Dan Heil

Stride rate recommendations for moderate-intensity walking.

UNLABELLED Current physical activity guidelines recommend physical activity of at least moderate intensity to gain health benefits. Previous studies have recommended a moderate-intensity walking cadence of 100 steps per minute for adults, but the influence of height or stride length has not been investigated. PURPOSE the purpose of the current study was to determine the role of height and stride length in moderate-intensity walking cadence in adults. METHODS seventy-five adults completed three treadmill walking trials and three overground walking trials at slow, medium, and fast walking speeds while V˙O2 was measured using indirect calorimetry. Five stride length-related variables were also measured. RESULTS mixed model regression analysis demonstrated that height explained as much variability in walking intensity at a given cadence as did two different measures of leg length and two different stride length tests. CONCLUSIONS the previous general recommendations of 100 steps per minute were supported for use where a simple public health message is needed. Depending on height, moderate-intensity walking cadence can vary by more than 20 steps per minute, from 90 to 113 steps per minute for adults 198 to 152 cm tall, respectively. Height should therefore be taken into consideration for more precise evaluation or prescription of walking cadence in adults to provide health benefits.

CHANGES IN SALIVARY IGA DURING ARDUOUS WILDFIRE SUPPRESSION RELATIVE TO WORK SHIFT LENGTH

PURPOSE: The purpose of this study was to compare the reduction in salivary IgA in response to a 14 and 21 hour work shift. METHODS: Seventeen (15M, 2F) wildland firefighters (WLFF) from two Interagency Hotshot crews served as subjects. Subject groups were studied in response to a short or long work shift (13.9±0.7 and 21.4±0.3 hrs). Work shift energy expenditure (EE) was estimated using the method established by Heil (MSSE 33(5):s168, 2001). Unstimulated saliva (4 min) was collected prior to work at 0530 and immediately post shift. Salivary IgA secretion rate (mg.min-1) was calculated from an ELISA assay procedure. RESULTS: Work shift EE was significantly higher (p<0.05) for the long work shift (4101±578 and 3322±478 kcals for the long and short work shift, respectively). Crew (work shift hrs) Pre Post Next AM 13.9±.7 hr (n=6) 76.9±23.1 14.0±8.0* 82.4±55.9 21.4±.3 hr (n=11) 54.1±30.3 24.7±26.5* 40.3±29.5† Values are salivary IgA (mg.min-1), * p<0.05 vs. pre, †p<0.05 vs. the 13.9 hr work shift group Salivary IgA was significantly decreased post work shift yet returned to pre shift values by the following AM in both groups. The 14 hr work shift group showed a recovery in sIgA (AM values at days 5 and 6 were 85.8±40.5 and 67.4±32.0 mg.min-1, respectively). In contrast, the 21 hr work shift group showed suppressed values for sIgA six days after the extended shift (27.3±13.3 mg.min-1). CONCLUSIONS: These data indicate that the sIgA secretion rate is significantly depressed following a single day of wildfire suppression. Work shift EE and/or duration may impair recovery and increase the risk for upper respiratory infection during extended operations. Introduction Recently our laboratory has demonstrated that the tee associated with wildfire suppression may exceed 6000 kcal/day (26.4 MJ/day) resulting from consistent work outputs and extended work shifts (Ruby et. Al, 2002). Forest fire suppression involves arduous work (average = 7.5 kcal/min) for prolonged periods (12 to 24 hours) in difficult environmental conditions (heat, altitude, smoke). The job routinely includes packing heavy loads, building fireline with hand tools, and emergency responses, often under arduous and dangerous field conditions. Firefighters perform their duties while wearing personal protective gear that balances the needs for protection, performance, and mobility. In busy fire seasons, firefighters may work as many as 21 days without relief, with meals provided by field rations, a remote fire camp, or an organized camp with full field kitchen. When combined with pre and post-season work on prescribed fires, firefighters may log over 100 days of work in a season and over 1000 hours of overtime. There is also an element of real danger as demonstrated by the 1994 Storm King Mountain fire in Glenwood Springs, CO, where 14 firefighters were killed. An additional 4 firefighers were killed during the 2001 fire season in Washington state. Medical units in large fire camps treat a number of injuries and medical emergencies. Although the work predictably leads to numerous cases of dehydration, upper respiratory tract irritation and infection is the most common complaint during extended wildfire assignments. Previous research has associated extended work with a suppression in the salivary protein salivary immunoglobulin A (sIgA) in response to running (Neiman, et. Al, 2002) and physical training (Gleeson, 2000). Previous research has also demonstrated that dietary intervention (supplemental liquid carbohydrate) may attenuate immunosuppression as measured by sIgA (Bishop et. Al, 2000). Purpose The purpose of this study was to determine the transitory changes in the secretion rate of sIgA (μg.min-1) in response to arduous wildfire suppression efforts. As a secondary purpose, the HR response to a standardized work bout was also evaluated to determine its association with fatigue.

Fitness, Workrates And Fatigue During Arduous Wildfire Suppression

PURPOSE: The purpose of this study was to investigate work and recovery rates in wildland firefighters of differing levels of sustainable aerobic fitness. METHODS: Eleven (9M, 2F) wildland firefighters (WLFF) from one Interagency Hotshot crew served as subjects. Subjects were grouped by levels of sustainable aerobic fitness (SAF) as measured by VO2 at the ventilatory threshold and studied in response to work rates and recovery after single or multiple arduous days of wildland fire suppression. Work shift energy expenditure (EE) was estimated using the method established by Heil (MSSE 33(5):s168, 2001). Recovery rates were measured using a submaximal HR step test index. RESULTS: * p<0.05 between groups. SAF EE EE ∆HR Index ∆HR Index (ml•kg-1 • min-1) Kcal • day-1 Kcal • day-1 • kg-1 1 arduous day 3 arduous days Low SF 34.6±3.5 2735±199* 39.7±3.8 5.1±18.0 37.3±14.7* High SF 43.7±3.9 3425±354 44.6±2.1 10.6±18.7 16.1±21.0 CONCLUSIONS: Wildland firefighters with higher SAF (SAF range=40-49 ml • min • kg) are able to do more absolute and relative work during each day of a duty cycle compared to lower fit firefighters (SAF range=30-39 ml • kg • min). The more fit individuals recovered more quickly after multiple days of arduous work in spite of higher workloads. Since the lower fit group met or exceeded the minimal job specific fitness requirements for wildland firefighters as determined by the arduous pack test (SAF > 23-25 ml•kg•min) these data suggest that the current fitness requirements may need review in that elevated sustainable fitness improves work output and recovery. Supported by the U.S. Forest Service Missoula Technology and Development Center.

Liquid and Solid Carbohydrate Feedings Increase Self-Selected Work Rates During Arduous Wildfire Suppression

Our laboratory has demonstrated that the total energy expenditure of extended wildfire suppression may exceed 26 MJ.d (MSSE 34(6):1048-54, 2002). Moreover, the majority of the total daily energy intake is provided before and after the extended workshift and supplemented with a mid-day sack lunch. PURPOSE: To determine the effects of supplemental carbohydrate (CHO) feedings on self-selected work rates during extended arduous wildfire suppression. METHODS: Subjects included wildland firefighters (N=20 Hotshots) during two wildfires in ID and OR. Subjects consumed a CHO (200 ml.hr, 20% maltodextrin (40g.hr)) or placebo (PLA) drink in a counter-balanced crossover design and were allowed ad lib water intake during the day. Blood samples were collected at two-hour intervals with automated glucometers. Self-selected hourly work rate was determined using CSA accelerometer count data. Data were analyzed using a priori planned comparisons across trials. RESULTS: During the 20% CHO trials, blood glucose (mM) was significantly higher (p<0.05) immediately prior to lunch (CHO=5.9±1, PLA=5.4±0.9) and at four (CHO=6.7±1.1, PLA =5.6±.8) and six hours (CHO=6.2±1.0, PLA=5.0±0.6) post lunch, and immediately post shift (CHO=5.8±1.3, PLA=4.7±0.6). Self-selected work rate was similar across trails during the initial hours post breakfast but were significantly different later in the morning (mean 4-6 hour post breakfast: CHO=58,088±15,730, PLA=40,191±20,070 counts.hr). Work rate was also similar across trials during the early hours after lunch but was significantly different later in the afternoon (mean 4-6 hour post lunch: CHO=64,172±30,501, PLA=47,528±26,541 counts.hr). CONCLUSIONS: Self-selected work rate is higher during arduous wildfire suppression when CHO is delivered at a rate of 40 g.hr (200 ml.hr, 20% solution). Supported by the U.S. Forest Service – Missoula Technology and Development Center.