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To the Editor

Brown, Peter1; McConnell, Alison2

Journal of Strength and Conditioning Research: March 2013 - Volume 27 - Issue 3 - p E1
doi: 10.1519/JSC.0b013e31825b7a88
Manuscript Clarification

1Department of Sport and Exercise Derby University, Derby, United Kingdom

2School of Sport and Education, Center for Sports Medicine and Human Performance Brunel University, London, United Kingdom

We read with interest the recent review article in this journal entitled “A Systematic Review of the Effects of Physical Training on Load Carriage Performance” by Knapik et al. (4). This article provides an excellent review of training methods used to improve performance while carrying a heavy load within a backpack. Within this review, physical training was defined as a routine and systematic set of physical movements (i.e., exercise) designed to improve one or more of the components of physical fitness (2). The authors selected and analyzed a number of publications, concluding that upper-body resistance training, in combination with aerobic training and progressive load carriage exercise, provide the largest improvements in load carriage performance (4). Knapik et al. acknowledged that in particular, resistance training of the upper body imparts large benefits on load carriage performance, possibly because of improved stabilization of the torso.

We wish to draw attention to a factor that was not considered in Knapik et al.'s helpful summary but which we believe may be important. We recently highlighted that respiratory muscle function is compromised because of the thoracic restriction imposed by carrying loads on the trunk and speculated that this may impair load carriage performance (1). Thoracic restriction increases the work of breathing and reduces cardiac output due to elevated intrathoracic pressure swings throughout the breathing cycle. Thoracic restriction has been shown to precipitate significant respiratory muscle fatigue (7), which is associated with impaired exercise performance (6). The mechanism underlying the performance impairment is a sympathetically mediated reduction in limb blood flow, which accelerates locomotor muscle fatigue. Ongoing work from our laboratory corroborates these findings demonstrating approximately 16% reduction in force output of the respiratory muscles after 60-minute load carriage (25 kg) at 6.5 km·h−1. Interestingly, this reflex appears to be attenuated by specific training of the respiratory musculature (9), which has also been shown to significantly improve whole-body performance during unloaded exercise (e.g., running, cycling, and rowing) (3,5,8). Whether an ergogenic effect exists during load carriage is currently under investigation by our research group. Thus, the ongoing studies within our laboratory indicate a fruitful and intriguing avenue for reducing the impairments induced by load carriage using specific respiratory muscle training in addition to whole-body aerobic and resistance training.

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1. Brown PI, McConnell AK. Respiratory system limitations in physically demanding occupations. Aviat Space Environ Med 83: 424–430, 2012.
2. Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: Definitions and distinctions for health-related research. Public Health Rep 100: 126–131, 1985.
3. Johnson MA, Sharpe GR, Brown PI. Inspiratory muscle training improves cycling time-trial performance and anaerobic work capacity but not critical power. Eur J Appl Physiol 101: 761–770, 2007.
4. Knapik JJ, Harman EA, Steelman RA, Graham BS. A systematic review of the effects of physical training on load carriage performance. J Strength Cond Res 26: 585–597, 2012.
5. Leddy JJ, Limprasertkul A, Patel S, Modlich F, Buyea C, Pendergast DR, Lundgren CE. Isocapnic hyperpnea training improves performance in competitive male runners. Eur J Appl Physiol 99: 665–676, 2007.
6. Romer LM, Polkey MI. Exercise-induced respiratory muscle fatigue: Implications for performance. J Appl Physiol 104: 879–888, 2008.
7. Tomczak SE, Guenette JA, Reid WD, McKenzie DC, Sheel AW. Diaphragm fatigue following sub-maximal exercise with chest wall restriction. Med Sci Sports Exerc 416–424, 2010.
8. Volianitis S, McConnell AK, Koutedakis Y, McNaughton L, Backx K, Jones DA. Inspiratory muscle training improves rowing performance. Med Sci Sports Exerc 33: 803–809, 2001.
9. Witt JD, Guenette JA, Rupert JL, McKenzie DC, Sheel AW. Inspiratory muscle training attenuates the human respiratory muscle metaboreflex. J Physiol 584: 1019–1028, 2007.
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