Research Review: Two Mile Performance and Fat Free Mass – Indicators for 12 Mile Ruck Performance

BLUF (Bottom Line Up Front)

This study examined what physical traits and fitness scores predicted success in a 12-mile (19.3-km) ruck march with a 35-lb (15.9-kg) load. Cadets who finished under the Army’s 3-hour standard had lower body fat, greater fat-free mass, and higher Army Combat Fitness Test (ACFT) scores. The strongest predictors of finishing were aerobic fitness (2-mile run) and fat-free mass. Each 1 kg increase in fat-free mass raised completion odds by 24%. Each 1-point increase in ACFT score raised them by 6.1%. Body weight, BMI, and muscular power were not strong predictors.

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Purpose of the Study

To identify body composition and fitness measures associated with completing a 12-mile ruck march under the Army’s 3-hour standard.

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Subjects and Methods

• Subjects: 65 ROTC cadets (36 finishers, 29 non-finishers)
• Ruck March: 12 miles (19.3 km) with 35-lb (15.9 kg) minimum load, self-paced, time cap of 3 hours
• Testing:
  • Body composition via bioelectrical impedance
  • Countermovement jump (CMJ) on force plates
  • Full Army Combat Fitness Test (ACFT)

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Key Findings

1. Fat-Free Mass and ACFT Score Predict Completion

• Each 1 kg increase in fat-free mass increased odds of completion by 24%
• Each 1-point increase in ACFT total score increased odds by 6.1%
• The model using fat-free mass and ACFT score predicted ruck completion with 91% classification accuracy (AUC = 0.91)

2-Mile Run Was the Strongest Individual ACFT Predictor

• When each ACFT event was tested on its own, only the 2-mile run score significantly predicted ruck completion
• Each 1-point increase in 2-mile run score raised the odds of finishing by 9%
• This reinforces that aerobic capacity—not strength or power—was the most critical fitness trait for completing a 12-mile ruck under load

3. Body Composition Matters More Than BMI

• Finishers had:
  • Lower body fat percentage (16.5% vs. 21.7%, p = 0.021)
  • Higher fat-free mass (67.1 kg vs. 60.4 kg, p = 0.032)
• No significant difference in body weight or BMI
• Reinforces that lean mass is a better readiness indicator than scale weight

4. Jump Performance Differed, but Didn’t Predict Success

• Finishers had higher CMJ height and concentric impulse
• But jump metrics didn’t predict ruck success when aerobic fitness and FFM were included in models

5. ROTC Experience Didn’t Predict Performance

• Military science class level (1–4) had no statistical impact
• Suggests that experience alone doesn’t prepare cadets for rucking if fitness isn’t there

6. Female Cadets Faced Higher Barrier to Completion

• Only one female cadet completed the ruck under the 3-hour standard
• On average, female cadets had higher body fat and lower fat-free mass
• These differences likely contributed to lower completion rates
• Authors note that prior research shows trained females can meet standards, but likely require focused conditioning

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Conclusion

Cadets with higher fat-free mass and better aerobic fitness—especially faster 2-mile run times—were more likely to finish a 12-mile ruck march in under 3 hours. BMI and body weight were not predictive. While power metrics like jump height were higher in finishers, they did not independently predict success. Programs focused on increasing lean mass and improving run performance may be most effective for improving ruck outcomes, particularly for underprepared or lower-mass cadets.

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Bibliography

Sax van der Weyden, M., Merrigan, J.J., & Martin, J. (2024). Long Duration Load Carriage Performance Is Associated With Army Combat Fitness Test Scores and Fat-Free Mass. Journal of Strength and Conditioning Research, 38(11), 1959–1966. https://doi.org/10.1519/JSC.0000000000004712