BLUF (Bottom Line Up Front)
This study examined how fatigue changes soldiers’ movement mechanics during a loaded ruck march. Using wearable sensors during a 7+ mile field march, researchers tracked gait and torso movement among 70 infantrymen from the 82nd Airborne Division. As the march progressed, fatigue led to clear biomechanical changes: stride length shortened, stride width and variability increased, and torso sway increased both forward-back and side-to-side. These changes may impact performance and raise injury risk. The study also showed that such changes can be measured in real time using wearable sensors in the field.
Purpose of the Study
The study aimed to:
- Determine how fatigue affects gait and torso movement during a real-world, long-distance ruck march.
- Track both average values and variability of movement across four segments of the march.
- Identify fatigue-related movement patterns that could help predict injury or performance decline.
Subjects and Research Methodology
- Participants: 70 male infantrymen (ages 18–23) from the 82nd Airborne Division.
- Protocol: Soldiers completed a 7+ mile ruck march in full combat gear (average load: 86.9 lbs) at the conclusion of a 72-hour field training exercise.
- Instrumentation: Each soldier wore three inertial measurement units (IMUs)—one on the chest and one on each boot—recording 3D motion data at 256 Hz.
- Data Processing:
- Rest periods were removed.
- March data was divided into four equal quarters (Q1 to Q4).
- Researchers analyzed both gait and torso motion patterns using validated algorithms.
- Changes were assessed using repeated measures ANOVA and post-hoc Bonferroni tests.
Key Movement Changes Across the Ruck March
| Variable | Start of March | End of March | What Changed |
|---|---|---|---|
| Stride Length (step length) | ~0.78 meters | ~0.76 meters | Steps became shorter (decreased, p < 0.001) |
| Stride Width (distance between feet) | ~0.12 meters | ~0.13 meters | Feet moved wider apart (increased, p = 0.003 to 0.012) |
| Stride Width Variability (step-to-step sway) | ~0.03 meters | ~0.04 meters | Steps became less consistent (increased, p = 0.003 to 0.006) |
| Torso Lean Variability (front-to-back sway) | ~3.9 degrees | ~4.4 degrees | More upper body movement front-to-back (increased, p = 0.025) |
| Torso Lean Variability (side-to-side sway) | ~3.1 degrees | ~3.6 degrees | More swaying left-to-right (increased, p = 0.008 to 0.031) |
Analysis and Interpretation of Findings
The data confirms that as fatigue builds during loaded movement, soldiers instinctively adapt their gait. Shortened stride length may reduce limb force demands, while widened and more variable steps suggest compensatory efforts to maintain balance under load. These adjustments could increase the likelihood of overuse injuries, especially in the lower legs and feet.
Torso lean variability also increased over the course of the march—both forward/backward and side-to-side. While average torso lean remained unchanged, step-to-step control became less consistent, indicating reduced trunk stability as fatigue progressed.
The study also validates wearable sensors as a practical tool for monitoring real-time changes in gait and torso motion in field conditions. These tools could help identify early signs of fatigue and support better load management or injury prevention protocols during tactical training.
Conclusion
This study observed clear changes in both stride and torso movement patterns over the course of a loaded ruck march, suggesting that fatigue leads soldiers to alter how they move under load. Specifically, fatigue was associated with shorter strides, wider steps, increased variability in step width, and greater sway in torso movement.
The decrease in stride length may contribute to overuse injuries by increasing the number of foot strikes. Increased movement variability may reflect reduced stability or control. While variability is sometimes associated with healthy movement, in this context it may signal increased injury risk under heavy load and fatigue.
Non-combat injuries are now recognized as a significant health concern in the military. This study helps clarify how fatigue affects movement during sustained rucking and may contribute to future modeling efforts aimed at understanding how the body responds to load and fatigue in tactical settings.
Bibliography
Bloch, A.E., Steckenrider, J.J., Zifchock, R.A., Freisinger, G.M., Bode, V.G., & Elkin-Frankston, S. (2024). Effect of Fatigue on Movement Patterns During a Loaded Ruck March. Military Medicine, Vol. 189(1–2), e15–e20. https://doi.org/10.1093/milmed/usad086