Author ORCID Identifier

Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation




Kinesiology and Health Promotion

First Advisor

Dr. Michael Samaan

Second Advisor

Dr. Fan Gao


Background: Low back pain is a significant problem and one of the primary musculoskeletal-related conditions affecting active-duty service members [1]. Since more than two-thirds of U.S. soldier occupational tasks involve lifting from floor level to waist height [2], increasing individual lifting capacity is strongly correlated with performance with these tasks [3]. Training for the first Army Combat Fitness Test event (3RM deadlift) to improve lifting capacity directly relates to performing carrying/lifting military tasks. One training modality that can be used in training programs to improve lifting capacity is repetitions-to-failure (RTF) or repetitive lifting to volitional failure [4-8]. RTF is widely used in resistance training to improve muscle strength and endurance; hence, performing RTF deadlift improves deadlift 3RM [9]. Although RTF deadlift are readily utilized in fitness training to increase lifting capacity, little is still known about the effects of such fatiguing repetitive deadlifting on the biomechanics of the lumbar spine.

Objective: The objective of this study is to investigate how implementation of RTF training for the new ACFT deadlift event will alter lumbar spine biomechanics in a healthy population with and without body armor. Aim 1) Compare lumbo-pelvic coordination and variability while performing RTF deadlift, with and without body armor. Aim 2) Investigate changes in lumbar loads while performing RTF deadlifts, with and without body armor. Aim 3) Identify strength and core muscle endurance tests that are best associated with changes in lumbar loads during RTF deadlifts.

Method: Kinematics of the trunk and pelvis were measured using a 3D motion capture system, a 68 kg low-handle hexagonal bar deadlift, and a 22.68 kg weighted vest (to simulate a tactical vest weight). Aim 1) Coordination was defined by using a vector coding method to identify the coordination patterns and circular statistics were used to quantify the coupling angle variability between the initial 10% to the final 10% of repetitions performed. Aim 2) A kinematics-drive finite element model was utilized to quantify lumbar spine compression and shear forces, at the L5/S1 level, during deadlift RTF. The initial and final 10% of repetitions were compared with and without the weighted vest. Aim 3) The relationships between the change in spinal loads with strength and core muscle endurance were further explored to identify which tests best predicts changes in lumbar loads without the weighted vest.

Results: Aim 1) Repetitive deadlifting to failure resulted in the re-organization of spatial orientation between the pelvis and trunk where the initial and final thirds of the lift were most affected. The initial third of the lift followed an increased in-phase pattern of coordination with increased coupling angle variability. Aim 2) Peak compression force at the L5/S1 level increased from 14 kN to 15 kN without vest and 15 kN to 17 kN with vest, while shear forces decreased from 3.8 kN to 3.4 kN without the vest and 4.2 kN to 3.8 kN with the vest, when deadlifting is performed to failure. Aim 3) Grip strength and total number of repetitions were positively correlated with changes in lumbar compression force, while the side plank test was negatively correlated with changes in shear forces.

Conclusion: High load RTF deadlifts, with and without body armor, places significant mechanical stress on the lumbar spine, specifically at L5/S1. Compression and shear forces surpass injury threshold guidelines for civilian occupational work force, however these injury thresholds are not identified for the military population (or tactical athlete such as firefighters and police officers). Training for the ACFT will not only require soldier to increase their 3RM deadlifting through varying training modalities (such as RTF), but they will also need to spend time increasing total strength and core muscle strength. Having a better understanding of which exercises affect spinal loads can aid clinicians and strength and conditioning specialist to be specific in their intended outcomes and injury prevention strategies.

Digital Object Identifier (DOI)

Funding Information

This study was supported by the Kinesiology and Heath Promotion Graduate Student Grants from 2019 to 2020.