In order to be awarded the Special Warfare Operator Naval Rating and the Navy Enlisted Classification 5326 Combatant Swimmer (Navy SEAL), candidates must undergo more than a year of intensive physical, mental, and battle-ready training.

The extremes of fatigue and torment that would-be SEALs have to endure is difficult to comprehend. It’s not just grueling physical conditioning—it’s pushing through mentally demanding tasks on virtually no sleep.

Running in combat boots with bleeding, chafing, infected feet. Carrying heavy loads despite dislocated joints and ruptured tendons. Swimming through dark, shark-infested, freezing cold waters—for hours. Being tied up and tossed into a pool as part of “drown proofing” training. Remaining a team player and helping the person next to you even though every instinct and molecule of energy you have has been spent on self-preservation. Beyond the physical and mental conditioning, it’s learning the ins and outs of some of the most challenging warfare skills, including underwater demolition, parachuting, and weapons training.

Despite the impressive performance feats, I find myself marveling most at the resiliency a SEAL’s body must have to survive training without traumatic injury. The truth is, overuse injuries and acute injuries do contribute to the high candidate dropout rate. Those who make it through must be genetically superior, with superhuman, unbreakable joints, right? While genetics does play a role in predisposition to connective tissue and muscle injuries, it’s what happens before initiation that prevents debilitating injuries on Day 1 of training.

The Navy makes it clear that SEAL training isn’t designed to get you in shape. You must be in excellent condition and pass the Physical Screening Test before even being considered a SEAL candidate. The importance of preparing physically to enter SEAL training can’t be understated. In Marcus Luttrell’s book Lone Survivor, he describes how a “farm boy from the backwoods of East Texas” came to be a SEAL:

“In fact, my natural-born assets are very average. I’m pretty big, which was an accident at birth. I’m pretty strong, because a lot of other people took a lot of trouble training me, and I’m unbelievably determined, because when you’re as naturally ungifted as I am, you have to keep driving forward, right?”

Marcus describes in detail his pre-SEAL training regimen, which didn’t start a few months before formal training. It started years prior when he was still a kid in East Texas. Marcus spent the summers, along with his twin brother Morgan, under the tutelage of his father—swimming, racing, and diving through a lake near their home.

They were taught discipline and survival. It was at about age 12 when Marcus realized “beyond doubt” that he was going to become a Navy SEAL. Later in their teen years, Marcus and Morgan began more intensive training under a former Green Beret sergeant named Billy Shelton who lived close by. Shelton operated a full-scale pre-SEAL training program for teenagers complete with weightlifting, 12-mile runs, and obstacles performed while carrying concrete blocks overhead. At Shelton’s advising, Marcus had studied martial arts to master unarmed combat starting in grade school.

By the time Marcus and Morgan were ready to enter formal SEAL training, they had a decade of preparation under their belts. The insanely high volumes of running and resistance training wouldn’t be a shock to their system. The resilience of their muscles and load capacity of their tendons had already been forged years before being assaulted by SEAL training on the beaches of Coronado.

For military candidates and recreational fitness enthusiasts alike, building load tolerance in the lower legs and ankles is one of the most effective injury prevention tactics. With strong, mobile calf muscles and resilient tendons and ligaments, you can withstand long runs, heavy rucksack hikes, and other endurance activities that test the limits of your connective tissue.

Here are the two most important tactics to employ before diving into hardcore endurance training if you want to avoid career-ending lower leg injuries:

  1. Lengthen your calf muscles — The Achilles tendon is the largest tendon in the body. It connects your calf muscles to your heel bone and is stressed heavily during running, jumping, and loaded carries. And, it’s one of the most difficult injuries to recover from (most doctors will tell you to expect a full year of active recovery, at minimum). Virtually every Achilles tendon injury is linked to tight calf muscles. If there’s one joint problem you want to avoid, this is it. Tight calves also contribute to foot pain (plantar fasciitis) and knee tendinopathies. Spend time daily stretching your calves with a straight knee to lengthen tight calf muscles. Use a mix of static holds (30-60 seconds) and dynamic movements where you move slowly in and out of a fully stretched position.

  2. Build Achilles tendon load tolerance — To fortify your Achilles tendon against injury, you need the ability to withstand heavy loads and fast movements. To build load tolerance, perform 3-6 sets of heavy calf raises on at least two days per week. Use a 5 second eccentric (lowering phase of the movement) to further bolster collagen remodeling within the tendon. 

To build elasticity within your Achilles tendon, devote at least one training session each week to performing movements that trigger the stretch-reflex within your lower legs (e.g. rope skipping, single leg hopping, or fast-paced calf muscle training).

This article contains excerpts from Built from Broken: A Science-Based Guide to Healing Painful Joints, Preventing Injuries, and Rebuilding Your Body. Visit to learn more about this book and the author.