Relationship of Daily Work Output to Salivary IgA and Fatigue
Steven E. Gaskill and Brent C. Ruby, Human Performance Lab, University of Montana
Relative work per day
Salivary IgA
Fatigue Index
40%
20%
-5,000
0%
-4,000
-20%
-3,000
-40%
-2,000
-60%
-1,000
0
am10
pm9
am9
pm8
am8
pm7
am7
pm6
am6
pm5
am5
pm4
am4
pm3
am3
pm2
am2
pm1
am1
-80%
Daily Energy Expenditure of Activity (Kcal)
% Change in sIgA and Fatigue Index
60%
Day into Duty Cycle
Figure 1: Percent Change in Salivary IgA and Fatigue Index following days of variable arduous work/
• Red Bars represent relative work during each work shift. Bars at the height of the red line represent about 4,000 Kcal of activity energy expenditure.
• Light blue line represents the percent change in the fatigue index (sum of resting heart rate, 1 minute submaximal step test heart rate and 30 second
recovery heart rate).
• Green line represents change in salivary IgA (an immunogobulin related to upper respiratory tract infection). SIgA was measured in mg/ml.
NOTE: Fatigue and saliva samples were collected in the morning before breakfast and again post-12 to 15 hour shift. Values close to 0% change reflect
recovery and larger absolute values represent fatigue (fatigue index) or immune function suppression (larger negative values in sIgA).
Health during arduous and long duration work during wildland firefighting is important for the efficacy of crews. In
this study we monitored a group of 11 elite (hotshot crew members) wildland firefighters during the first 10 days of a 14 day
duty cycle. Average caloric activity output was about 3,800 Kcal per day with activities including fire-line construction in
steep mountainous terrain, chainsaw work, brushing, mop-up duties and two days of lighter duties (days 3 and 4). Actical
activity monitors were used to measure daily work which is represented by the red bars. Bars reaching the center red line
represent 4,000 kcal of daily activity energy expenditure (right hand scale).
Salivary IgA was depressed at the end of each workshift and recovered, partially or fully, following 10-12 hours of
sleep and inactivity before the next morning. Note that following moderate and light work shifts of less than 3,000 kcal (days
3, 4, and 5), immune function recovery reached baseline values. Following days of heavy work, greater than 3,000 Kcal
(days 1, 2, 6, 7 and 8), recovery was incomplete and by the 10th morning, even following a moderate workday, immune
function remained depressed.
The fatigue index was calculated as the sum of heart rates collected after five minutes of rest, following 1 minute of
stepping on an 8 inch (20cm) bench at a cadence of 120 beats/min. and following 30 seconds of recovery post stepping. The
fatigue index mirrored (Pearson product correlation between sIga and Fatigue Index r =-0.8189) the sIgA values showing a
similar pattern of recovery to baseline following easy and moderate workdays, but not following sustained periods of multiple
hard days of work.
These data suggest the wildland firefighters can sustain multiple days of arduous activity, but require lighter activity
to fully recovery immune function and reduce fatigue. Further, the fatigue index test, using bench stepping, is a good indicator
of fatigue and suppressed immune function.