Background
Ever since the “Battle of Marathon” between Greece and Persia was recorded [
1], the accomplishment of running a 42 km marathon is seen as the ultimate achievement for a distance runner. However, the last two decades has seen both recreational and elite level runners striving for distances well beyond a standard marathon. Interest and participation in ultra-endurance races [
2], multi-stage racing events [
3], and off-terrain trail and adventure races have risen [
4] as individuals seek new ways to challenge human limits. These races not only test a runner’s speed and endurance, but also their ability to navigate and survive over undulating terrains and harsh environments [
3]. Navigation and survival requires routine access to specialized equipment and sustenance. This requirement necessitates athletes to compete with externally carried loads [
5]. Few studies have considered the role of load carriage on the potential impact on a runner’s health and performance [
6]. In addition, no studies have considered if runners can be trained to adapt to external loads in running.
Load carriage in running poses two fundamental problems to athletes and occupational personnel: 1) an increased injury rate, 2) and increased metabolic energy expenditure that may reduce performance [
7]. In the adult population, the increased overuse injury rate associated with load carriage has largely been investigated in the military setting, where load carriage biomechanics have been predominantly investigated while walking [
7]. A previous study reported that 8 % of the 5000 injuries reported in the Australian Defence Force from January 2009 to December 2010, were related to heavy load carriage [
8]. Of these injuries, 56 % affected the lower limb and were classified as muscular stress related [
8]. Although no causative studies have been performed, it is likely that load carriage while running may exacerbate the already high incidence of running related injuries [
9]. In addition, when an individual runs with load, the energy demand involved in maintaining constant running speed is increased [
10]. Minimising the reduction in running speed associated with load carriage is important for the survivability of military personnel, the performance of athletes, and the overall efficiency of movement in recreational runners [
11,
12].
The risk of injury and reduced performance associated with load carriage in running, points to the need for a preconditioning program for these athletes. There is convincing evidence that resistance based neuromuscular training programs are effective at reducing running related injuries (RRI) during body weight (BW) running (i.e. running with no external load) [
13], and improving BW running performance [
14]. However, current training programs have been developed using BW running research [
15,
16], rather than loaded running research. The only studies that have attempted to define best training practices for load carriage gait has been performed in the military setting [
17]. A limitation in existing training studies has been that exercise prescription has not been explicitly informed from biomechanical studies of load carriage gait. Rather, training was of a generalised nature, targeting the large muscle groups of the lower limb [
17]. The type of exercises and mode of contractions used for preconditioning programs should be specific to the gait pattern required for athletes, and be based on prior knowledge of biomechanical adaptations during load carriage.
Potential adaptive and mal-adaptive biomechanics
Studies using computed muscle control and induced acceleration analysis have identified the integrated roles of lower limb muscles in BW running. Collectively, the functions of these muscles are to provide a vertical force to accelerate/decelerate body weight, and horizontal forces to accelerate/decelerate inertial mass [
18,
19]. When additional load is imposed on a runner, greater vertical and horizontal forces are needed to accelerate and decelerate an increased total weight and total inertial mass, respectively. Biomechanical changes to running with load are classified as adaptive if they enable an increase in baseline motor function (Table
1). For example, an increase in ankle power absorption in mid stance with load may be adaptive as it transfers power away from proximal segments to the foot [
6]. This may aid in increased elastic-energy recovery at the ankle plantar flexor muscle-tendon unit, which may be essential to sustain faster running velocities during load carriage.
Table 1
Biomechanical adaptations of load carriage to potentially optimize metabolic cost and minimise injury risk
• Transfer energy from proximal to foot segment [ 64] • ↑ Energy stored as elastic energy [ 65] | ↑ Ankle negative power mid-stance [ 6] | |
• Accelerates leg into extension to ↑ energy transferred to proximal segments [ 64] | ↑ Knee positive power late stance [ 6] |
• ↑ Hip extension deceleration of trailing thigh segment for preparation into hip flexion swing [ 66] • Transfers energy from trunk to trailing stance limb to prepare into swing [ 64] | ↑ Hip negative power late stance [ 6] |
• ↑ Elastic energy recovery [ 67] • Avoid excessive vertical COM excursion and maintain ground reaction force alignment to stance limb [ 68, 69] | |
• Architecture of triceps-surae muscle tendon unit makes it an efficient force generator [ 65] | Small role for inter-joint work redistribution [ 71] |
| ↑ Hip adduction late stance [ 6] | • Asymmetrical loading on knee soft tissues [ 72] |
↑ Knee and ankle flexion mid-stance [ 6] | • ↑ COM vertical excursion [ 70] • ↑ Patellofemoral joint compression pressure and ↑ Achilles tendon compression [ 73, 74] |
On the contrary not all biomechanical changes with load may be adaptive. Some mechanical changes are likely to be mal-adaptive as they may contribute to a greater risk of incurring RRI or represent an inefficient running style. Poor hip control of non-sagittal plane rotations has been documented to increase the risk of developing RRIs [
20]. At the kinematic level, load carriage has been associated with increased hip adduction at terminal stance [
6] (Table
1). This increase in non-sagittal plane movements may represent suboptimal muscle capacity and motor control [
21]. In addition, poor proximal trunk-pelvis control in running may result in energy being wasted in maintaining postural balance and inter-segmental alignment. Remediating mal-adaptive mechanical changes whilst enhancing adaptive changes could improve biomechanical indices of running performance and injury risk during load carriage.
Rationale
Load carriage in running is increasingly common in running related sports. The ability to positively and predictably adapt to the imposed load when running necessitates an evidence-based training program. Existing training studies for load carriage performance in the military setting cannot be immediately applied to load carriage running, as most studies investigated performance in walking. This is because running and walking involve different movement dynamics, making the extrapolation of results from walking studies problematic when applied to running. For example, the hip contributes approximately 20 % of total positive power in the stance phase of BW walking, but less than 10 % of total positive power in the same phase of BW running [
22]. Second, studies that have investigated ways to improve load carriage performance have adopted a non-randomized design [
17]. Reported effect sizes of benefit in intervention studies were larger in trials without a randomized design compared to one with a randomized design [
23]. Lastly, studies on load carriage do not appear to specifically target the known neuromuscular demands of load carriage gait patterns [
17]. Therefore, the purpose of this investigation is to compare the effects of a biomechanically informed neuromuscular training program to a generic standard best-practice resistance training program on changes in the biomechanics of running with load.
Objectives
To compare changes in (1) self-determined running velocity with and without load carriage, (2) lower limb running kinematics and kinetics, (3) jumping power and hopping stiffness, (4) and isokinetic knee and ankle extensor strength in healthy adult runners participating in a biomechanically informed training program compared to a generic resistance training program. This generic resistance training program may be seen as the current “gold-standard” program based on current best evidence [
17].
Discussion
Carrying some form of external load is becoming increasingly ubiquitous in running related sports, such as adventure racing and ultra-endurance events. Running mechanics alterations with load carriage could represent adaptive or mal-adaptive mechanics. Mechanical changes like increased joint power may represent attempts at maintaining constant running velocity, support an increased weight, maintain postural control and/or attenuate excessive impact shocks. In addition, some mechanical changes are likely to represent a failed capacity of lower limb muscles to cope with the additional load, that result in a reduction in running performance and an increased risk of future injuries. The long term sequela not only has an effect at an individual level, but could affect long term sporting participation and health care costs. In addition, runners may have to compromise running economy and running velocity when load carriage is involved if lower limb muscles are not tuned to the specific neuromuscular demands. Velocity decrements as a result of load carriage would result in compromised survivability in combat soldiers, and reduced performance in competing running athletes. This study will provide preliminary evidence of the potential efficacy of a targeted neuromuscular training program or a best-practice strength training program on improvements in strength, stiffness, running velocity and biomechanics during load carriage running.
Acknowledgements
Not applicable.
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