This study utilizes a crossover design, with participants acting as their own controls
and participating in both the control (gradual uncompensable heat) and experimental
(rapid heat stress (RHS)) conditions. The independent variable is the type of heat stress
(uncompensable or rapid). The dependent variables are: peak concentric muscle torque (hip
abductors, hip adductors, knee flexors, knee extensors), dynamic balance (Y-Balance Test,
Firefighter-Specific Functional-Balance Test (FFSFBT)), and frontal plane movement
quality (Forward-Step-Down Test (FSDT)). These will be measured three times, once during
pre-testing and twice during post-testing as described below.
The study will consist of four visits. Visit one will be the informed consent and
familiarization to the outcome measures. Participants will be educated on filling out the
24-hour food/drink diary, and asked to standardize their diet as close as possible to
this record the 24 hours before visits 2, 3, and 4. Injury history and demographic data
will be collected at visit 1.
Visit two will be the pre-test data collection, which will occur at approximately 22°C.
Participants will complete all outcome measures (torque, balance, movement quality
measures). Body composition will also be measured in the BodPod at this time.
Before visits 2, 3, and 4, participants will follow the safety guidelines outlined below
to ensure proper hydration:
48 hours before: no heat stress (will contact participant to determine if they have
been called out to a live fire. Data collection will be postponed if so)
24 hours before: no alcohol consumption, no energy drink consumption (normal
coffee/tea is allowed)
24 hours before: no physical activity (exercise or vigorous play)
24 hours before: drink 3.7L of total water to make sure to be well-hydrated
2 hours before: eat last meal
Visits three and four will consist of a steady-state treadmill protocol inside the
environmental chamber (37-38°C, 50% humidity) followed by post-test measures. During
either session three or four, the participant will don his/her firefighting gear (jacket,
pants, hood, boots, gloves) and perform the treadmill test with the gear on. This is the
RHS trial and will be denoted GEAR. In the other session, the participant will wear
exercise clothes (shirt and shorts) plus a weighted backpack to replicate the weight of
the gear. This is the gradual uncompensable heat stress trial and will be denoted NOGEAR.
The order of the GEAR and NOGEAR sessions will be randomized and counterbalanced to
prevent crossover effects.
Upon arrival for the pre-testing visit (session two), participants will be measured for
height using a stadiometer and weight. Body composition will be measured once with the
BodPod (Cosmed, USA). They will warm up for five minutes on an exercise bike or treadmill
then proceed with taking the outcome measures.
Upon arrival for visits three and four, participants will be weighed again. A urine
sample will be collected to assess hydration status. They will swallow the core
temperature (Tc) capsule 40-45 minutes prior to the start of the session using a small
amount of 20°C water, with no further water ingestion throughout the protocol to avoid
artificially lowering Tc. Participants will be fit with the Equivital EQLife Monitor
(Equivital, NY) system to record heartrate (HR) and Tc. They will then perform the
treadmill protocol, and post-test measures will be completed immediately following the
cessation of the treadmill test with removal of their gear. They will be weighed
following exercise to determine changes in body weight due to sweating.
Sessions three and four will be separated by at least seven days to reduce the influence
of heat acclimatization and fatigue. Continuous monitoring of Tc and HR will help ensure
participant safety.
The treadmill exercise protocol begins with a graded approach to reach aerobic steady
state. The treadmill protocol includes an initial five-minute stage at 3 miles per hour
and a 0% grade, followed by increases in percent grade of 4% at minute 5 and 8% at minute
10 and continued throughout the test. Following the final planned incremental increase in
treadmill incline, the incline will then be adjusted to ensure that the subject remains
at steady-state for the duration of the protocol, defined by a respiratory exchange ratio
between 0.9-0.99 using the lab's metabolic cart (Parvomedics, USA). HR, rating of
perceived exertion (RPE), thermal comfort, and thermal sensation will be recorded at each
0.5°C interval. The treadmill test will be stopped when participants either reach a Tc of
39°C or a volitional maximum (participant is unable to continue exercise). The
investigators will also halt the test if signs of excessive heat stress are present
(unsteady gait, confusion, dizziness, slurred speech, etc.) Should the participant not
meet either termination criteria, there will be a 50 minute time limit for the treadmill
protocol for the GEAR condition and 2 hour limit for the NOGEAR condition. Both GEAR and
NOGEAR sessions will begin with the same treadmill test.
Following the treadmill test, the participants will complete the post-test outcome
measures. These consist of isokinetic torque of the hip abductors and adductors,
isokinetic torque of the knee flexors and extensors, the Y-Balance Test, the FFSFBT, and
FSDT. Participants will doff their gear immediately following cession of the treadmill
test. For unilateral tests, the dominant leg will be recorded and used for these tests;
this is the leg used to kick a ball. Outcome measure order will be standardized for each
participant with testing order based on primary outcomes and difficulty of each test. The
FSDT will be performed first, as this is the primary outcome and previous research showed
that recovery after a maximal exertion stimulus happens quickly. The FFSFBT will be
performed next, followed by the Y-Balance Test. Isokinetic testing will be last due to
the fatiguing nature of this testing. Hip muscles will be tested first, followed by thigh
muscles. Three minutes will be allotted for rest and instruction between each outcome
measure. Five minutes of rest will be given between isokinetic tests to allow for machine
set-up and instructions. One minute of rest will be given between each isokinetic speed,
with three speeds tested for the hip (30°, 60°, and 120° per second) and knee (60°, 180°,
and 300° per second) muscles.
Participants will be followed for 2 years, with 6-month follow-up calls to track injury
status. Details on type of injury, location of injury, and time lost to work will be
collected.