Military personnel are required to carry loads as part of their occupation – loads that, in excess, have altered battle tactics and led to soldier deaths in previous conflicts (1). With recent evidence suggesting that soldiers are now carrying more load than ever before (2), there is potential for the injuries and casualties caused by load carriage practices to impact on force generation (the pool of personnel undergoing training and development) and force maintenance (the pool of deployed and deployable personnel). Acknowledged as placing stress on the musculoskeletal system of the carrier (3), load carriage tasks have the potential to cause a variety of injuries ranging from blisters, lower back injuries and knee and foot pain(2,4-6), to stress fractures, and brachial plexus palsy (2,6,7). With low fitness levels associated with an increase in the risk of injury during general military training (8) and load carriage tasks in particular (2), physical conditioning to increase fitness levels can provide a means of limiting load carriage injuries (2). This concept of conditioning soldiers to carry loads is not new and can be traced back to the Roman Legionnaires (9). What is lacking however, are practical guidelines on how to condition military personnel for load carriage tasks; a translation of research findings into practice.
Literature Search: Training for Load Carriage
Research papers and articles that included key search terms related to training and conditioning for load carriage were gathered from numerous sources in two stages. The first stage entailed using databases as an initial starting point and entered key search terms. These databases and key search terms, which varied slightly depending on the specifics of the databases’ search engine, are detailed in Table 1. No language restrictions were applied and, where possible, searches were limited to “human” subjects. In an attempt to identify further research publications of relevance to this literature review, both military and civilian colleagues were contacted.
The research reports yielded by the literature search were reviewed using a narrative approach. In order to contextualize these reports within a practical implementation framework, the reports were considered in the light of pertinent information from the broader fields of physical training. On this basis, the literature review synthesized key findings from the identified reports with information gathered from a wide range of published physical training literature. The findings are presented utilizing a funneled approach, whereby general physical training concepts are presented initially, in order to provide the necessary framework for presenting the findings of the load carriage literature review. Search Results
Following the first stage of the literature search, 8,053 papers were identified from the databases search and 36 additional papers were gathered from colleagues and journal article reference lists. The initial exclusion of clearly non-relevant and duplicate articles reduced the number of papers to 291. From these papers, three full text articles could not be obtained through library, peer or military sources and were therefore also excluded. Judging from the article titles, it is highly unlikely that these papers would have met the inclusion critera and were therefore deemed non critical papers. Following the implementation of the listed exclusion criteria, the number of articles was further reduced to 215 (124 primary research; 32 conference; 59 secondary source). The second stage of the literature search further reduced the total number of papers to 12 (7 original research; 1 conference; 3 secondary source these add up to 11, not 12). A tabulated overview of these original research papers and the conference paper is presented in Table 3. The secondary source articles included a non-experimental military report (6), a peer reviewed load carriage review (2) and a military journal article (12).
The Principle of Specificity
The principle of specificity has as its essence the need to conduct task specific physical conditioning (13,14), supporting claims that load carriage tasks need to be included in a conditioning program designed to improve load carriage ability (2,10,11). As an example of the specificity concept, a study by Genaidy et al. (1989) (10) had an experimental group participate in eight training sessions (2.5 weeks) replicating a repetitive lift-and-carry task (20 kg load). Following completion of the program, the experimental group significantly improved their ability to continue the repetitive lift-and-carry task by 50 % more time than the improvement time observed in the control group.
Concurrent training involves training for more than one physiological response (e.g. strength and aerobic endurance) at the same time (21). The results of Kramer et al (2001, 2004), noted above, suggest that, contrary to the findings of some research, which questions the value of concurrent training in contexts other than load carriage (21), a combination of resistance training and aerobic training may be of value for load carriage conditioning. Supporting the use of concurrent training in load carriage training are the research findings of Harman et al (2008) (22), who compared two physical conditioning programs. The first program followed a new U.S. Army Standardised Physical Training regime (including weight load marching, stretching, calisthenics, sprints, shuttle runs, and medium-distance runs (12-18 mins runs) and the second a weight-based training program with an increased resistance training focus (including weight load marching, full body resistance, longer-distance, ability based, runs (20-30 min runs), sprinting, and agility training). Both groups were found to make similar, significant improvements in short duration load carriage abilities (400 m with 18 kg load and 3.2 km with 32 kg load).
With studies by Kraemer et al (2001, 2004), (19,20) suggesting that load carriage performance can be improved with concurrent training that excludes specific load carriage training, two factors need to be considered. Firstly, it should be noted that subjects with lower levels of fitness and exercise make greater initial gains regardless of the type of training employed, after which specific training is needed to improve performance for a specific task (11). Secondly, specific training can impart gains other than those measured by objective means. The aforementioned study by Rudzki (1989) (5) identified that, while both the run group and the load-marching group made similar gains in aerobic fitness, the load-marching group were subjectively rated by staff as performing better at military tasks than the run group.
Just as the training frequency, volume and intensity are important factors to consider in the design of a load carriage conditioning program, so too is the concept of recovery. The principle of recovery highlights the need for the systems of the body to have a sufficient recovery from the training stimulus to prevent overload and injury (13). Failure to provide recovery in the training program and to instead employ high volumes of vigorous weight bearing activities continuously has been identified as a causal factor in high injury rates among military personnel (26). One means of providing recovery from a progressive training stimulus is to reduce the total volume (distance) of conditioning (13). This volume reduction strategy has been found to dramatically reduce injury rates during recruit training, without negatively influencing fitness (7).
While the above findings suggest that physical conditioning may improve load carriage task performance, to be of a practical value, the load carriage research needs to be presented and applied in a manner consistent with the programming approach used for traditional physical conditioning. One such approach is the F.I.T.T. (frequency [how often], intensity [how hard], time [how long] and type of training) principle or a derivative thereof (14). In this section, we attempt to present the key information for load carriage conditioning using this approach.
As a result of their findings, Knapik et al. (1993, 2004) (2,29) recommended that weight load marching be conducted at least two times a month with loads that soldiers are expected to carry in a unit on operations. Visser et al. (1995) (16) however found greatest improvements with sessions conducted weekly versus fortnightly. Considering both of these findings, the 10-day load carriage conditioning cycle implemented in the Netherlands (6) may in fact be the optimal frequency in the training dose. This frequency may however vary depending on training intensity (load, speed) and training volume (time or distance).
To stimulate aerobic fitness adaptations, the load carriage conditioning intensity (eg. load, speed) needs to be sufficient enough to elicit a training response. While research has suggested higher intensity training to be of particular benefit for improving load carriage performance (5, 16), the potential for injury following a long period of high intensity load carriage (2) must be considered. Ultimately, the conditioning program needs to ensure that personnel are being conditioned to carry loads at the intensities required for military exercises and operational tasks, whilst being cognisant of the fact that, no matter how much conditioning is undertaken, there is still a point beyond which the load carriage task will become too much for the carrier to physiologically withstand (12).
The conditioning stimulus time (or distance) must be considered against both the intensity of the task and the outcome requirements. Just as short duration, high intensity sessions can be used to develop the ability to move rapidly for short durations (under direct fire for example) (14, 30), longer duration sessions are needed to develop the physical and mental stamina to endure long duration tasks (dismounted patrols, for example) (30).
The principle of specificity identifies the need for the conditioning context to meet the requirements of the performance context. However, the concept of concurrent training also suggests that other forms of physical conditioning may be useful to supplement the conditioning program, especially for the less fit. The results of the reviewed research suggest that exercises which stimulate upper body strength and increase aerobic fitness, in particular, may be of benefit for load carriage, provided they do not become the focal point of the training and reduce time allocated to load carriage specific training.
Finally, the principle of recovery demands that the overall load carriage conditioning program be structured and progressive, and include musculoskeletal recovery periods to help mitigate overuse injuries. Each of these concepts is supported by the review of pertinent research results, presented above.
Implementation Guide In summary the author’s recommend that military physical conditioning programs include:
- two to four evenly spaced load carriage sessions per month;
- carried loads that are initially light yet progress in weight to meet that required for given military tasks;
- load carriage task durations and distances that gradually increase (yet not at the same time as increase in load) to meet military requirements;
- periods of recovery spaced throughout the program to allow the body to recover from the conditioning stimulus; and
- supplemental conditioning (muscle strength and aerobic training) sessions utilising functional movement patterns to provide adaptation to a broad spectrum of load carriage duties and tasks.
Limitations of this review
Several limitations to the establishment of evidence of best practice and the subsequent guidelines issued in this paper are acknowledged. The heterogeneity of the populations in the identified research is high. While differences in motivation and experience can be found when comparing military and civilian participants, so too can differences be found across defence groups (comparing recruits in training and fully qualified soldiers, for example). Due to the limited number of core research papers focused on load carriage conditioning, the data could not be limited to only one such group and so all researched groups that undertook load carriage utilising systems akin to those in the military were included. Another limitation lies in the possibility that not all relevant papers may have been identified during the literature search. Likewise, military research papers that were rated above ‘unclassified’ could not be used in this public-domain review, due to security restrictions. Potentially limiting the application of the findings of this paper is its specificity. With the focus being on military load carriage, the generalisability of the results to other occupations which include load carriage will be limited primarily to those required to carry heavy loads on their backs, including fire fighters wearing breathing apparatus, special operations police and trail porters. The conditioning guidelines may, however, also be of use for recreational activities like distance hiking and mountaineering, where heavy loads are carried on the back.