Physiological and perceptual responses to load–carrying in female subjects using internal and external frame backpacks

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Abstract
Eleven female subjects (ages 18–33 years) walked on a motor–driven treadmill at 86 m/min for 1 h carrying 33% of their body weight in a backpack. The grade of the treadmill alternated every 15 min between 0 and 3%. Each subject carried an internal frame backpack for one trial and an external frame backpack for another trial on a separate day. The variables measured during the two load–carrying experiments included oxygen consumption (V˙O2), heart rate (HR), respiratory exchange ratio (R), minute ventilation V˙E , and the ratings of perceived exertion for the chest (RPE–chest), shoulders (RPE–shoulders), and legs(RPE–legs). When oxygen uptake measured during load–carrying was expressed as a percentage of V˙O2max, the average values were 40·1% (63·5% HRmax) at 0% grade and 49·0% (69·6% HRmax) at 3% grade for both backpacks. No significant differences were found between the two packs for any of the metabolic, cardiorespiratory, or perceptual variables measured. Changes in treadmill grade had a significant effect on V˙O2, HR, and V˙ E, regardless of the type of pack carried. Minute ventilation was the only physiological response to load–carrying that increased significantly as exercise duration increased. The values for RPE–chest, RPE–shoulders, and RPE–legs were significantly increased by both exercise time and treadmill slope, regardless of the type of pack frame carried. Thus despite relatively constant metabolic responses over time, increased perception of effort could compromise completion of the load–carrying task. It was concluded that differences in backpack frame design were not great enough to produce significant differences in the energy cost or perception of carrying a moderately heavy load on the back.