(36) APPLICATION OF A MODIFIED ETRIMP TO DETERMINE INTERNAL LOAD RESPONSE TO FIRE TONE CALLS IN FIREFIGHTERS

Introduction: Heart rate (HR) has been used to quantify the internal load, or physiological response, in sport-athletes and more recently to a task in the firefighter population where there is a risk for overexertion-based injury.  Edward’s training impulse (ETRIMP) is a measure that utilizes time spent in various HR intensity zones up to 100% of age predicted maximum heart rate (MHR) to determine the internal load of a task.  In the firefighter population, however, a fire tone call (e.g., fires [rubbish, garage, or structural] and auto extrication) may result in an internal response exceeding 100% of age-predicted MHR due to the intensity of the tasks.  Thus, the traditional ETRIMP may underestimate the internal load of a fire tone call in a firefighter. 

Purpose: To compare the internal load from the traditional ETRIMP to a modified version (ETRIMP_MOD) that accounts for time spent above 100% MHR in fire tone calls. 

Methods: 33 active-duty firefighters (29 males, 4 females; 36.5 ± 9.1 yrs, 179.9 ± 7.0 cm, 91.4 ± 12.8 kg) participated in this study.  Each participant wore a remote physiological chest-strap sensor to record heart rate for an entire 24-hour shift.  Department call logs were used to determine the time for each fire tone call and corresponding time-stamped HR data was extracted to post-hoc determine the total time (TIME) in minutes (mins) during the fire call spent in 50-59% (TIME₅₀) maximal HR (HR_MAX), 60-69%HRmax (TIME₆₀), 70-79% HR_MAX (TIME₇₀), 80-89%HR_MAX (TIME₈₀), and ≥90%HR_MAX (TIME₉₀₊) zones for ETRIMP (AU).  For ETRIMP_MOD, the 5^th HR_MAX zone was adjusted to reflect 90-99%HR_MAX (TIME₉₀) and a 6^th HR_MAX zone was created to reflect ≥100%HR_MAX (TIME₁₀₀₊).  MHR was estimated using Tanaka’s age-based prediction equation. A paired t-test examined for differences between the two internal load measures (Arbitrary Units; AU) with an alpha of p < 0.05 representing statistical significance.  

Results: ETRIMP_MOD (38.5 ± 35.9 AU) was significantly (p=0.04) greater that ETRIMP (34.3 ± 27.6 AU).  The average fire suppression call time was 15.7 ± 2.6 mins and the time spent in each ETRIMP zone was TIME₅₀ (5.3 ± 3.2 mins), TIME₆₀ (4.0 ± 3.1 mins), TIME₇₀ (2.8 ± 2.4 mins), TIME₈₀ (1.7 ± 2.1 mins), and TIME₉₀₊ (1.9 ± 2.2 mins).  For ETRIMP_MOD, the only differences were for TIME₉₀ (1.2 ± 2.2 mins) and TIME₁₀₀₊ (0.7 ± 2.2 mins). 

Conclusions:   The traditional ETRIMP may underestimate the internal load in a firefighter when responding to a fire tone call.  Although the absolute time spent performing work at ≥100%HR_MAX in this study is approximately 1 minute, that is nearly 1 minute spent performing work at a supramaximal intensity.  Future research should explore ETRIMP_MOD in medical service calls, as well as structural fire suppression tasks only.  PRACTICAL APPLICATION: The relative intensity for the work done in fire tone calls can exceed 100% of age predicted MHR and accounting for the time spent in performing work ≥100%HRmax may be important to fully characterize the internal load response in firefighters.  Further, recognizing the time demands in the upper internal load zones could help inform the development of training programs aimed at firefighter performance, as well as recovery strategies, to optimize cardiovascular health and minimize post-call cardiac injuries.

Acknowledgements: None