7 heart rate monitoring and can pick up on sleep passively (which is an important feature for studies—though both features work better with activation during workouts or sleep sessions), and it is significantly less expensive than many other trackers with similarly strong heart-rate monitoring.

thank you kindly for your response and time. I appreciate your expertise and input.

This is not in direct response to your question, but you might find this new review useful:

Article Accelerating Accelerometer Research in Aging