Gait & Posture 40 (2014) 237–242

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Body borne loads impact walk-to-run and running biomechanics T.N. Brown a,b,*, M. O’Donovan b, L. Hasselquist b, B.D. Corner b, J.M. Schiffman b a b

Oak Ridge Institute for Science and Education (ORISE), Belcamp, MD, USA U.S. Army Natick Soldier Research, Development and Engineering Center, Natick, MA, USA

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 September 2013 Received in revised form 29 March 2014 Accepted 1 April 2014

The purpose of this study was to perform a biomechanics-based assessment of body borne load during the walk-to-run transition and steady-state running because historical research has limited load carriage assessment to prolonged walking. Fifteen male military personnel had trunk and lower limb biomechanics examined during these locomotor tasks with three different load configurations (light, 6 kg, medium, 20 kg, and heavy, 40 kg). Subject-based means of the dependent variables were submitted to repeated measures ANOVA to test the effects of load configuration. During the walk-to-run transition, the hip decreased (P = 0.001) and knee increased (P = 0.004) their contribution to joint power with the addition of load. Additionally, greater peak trunk (P = 0.001), hip (P = 0.001), and knee flexion (P < 0.001) moments and trunk flexion (P < 0.001) angle, and reduced hip (P = 0.001) and knee flexion (P = 0.001) posture were evident during the loaded walk-to-run transition. Body borne load had no significant effect (P > 0.05) on distribution of lower limb joint power during steady-state running, but increased peak trunk (P < 0.001), hip (P = 0.001), and knee (P = 0.001) flexion moments, and trunk flexion (P < 0.001) posture were evident. During the walk-to-run transition the load carrier may move joint power production distally down the kinetic chain and adopt biomechanical profiles to maintain performance of the task. The load carrier, however, may not adopt lower limb kinematic adaptations necessary to shift joint power distribution during steady-state running, despite exhibiting potentially detrimental larger lower limb joint loads. As such, further study appears needed to determine how load carriage impairs maximal locomotor performance. Published by Elsevier B.V.

Keywords: Locomotion Load carriage Lower limb biomechanics Mechanical work

1. Introduction Locomotion with body borne loads has a deleterious effect on the load carrier’s capacity to run, jump, and maneuver [1]. This reduced physical capacity may be further exacerbated with greater load mass [2] and attributed to significant trunk and lower extremity biomechanical adaptations [3]. Specifically, during prolonged walking, greater sagittal plane trunk, hip, and knee joint motions [4–6] and moments [7] occur while supporting body borne loads. Yet during dynamic locomotor activities, such as movements that require a quick increase of speed, mechanical adaptations to body borne load may be greater than exhibited

* Corresponding author at: U.S. Army Natick Soldier Research, Development and Engineering Center, Department of Army, 15 Kansas Street, Natick, MA 01760, USA. Tel.: +1 508 233 5705. E-mail addresses: [email protected], [email protected] (T.N. Brown). http://dx.doi.org/10.1016/j.gaitpost.2014.04.001 0966-6362/Published by Elsevier B.V.

during walking, further impairing performance. To date, biomechanics-based load carriage research has limited its assessment to prolonged steady state walking [3,4], despite the fact that soldierrelevant body borne loads, which often exceed 45 kg [8], may significantly reduce the physical capacity to successfully perform dynamic locomotor activities. To successfully perform a dynamic locomotor activity, muscles must generate energy to accelerate the center of mass. It may be, however, that accelerating the center of mass with body borne load results in large biomechanical adaptations of the lower limb. Previous experimental evidence suggests a unique set of kinematic [9] and kinetic [10] criteria define the transitional period of accelerating from a walk to a run. Segers et al. [10] concluded that the walk-to-run transition is realized in one step. During this transitional step, the stance leg exhibited greater flexion posture [9] and required three times the mechanical energy [11] to propel the body into the flight phase that demarcates running. When impaired with load, this transition may require greater power and

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larger attenuation of forces to initiate running, further inhibiting performance. With body borne loads, the lower limb musculature may take substantially longer to stabilize the external load and increase the time required to generate the mechanical energy needed to initiate running. Currently, however, it is not understood how body borne load may impact the joint kinematics, kinetics, and power distribution during a walk-to-run transition. When running the body alternates between periods of energy generation and absorption. With steady-state locomotion, i.e. constant speed of running, muscles do not perform net mechanical work, as neither potential nor kinetic energy change from step to step. Experimental evidence suggests distribution of joint power, the rate at which mechanical energy is added or removed from the body via either concentric or eccentric muscular contractions, does not significantly shift during steady-state running [12]. Joint power may be redistributed from proximal to distal or distal to proximal among the lower limb joints when kinematic adaptations, such as increased flexion posture, change the mechanical advantage of the lower limb musculature [13,14]. As such, transporting body borne loads, where the load carrier demonstrates greater lower limb flexion posture, may substantially shift joint power production, but to date the effect of load carriage on joint power distribution during steady-state running is unknown. The purpose of this study was to perform a biomechanics-based assessment of body borne load during the walk-to-run transition and steady-state running. We hypothesized that trunk, hip, and knee flexion angle and moments would increase, and joint power production would shift proximally up the kinetic chain as load mass increases during a walk-to-run transition and steady-state running. 2. Methods Fifteen male (age: 20.9  3.1 years, height: 1.8  0.1 m and weight: 75.6  11.6 kg) military personnel volunteered for this study. Participants were between the ages of 18–40 years and had the ability to safely carry loads up to 43 kg. Participants who reported: (1) a history of [(Fig._1)TD$IG]previous back or lower extremity injury or surgery, (2) any recent pain

or injury to the back or lower extremity (previous 6 months) and/or (3) any known neurological disorder were excluded from participation. Prior to testing, research approval was obtained from the local institutional review board and all participants gave written consent. All participants completed three test sessions. During each session, participants performed the study procedures with a different load configuration (light, medium or heavy) (Fig. 1). For the light load (6 kg), participants wore a helmet and carried a mock weapon. The medium load (20 kg) consisted of the light load plus body armor with a fabric ammo panel attached on the anterior of the participant. The heavy load (40 kg) added a standard issue military backpack to the medium load. To randomize and balance the testing order, each participant was randomly assigned a sequence of load configurations prior to beginning the study from a 3  3 Latin Square scheme. Participants had synchronous three-dimensional (3D) joint (trunk, hip, knee and ankle) biomechanical data recorded during a series of dynamic movements. Two force platforms (AMTI Optima, Advanced Mechanical Technology Inc., Watertown, MA) synchronously captured ground reaction force (GRF) data (1200 Hz), while twelve high-speed (240 fps) cameras (Oqus, Qualisys AB, Gothenburg, Sweden) captured motion data during the stance phase of all dynamic movements. For each movement, participants accelerated (walk-to-run) or maintained (run) the velocity of movement, while two sets of infrared photocell timing lights (Brower Timing, Draper, UT, USA), captured their velocity immediately prior to contacting the force platforms. For the walk-to-run task, participants walked at 1.3 m/s (5%) before transitioning to a 3.5 m/s run while contacting the force platform. The run task required participants run 3.5 m/s (5%) across the force platform. For all tasks, participants ran a total of 10–15 m by starting between 5 and 8 m from the edge of force platforms and running out of the motion capture volume, approximately another 5–8 m, after contacting the force platform. A trial was considered successful if the dominant limb contacted only the force platform. Participants repeated each task until three successful trials were obtained. During all movements, joint rotations were quantified from 3D coordinates of thirty-six (14 mm diameter) reflective skin markers.

Fig. 1. Load carriage equipment for the three configurations. For the light load (6 kg), participants wore tight spandex top and shorts, combat boots, helmet and carried a mock weapon. For the medium load (20 kg), participants added body armor and ammo panel to the light load. For the heavy load (40 kg), participants added a rucksack to the medium load. Prior to testing, participants were properly sized for load carriage equipment to ensure fit and confirm components were not restricting movement or contacting the lower extremity during testing.

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Markers were attached by a single experimenter and secured with double-sided elastic tape (Cover-Roll Stretch, BSN medical GmbH, Hamburg, Germany) to pre-determined anatomical landmarks. After marker placement, the participant stood in a neutral (static) position while a high-speed (240 fps) recording was taken. From the stationary recording, Visual 3D v4.00 software (C-Motion, Rockville, MD) defined a kinematic model comprised of eight skeletal segments (bilateral foot, shank and thigh, and pelvis and trunk segments) with 27 degrees of freedom. The kinematic model defined the pelvis with respect to the global (laboratory) coordinate system and assigned it six (three translational and three rotational) degrees of freedom, while the trunk, hip, knee, and ankle joint centers and local coordinate systems (three degrees of freedom) were defined in accordance with previous literature [15–17]. Synchronous GRF data and marker trajectories were low pass filtered with a fourth-order Butterworth filter at a cut-off frequency of 12 Hz [18]. Visual 3D software processed the 3D coordinates at each time frame and expressed trunk, hip, knee and ankle joint rotations relative to a participant’s neutral pose. Using a conventional inverse dynamics approach and segmental inertial properties defined in accordance with Dempster [19], the filtered kinematic and GRF data were processed to obtain intersegmental moments at the trunk, hip, knee and ankle [20]. Hip, knee and ankle joint moments were multiplied with the joint angular velocity to calculate joint power then integrated with respect to time to obtain discrete periods of positive and negative mechanical work. From the discrete periods of mechanical work, the following were calculated: total average power, average joint power at the hip, knee and ankle, and per cent contribution of each individual joint to total average power [12]. Joint moments were expressed as external moments and normalized to the product of participant body mass (kg) and height (m). The biomechanical data were time-normalized to 100% of the stance (run) or acceleration (walk-to-run) phase and re-sampled at

[(Fig._2)TD$IG]

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Table 1 Mean (SD) stance time (s) during the light, medium and heavy loads during the walk-to-run transition and steady-state running tasks.

Walk to run Run* *

Light

Medium

Heavy

0.41 (0.04) 0.26 (0.03)

0.42 (0.04) 0.28 (0.03)

0.42 (0.04) 0.30 (0.03)

Significant (P < 0.05) effect of body borne load on stance time.

1% increments (N = 101). For the run task, stance phase was identified as heel strike to toe-off, defined as the instant GRF first fell below and exceeded 10 N [18]. Only the acceleration phase of the walk-to-run was examined, which was defined as heel strike to the end of the propulsive impulse. Sagittal plane trunk, hip, and knee biomechanical parameters were submitted for statistical treatment. The dependent variables included stance time, peak of stance trunk, hip, and knee flexion– extension angles and moments, and percent contribution of individual joints to total average positive power. For each participant, the dependent variables were averaged across the three successful trials and submitted to repeated measures ANOVA to test the effects of load configuration (light, medium and heavy). In instances where statistically significant differences between load configurations were observed, a modified (Hommel) Bonferroni procedure was used [21]. All statistical analyses were performed using SPSS v18.0 software (IBM, Armonk, NY, USA) with an alpha level set a priori at P < 0.05 to denote statistical significance.

3. Results Stance time increased (P < 0.001) during steady-state running with the body borne load, where time was significantly longer for the heavy compared to the light (P < 0.001) and medium (P < 0.001) loads (Table 1). Similar differences in stance time were not demonstrated between load configurations during the walk-to-run transition.

Fig. 2. Stance phase (0–100%) plots for trunk, hip and knee flexion angle (8) and moment (N m/kg m) during the walk-to-run transition for the light, medium and heavy loads. Peak trunk flexion posture was 35.8, 39.2 and 46.7, while peak trunk flexion moment was 0.4, 0.4 and 0.5 for the light, medium and heavy loads, respectively. Peak hip flexion posture was 54.2, 45.9 and 47.3, while peak hip flexion moment was 1.3, 1.5 and 1.6 for the light, medium and heavy loads. Peak knee flexion posture was 60.3, 56.5 and 56.3, while peak knee flexion moment was 0.9, 0.8 and 1.1 for the light, medium and heavy loads.

[(Fig._3)TD$IG]

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Fig. 3. Stance phase (0–100%) plots for trunk, hip and knee flexion angle (8) and moment (N m/kg m) during steady-state running for the light, medium and heavy loads. Peak trunk flexion posture was 14.3, 20.8 and 27.0, while peak trunk flexion moment was 0.4, 0.4 and 0.6 for the light, medium and heavy loads, respectively. Peak hip flexion posture was 38.8, 37.1 and 38.7, while peak hip flexion moment was 1.1, 1.4 and 1.3 for the light, medium and heavy loads. Peak knee flexion posture was 48.3, 49.5, 47.8, while peak knee flexion moment was 1.6, 1.7 and 1.9 for the light, medium and heavy loads. During the walk-to-run transition, load had a significant effect on peak trunk (P < 0.001), hip (P = 0.001), and knee flexion (P = 0.001) posture (Fig. 2). While donning the heavy load, trunk flexion increased (P < 0.001) and knee flexion decreased (P = 0.008) compared to the light configuration, while both hip (P < 0.001) and knee flexion (P = 0.006) posture decreased with the medium relative to light load. Body borne load also significantly increased trunk flexion (P < 0.001) posture during steady-state running, but similar adaptations were not noted for hip (P = 0.349) or knee flexion (P = 0.253) (Fig. 3). Specifically, greater peak trunk flexion was exhibited for the heavy compared to the light (P < 0.001) and medium (P = 0.001) loads, and for the medium load (P = 0.002) with respect to the light configuration. Body borne load significantly increased peak trunk (P = 0.001), hip (P = 0.001), and knee flexion (P < 0.001) moments during the walk-to-run transition (Fig. 2). While donning the heavy load, participants exhibited greater trunk (P = 0.025), hip (P = 0.002) and knee flexion (P = 0.001) moment compared to the light configuration, and larger trunk (p = 0.010) and knee flexion (P < 0.001) moments relative to the medium load. Further, greater hip flexion (P = 0.014) moment was noted for the medium load with respect to the light configuration. During steady-state running, the addition of load had a significant impact on peak trunk (P < 0.001), hip (P = 0.001), and knee (P = 0.001) flexion moments (Fig. 3). Specifically, while donning the heavy load, greater peak trunk (P < 0.001) and knee flexion (P = 0.005) moments were evident compared to the light configuration, whereas, a larger peak hip flexion (P < 0.001) moment was exhibited with the medium relative to the light load. During the walk-to-run transition, load demonstrated a significant effect on percent contribution of the hip (P = 0.001) and knee joints (P = 0.004) to total average positive power, but substantial differences were not found at the ankle joint (P = 0.054) (Table 2). Specifically, the hip significantly decreased contribution to

Table 2 Mean (SD) percent contribution of hip, knee and ankle joints to total average positive power for the light, medium and heavy loads during the walk to run transition and steady-state running tasks. Steady-state running Light Hip Knee Ankle

Medium

Walk to run Heavy

Light

Medium

Heavy

22.7 (10.1) 26.3 (10.2) 21.6 (9.5) 40.6 (5.5) 40.4 (7.4) 35.4 (7.1) 20.3 (6.1) 19.3 (5.8) 23.8 (8.5) 16.3 (2.8) 16.4 (4.5) 19.6 (6.2) 56.9 (7.1) 54.3 (6.8) 54.5 (5.6) 43.1 (5.3) 43.2 (6.2) 45.0 (5.3)

total average positive power with the heavy compared to the medium (P = 0.011) and light (P = 0.012) loads. For the knee contribution, however, post hoc analysis revealed no substantial difference between any of the loads (P > 0.036 and corrected a = 0.017) during the walk-to-run transition. During steady-state running, load configuration did not substantially alter the percent contribution of the hip (P = 0.218), knee (P = 0.057) or ankle (P = 0.078) to total average positive power (Table 2).

4. Discussion To successfully transport body borne loads, the load carrier needs to effectively start locomotion. Increased body borne load during the walk to run transition resulted in a significant redistribution of joint power, but not a subsequent reduction in performance. Average positive power shifted distally down the kinetic chain as contribution decreased at the hip and increased at the knee. During a phase of power generation, such as the walk-torun transitional step, the stance leg is required to produce up to three times the mechanical energy [11] to initiate the flight phase that demarcates running. Generation of this positive work has been suggested to be primarily provided by the ankle and hip during walking [22], or hip during dynamic locomotor tasks [23]. While 80–85% of the positive work was provided by the hip and ankle, the current outcomes demonstrate an increased contribution (4%) to positive joint power is required at the knee with the addition of body borne load. Although, the knee is not generally a significant contributor to power generation, during the push-off phase of running, mechanical energy can be transferred down the kinetic chain to help extend the distal joints [24]. The load carrier, in fact, may transfer energy down the kinetic chain, i.e. from the hip to the knee, to counteract the increased lower limb moments evident during the loaded walk to run transitions. Increased lower limb loading may also necessitate an extended posture to prevent the ensuing collapse of the limb that might otherwise occur with greater flexion [25] and aid with power generation by utilizing the

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capacity of the knee musculature. These biomechanical adaptations may allow the load carrier to maintain performance of the walk to run transition, as substantially longer stance times were not evident with the addition of load. Further study is warranted to determine if load carriers are able to maintain locomotor performance during tasks that necessitate a larger change in velocity, such exiting the walk to run transition with greater speed. The current outcomes suggest that adding body borne load does not significantly shift lower limb joint power distribution during steady-state running. These findings are in agreement with previous experimental evidence that demonstrates redistribution of positive power output does not occur with level, steady-state running [12]. When a task requires no net positive work, such as the steady-state running, redistribution of joint power is attributed to a change in limb mechanical advantage [14] that results from kinematic adaptation of the lower limb, e.g. increased flexion of the hip or knee [13]. The fact that adding body borne load did not currently increase hip or knee flexion during steady-state running, suggests the load carrier may not exhibit the necessary kinematic adaptations to shift joint power production during such tasks. The current outcomes contradict previous experimental evidence that demonstrated greater hip [5] and knee flexion [4] posture occur during load carriage. While the reason for the current discrepancy is not immediately evident, it may stem from task differences. Walking, as used in previous biomechanical assessment of load carriage, exhibits periods of double support where the load carrier can distribute the increased demand of body borne loads across both limbs during the stance phase. When running, the load carrier must distribute the demand across a single limb, and thus may maintain an extended lower limb posture to prevent collapse of the stance limb [25]. The current analysis of stance limb mechanics may be limited because only the dominant limb was included. The non-dominant limb may exhibit a substantially different biomechanical profile than the dominant limb during dynamic activities [18], but research on limb differences are not conclusive [26]. Any potential limb differences, however, may be exaggerated by body borne load and warrant examination in future efforts. Adding torso borne loads to the load carrier increased joint loading of the stance limb that may impair physical performance. Running with the heavy load, for instance, increased knee flexion torque. In response to this externally imposed moment, an equal and opposite internal torque was generated by eccentric contraction of the knee extensors to effectively decelerate the body’s center of mass following heel strike. To attenuate elevated joint loads and maintain stability while donning body borne loads, participants exhibited a substantial increase in stance time. The increased stance time may indicate a reduction in maximal performance capability. During running, speed is related to the length of foot-ground contact [27], i.e. stance time, and muscular ability to support the body during that period [28]. With body borne loads, the lower limb musculature may take substantially longer to develop the vertical impulse required to provide sufficient flight time to reposition the swing limb for the next step, as it has to stabilize against greater external loading, reducing the capacity of the musculature to generate and transmit force to the ground. Further study, therefore, is warranted to determine the extent to which body borne loads limit the lower limb musculature capacity to generate maximal sprinting speed. Extending the analyses to such tasks may provide further insight into the detriment of body borne load. Body borne loads evoked greater trunk flexion posture during the current dynamic locomotor activities. In agreement with previous experimental evidence [6], the participants exhibited ever-increasing trunk flexion with load. The increased trunk lean may move load closer to the body’s center of gravity [6] and assist

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with forward advancement of the body by reducing the propulsive force required during locomotion [29]. The trunk flexion posture may also increase the demand placed upon the core musculature, particularly with posterior torso borne loads. The fact that significantly greater trunk flexion moments were evident with the heavy load supports this contention. Adequate core, i.e. lumbopelvic, musculature strength, for instance, is critical for providing a foundation for force production of the lower limb during dynamic movements [30]. If this musculature is compromised supporting posterior torso borne loads, it may weaken this foundation resulting in detrimental lower limb biomechanical profiles and potentially limit the load carrier’s ability to perform dynamic tasks. Further study to assess whether non-military posterior torso borne loads, such as equipment borne by first responders or recreational hikers, can compromise a load carrier’s lumbopelvic foundation and impair their ability appears warranted. Extending the analyses to these non-military loads appears necessary because the current study may be limited by the fact all load configurations were military specific and it would provide further insight into how body borne loads impact performance. In conclusion, the load carrier adopted a biomechanical profile to maintain performance of the walk to run transition. Specifically, the load carrier shifted joint power production distally down the kinetic chain, from the hip to the knee, and utilized an extended lower limb posture to overcome the impairment of the additional load. Further work is needed to determine if load carrier can maintain locomotor performance during dynamic tasks that necessitate a larger increase in velocity. The addition of body borne loads did not significantly alter the average positive power production required to maintain steady, forward advancement during running. This may be attributed to the fact that a substantial increase in hip or knee flexion was not evident during steady-state running. The load carrier, however, did exhibit an altered biomechanical profile, including greater trunk flexion and increased lower limb sagittal plane loading. Additional study is warranted to determine if these mechanical adaptations compromise physical performance during aggressive locomotor activities that require maximal, e.g. sprinting, running velocities. Acknowledgements The authors thank Ms. Marina Carboni and Mr. Albert Adams for their assistance with this study. Conflict of interest: None of the authors demonstrate any conflict of interest regarding this submission or received any external financial support.

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Body borne loads impact walk-to-run and running biomechanics.

The purpose of this study was to perform a biomechanics-based assessment of body borne load during the walk-to-run transition and steady-state running...
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