# Heart Rate Index as a Measure of Physical Workload in Chainsaw Operations
## 2 Materials and Methods
### Justification of Selected Wearables  
The reasons for using Garmin, Biostrap, and Whoop are primarily based on the following groups of parameters:  
As for users, all three devices have been used in practice for a long time to measure various parameters of exercise and exercise capacity. The devices are commercially manufactured, certified, and globally available to users. Their manufacturers provide full support and service to users. User interfaces are easy to use. As for measurement and data collection, all three devices used are, in principle, capable of measuring heart rate information with high accuracy both at rest and in motion; the combined use of green, red, and IR light sources is in the individual devices linked to their optimal placements. The Whoop and Garmin devices are placed on the wrist for measurement purposes; the Biostrap device is placed on the arm or forearm to potentially eliminate unwanted influences and measurement artifacts caused by grip, large grip, and dynamic wrist movements. The devices capture the heart rate signal at a relatively high sampling rate: Garmin HR 3 measures HR with two green LEDs 256/32 Hz sampling; WHOOP measures HR with five LEDs (three green, one red, and one infrared) 24/7 at a sampling rate of 100 times per second; and Biostrap measures HR with three green LEDs 24/7 at a sampling rate of 86 times per second. As for data collection, Whoop is known for advanced recovery and strain analysis, utilizing a red-light PPG sensor for improved accuracy during physical activity and varied conditions. Biostrap features a multi-sensor system with clinical-grade red and infrared light, prioritizing deeper tissue measurement and minimizing artifacts. Garmin HR 3 measures HR with two green LEDs with 32 Hz sampling during standard use and higher rates and 256 Hz during activity tracking. As for task suitability, these devices are wearable and minimally intrusive, ideal for extended observation in physically demanding work environments. As for data accessibility and versatility, each device supports cloud or app-based synchronization, enabling the collection of large datasets for subsequent analysis. Measured data from all three devices are time-coded globally for synchronization and possible further processing. Biostrap and Whoop applications allow data export via APIs and provide SDKs for future user application development. The individual work processing phases mentioned in our study are Preparation, Cutting, Chopping, Manual Handling, and Other activities. The specifications of the activities are Preparation--preparation of the working tools and machines, moving of the work stuff, preparation of the workplace; Cutting--cutting activities; Chopping--chopping of the wooden part to cut the part of the wood or managing the safety of the cutting; Manual Handling--manual handling of the wooden part to support tree-cutting activities or maintain the safety of the space; Other activities--other non-specified activities related to the working activities such as dressing/ undressing, safety activities, safety device checks, etc. Based on international recommendations for ergonomic and non-ergonomic working positions [26; 36], we used the following definition for the non-ergonomic position:  
For the trunk position, the forward tilt was greater than 60 degrees, the tilt was un-supported, and the tilt or rotation of the trunk was greater than 20 degrees; for the head and neck, the forward tilt of the head was greater than 25 degrees; the head tilt, tilt, and rotation were greater than 15 degrees; for the lower limbs, hips, knees, or ankles were in extreme positions (e.g., kneeling); and for the hands; it was the rotation of the forearm. All other working positions were, for use in this case study, considered ergonomically accepted. This evaluation aimed to monitor the most critical working positions, workload, and critical points for increased workload that is caused by working positions. We did not focuse on the ergonomic evaluation of the working position but on the load parameters.