The study protocol was approved by the ethics committees of the Israeli Defense Forces (1294-13-IDF) and the Sheba Medical Center (0529-13-SMC) and registered in the NIH research database (clinicaltrials.gov registration number NCT02091713).
Participants were new male recruits from five basic training companies of the Nahal infantry brigade, age 18 to 21 years-old. Before their induction each recruit had undergone two evaluations to determine in which IDF units they were suitable and qualified to serve:
1) Quality Group Score (Hebrew acronym KABA ): The score is calculated based on three components. Fifty percent is based on the results of psycho-technical computerized tests (Hebrew acronym DAPAR) which evaluate language, mathematical and logic skills. Thirty-three percent is based on a personal interview which evaluates suitability for military service. Seventeen percent is based on a score that represents the number of years and quality of the subject's formal education. Scores range from 41 to 56 with a score ≤ 43 considered unsuitable for regular military service . The average score for Israelis is 49. A minimum KABA score of 48 is necessary to serve in the Nahal Brigade.
2) Medical Profile: All citizens in Israel are members of one of four health maintenance organizations (HMO). Assessment of the subject's general health is based on records from their HMO, hospitals and individual treating physicians and medical interviews and physical examinations done in the framework of the pre-induction process in IDF centers. According to this evaluation the subject receives a numerical health profile . There is a minimum health profile requirement for service as an infantry soldiers in the Nahal Brigade.
All the study recruits had health profiles and KABA scores that allowed them to serve as infantry soldiers. Although they were assigned by the IDF to service in the Nahal Brigade, many had requested specifically to serve in the Brigade.
Following arrival at the basic training base, recruits in each induction group with appropriate KABA scores were offered to volunteer for service in the Nahal assault battalion (rather than service as regular infantry combatants in the regular Nahal battalions). In order to be accepted into the assault battalion they had to participate in an arduous 3-day physical and mental trial done under strict surveillance. Many candidates quit before the end of the trial. Those completing the trial and who had better scores were selected to serve in the assault battalion. The remaining recruits were assigned to the regular infantry battalions within the Nahal brigade. During the trial, observers looked for different traits such as perseverance, ability to perform individual and group physical and mental tasks as well as social interactions. Details of the trial are classified and not were not available to the research team. Recruits in the assault battalion have a different and more physically demanding training program than those who serve in the regular battalions.
Nahal recruits inducted in November 2014 (1 assault company & 1 infantry company) and March 2015 (1 assault company & 2 infantry companies) were recruited into the study just after their company assignment.
Following recruitment into the study, baseline data collection was done on two consecutive days.
Day 1: Data collected included questionnaires regarding demographics, life style, dietary and physical fitness habits as well as self-reporting of past or present orthopaedic injury or pain.
Day 2: A battery of physical tests, was performed in random order (apart from the final test, a shuttle run, to minimize the effects of physical exhaustion on the results of the other tests). To ensure measurement consistency, each physical test was performed on all recruits by the same trained member of the study team.
Physical testing: Measurements were made of height, weight, waist & hip circumferences, upper extremity length (most superior lateral point of acromion process to the lower and lateral border of styloid process of radius, palm facing forward), lower extremity length (anterior superior iliac spine to medial malleolus) and skinfold thickness at four sites: subscapular, biceps, triceps, iliac crest, three times each, (Mayer DC, Ontario, Canada). Body density was calculated according to Durnin & Rahaman  and Durnim &Womersly . Body fat percentage was calculated according to Siri . Total foot & heel to ball length were assessed with a Brannock device (Brannock Device Company, Liverpool, NY, USA). Navicular height was assessed with a standard caliper with the recruit standing in relaxed bipedal stance, as the vertical distance from the most medial prominence of the navicular to the supporting surface . Ankle Dorsiflexion (DF) was measured (mean of 3 repetitions, each side) with a digital inclinometer (1° increments, S-Digit mini, Geo-Fennel, Hesse, Germany).
Assessment of major movement limitations and asymmetry were assessed using the Functional Movement Screen (FMS) including seven tests (deep squat, hurdle step, in-line lunge, active straight leg raise, shoulder mobility, trunk stability push-up, and quadruped rotary stability [16, 17]. Rating are from 0 to 3 (0: pain present, 1: unable to complete test, 2: compromised/compensated performance, 3: correct movement without compensation).
Upper and lower quarter balance was assessed by the upper and lower quarter Y balance test (Functional Movement Systems, Inc., Virginia, USA [18, 19]. All examiners performing the FMS tests had formal training to administer the tests.
Dynamic tests: The timed 6-meter hop test , the crossover hop test (maximum distance of 3 hops alternating sides of a 15 cm wide tape  and a modified 240-meter shuttle run (6 times 40 meters, not paced) without load, followed, after a 10 minute recovery period by a loaded 240-meter shuttle run  were performed. Recruits ran wearing a weighted military vest fitted to their torso. The vest weight was set according to the body weight: 9Kg for BW < 60Kg, 11Kg for 60Kg < BW < 80Kg or 13Kg for BW > 80Kg [21, 22]. The time to complete the 240 meters with & without the weighted vest was recorded.
Jump tests: The single leg maximum vertical jump (SLVJ) displacement was performed. After two warm-up trials on each leg, three tests, with at least 20 seconds rest between each jump were performed for each leg. The jump height was assessed by the Optogait System (Microgate Corporation, New York, USA) based on the time in the air .
The socio-economic status (SES) of each recruit was defined as being level 1 to 10 based on their place of residence according to tables published by the Israel Central Bureau of Statistics . One represents the weakest and ten the strongest SES.
Recruits were then followed for one year for injuries and attrition. For the first 30 weeks of training a dedicated team of orthopaedists examined each soldier for signs and symptoms of overuse injuries, acute orthopaedic injuries and pain of musculoskeletal origin. All data was incorporated into a custom designed application on a relational database (MS Access®, Microsoft, Redmond, WA) detailing recruits’ presence (if absent, reason was determined and attrition documented as necessary) diagnoses, side involved, dates, etc. After the first 30 weeks, follow-up to 52 weeks was conducted by checking the computerized patient record (CPR) of the IDF Medical Corps and documenting all overuse and acute injury related information. This data as well as data from the IDF central manpower database was manually entered into the MS Access® database. Any diagnosed injury was treated according to the IDF Medical Corps guidelines.
Data collection and analysis:
Data collected from the initial phase (questionnaires, results of physical testing) were documented on a paper case report form, verified and assembled into a spreadsheet (MS Excel®, Microsoft, Redmond, WA). Data was imported from the excel and access files and analyzed with SPSS (version 20, IBM Corp., Chicago, IL) and SAS (version 9.4, SAS Institute Inc., Cary, NC, USA).
Injury data (possibly more than 1 per recruit) were flattened with the first incident giving the date. Recruits were divided into those with and those without each injury or injury group. Injuries were grouped into two types: objective injuries including stress fractures, ankle sprains, knee ligament and meniscus injuries, shoulder dislocations, etc., and pain-type injuries where the main manifestation of the injury is pain (back pain, anterior knee pain, shoulder pain, etc.). Attrition was defined as any case where there was positive evidence that the recruit was not serving as a combat soldier at one year following induction. Absolute attrition was defined as a case when the recruit was not serving in the IDF at one year following induction.