The investigators will conduct a controlled clinical trial with U.S. Army soldiers training to become combat medics. The purpose of this study is to determine if a 11-week, high intensity exercise program targeting the low back muscles using specialized equipment will result in a 25% increase in low back muscular strength and endurance compared with a lower intensity general core stability exercise.

Background Low back injury is responsible for the largest percentage of non-battle injuries in the theater of operations and is a large contributor to non-expiration of active service attrition in the US Armed Forces. Weakness and poor endurance of the back muscles are associated with low back injury. Targeted, high intensity exercise approaches using specialized equipment to develop the strength and endurance of the "weak link" muscle group (the lumbar extensors) have been shown to reduce risk for low back injury in high-risk civilian workers, but have not been widely implemented in military settings.

Objective/Hypothesis Specific Aim: In a controlled clinical trial, the investigators will assess the effectiveness of a high intensity progressive resistance exercise training program targeting the lumbar extensors to improve lumbar extensor muscular strength and endurance in US Army soldiers.

Hypothesis: A high intensity progressive resistance exercise for the lumbar extensors will result in a 25% increase in lumbar extensor muscular strength and endurance compared with control following the 11-week intervention.

Study Design A mixed methods, two-arm, controlled clinical trial with cluster randomization will be conducted. The sampling frame will be soldiers training to become combat medics from one domestic US Army base. Soldiers will be randomly assigned (by platoon) to one of two interventions - experimental or control. All participants at a given platoon will receive the same intervention and all interventions will be carried out at the US Army base, in addition to the soldiers' usual physical fitness training program. Participants randomized to the experimental group (strengthening exercise) will perform lumbar extensor muscle progressive resistance exercise using standardized protocols. Exercise training will consist of 1 set of high intensity progressive resistance exercise for lumbar extensors on specialized equipment. Participants in the active comparator control group (stabilization exercise) will perform 5 minutes of low intensity core stabilization exercises on the floor. Interventions will be carried out 1X/week for 11 weeks. Outcome measures that will be utilized to test the hypothesis of Aim 1 include validated physical fitness tests for lumbar extension muscular strength and endurance. Fitness tests will be conducted at baseline and following the 11-week intervention period.

Relevance Soldiers preparing for deployment are in need of advanced technology to help improve and optimize the functional capacity of the lumbar extensor muscles. Assuming positive results from this study and confirmatory trials, implementation of this targeted exercise protocol will maximize resilience in soldiers at high risk for low back injury, thereby helping them become more physically fit to counteract the extreme physical demands required in combat.

Inclusion Criteria:

• Age 18-35 years
• Active duty soldiers in the US Army training to become combat medics at Fort Sam Houston

Exclusion Criteria:

• Cardiovascular contraindications to resistance exercise as identified by history and physical examination
• Orthopedic contraindications to resistance exercise as identified by history and physical examination
• History of systemic inflammatory disease or spinal surgery
• Low back pain intensity > "mild"
• Disability >= 50% on the Roland Morris Disability Questionnaire
• Currently receiving care for spinal pain disorder/injury
• Currently disabled due to spinal pain disorder/injury
• Currently diagnosed with or receiving treatment for a psychological or psychiatric disorder
• Currently performing progressive resistance exercises for the lumbar extensor muscles other than those included in standard for military fitness programs
• Active workers' compensation or personal injury case
• Pregnant
• Simultaneously enrolled in another biomedical clinical trial
• Drug or alcohol abuse within the past year
• Any other condition, which in the opinion of the investigators or military medical authority, would put the candidate at increased safety risk or otherwise make the candidate unsuitable for this study
• Unable or unwilling to complete the study procedures

All enrolled participants who were deemed eligible, completed baseline assessments, and were randomized.

Adverse event information was collected for the time frame from baseline assessments through the 11-week intervention period through the follow-up assessments, up to 12 weeks.