Antigravity Treadmills and Stress Fractures: Do They Help?

The use of antigravity treadmills referred to as lower-body positive pressure treadmills (LBPP) in the literature, has increased quite a bit over the last decade. When I was going through PT school around 2010, I remember hearing about the Alter-G treadmill and how runners were increasingly using them to either cross-train or recover from an injury. Currently, there are many different types of treadmills including models with the “bubble” shapes like the Alter-G as well as models with straps and all other types of harnesses. The basic idea is to let you walk, run, or perform functional tasks at a percentage of your normal body weight.

Not everyone can tolerate full weight-bearing activities for a variety of reasons so these types of treadmills can help improve cardiovascular fitness, help people with neurologic conditions, and help post-operative patients return to their normal walking mechanics in the early stages after surgery.

Endurance athletes love their gadgets and toys. As appealing as they might look, are these helpful for runners recovering from a bone stress injury (BSI)? I think they can be useful, but I think we need to understand a few things before we start prescribing them to our athletes.

Standard of care

During my clinical career, I have been able to use a few of these devices with patients and I have certainly interacted with many runners and triathletes that were either currently using them or had experience with them in the past. Here is a typical use for a runner dealing with a stress fracture.

“I was diagnosed with a stress fracture in my (insert a specific bone). My healthcare provider (physician, chiropractor, etc) told me I needed to rest a few weeks before I started running again. I stopped running for a few weeks and then got released to run on the LBPP treadmill and now I am trying to run outside, but I am still feeling something (pain, tightness, cramping, fatigue) around where the fracture was.” Maybe they are given some low-level exercises, but the overall strategy for recovery is to simply begin a running progression in an LBPP treadmill.

We can do better than that. These devices are not magical solutions to managing running injuries. The goal of this article is to highlight why running on one of them is not the same as running outside and how we can properly use them during recovery from a BSI.

Proper BSI management includes investigating

These machines are small pieces of what should be a much broader management strategy. They are short-term tools that can be used to help runners once the appropriate management strategy has been established. Taking someone into an antigravity treadmill without the requisite changes in training philosophy, nutritional philosophy, or stress management is the same as plugging your ears and saying “La la la”.

It is very easy to chalk up a runner’s injury to running too much, running too far or because you think they are “crazy”. I’ve been guilty of these assumptions in the past. When dealing with any injury, particularly a BSI, a thorough investigation needs to happen into both their running specifically and their health status globally. Simply zipping someone into a treadmill and giving them a run progression is a disservice to the athlete even though it might “feel good” to the athlete.

Stress Fracture

Rehab must include progressive loading to the affected bone

Bone adapts in a site-specific manner to the demands we place on it. This means we have to create stress through the affected region once it’s appropriate. We need to progressively increase the stress on the area and eventually get it as close to the stresses of running as possible. I think this is the most forgotten principle in BSI management.

There is a time and place for unloading (rest, non-weight bearing, or working at a percentage of body weight) with these injuries, but progressively increasing load to the affected area is one of the hallmarks of proper rehab for BSIs. I wrote two posts on this last year on hip and foot fractures. You can check those out below.

Hip Stress Fracture Post

Ankle and Foot Stress Fracture Post

Running on an anti-gravity treadmill is not the same as actually running

This point is important for ALL running injuries. When you run on a treadmill like this you are not having to deal with the same forces, muscular strain, or joint stresses as when you are running outside. Let’s investigate our changes in running mechanics first. Neal and colleagues highlighted this well in the article below.

Leg Muscle Activation Running

They investigated whether our mechanics change when we run at different percentages of our bodyweight compared to our normal running mechanics. They found, “It is clear from these results that significant changes occur, most notably reductions in ankle and knee range of motion, shorter ground contact time, longer flight time, longer stride duration, and lower stride frequency.”

When we move differently we load our bodies differently. Typically, this means we see a decrease in our cadence, a shift to more of a forefoot strike, and a longer period of time when we are in flight. I’ve seen many athletes run in these devices that look more like they are taking an approach for a triple jump than running.

Now let’s take a look at what changes in our muscles. This article by Jensen nicely highlights what changes in how our muscles are stressed in these environments.

Alter G Muscles

One of their key findings was nicely summarized here. “Thus, the relative activation reduction of gastrocnemius and soleus was much less pronounced than the reduction of quadriceps activation, indicating different functions of the 2 muscle groups during running.”

Even though the force on the body was lowered, the change in force was different for the knee versus the ankle/foot. The number on the treadmill might say 85% body weight, but it doesn’t mean that’s being applied the same to the muscles of the entire leg. This has big implications for someone with a stress fracture of their femur versus a stress fractures in their ankle or foot.

When we run differently than we are accustomed to we load our entire body differently. Bones, tendons, and muscles all have to manage a different type of stress than they are use to. Depending on the site of the fracture, and how someone changes the way they run, these devices can develop an easy pathway for shying away from the area that we are rehabbing.

Imagine you are a runner that tends to run more up on your toes (“forefoot striker”) and you develop a BSI in one of the bones of your foot. You rest for a bit, take some time off, start some running at 80% BW in an antigravity treadmill, and once you start running outside you now display a rearfoot strike on the side of the fracture. You have changed your preferred running mechanics.

This is one example of “shielding” the affected area. Shifting to a rearfoot strike is one strategy for taking some loading away from your foot. Maybe you will be fine and adapt to your new style of running. Maybe at some point your mechanics will transition back to your previous mechanics and no issues will develop. Or maybe you weren’t ready to run yet and your recovery is delayed due to delayed healing, pain, or other clinical presentation. I’ve seen too many runners been told they were good to run when they hadn’t displayed the requisite capacity to load the affected tissue and they end up spinning their wheels with injuries for months or years.

When I teach the bone stress injury section of Chris Johnson and my Bones, Tendons, Weights, and Whistles Course, one of the key tenets I talk about is that we have to have proper management strategies in place before we run. Bone is affected by many different things, not just how far we run, how fast we run, or what shoes we wear. That means referrals to a dietician, an endocrinologist, a psychologist, or whatever resources we might need. We have to make sure the PERSON is ready for the demands of running. We also have to make sure the specific TISSUE is ready for the demands of running.

Practical advice

We have studies and clinical experience to tell us we run different in these environments, but what does the specific runner in front of you display when running in an antigravity treadmill? Are they showing altered mechanics? Are they showing you a strategy to offload the affected region? These are critical questions that need to be investigated with every athlete. More than anything, we have to PAY ATTENTION to how the runner is moving when they are using these devices and then determine are we creating the right environment for a successful recovery. If mechanics are normal and they have progressed through appropriate lower extremity drills, these treadmills can be a good tool to help increase their training volume.

If you are going to use a treadmill in your stress fracture recovery, here are two things you should implement before zipping into a treadmill.

Site-specific exercise prior to beginning treadmill running regimen

This is a non-negotiable for me. If you are coming off a stress fracture you have to load the affected bone throughout the rehab process before running. This is the biggest misstep I see people making when returning to running. Because we know running in these environments is the same as running outside, runners will often resume running before they are ready.

If you have a stress fracture in your hip, knee, or foot you better make sure you can load your hip, knee, or foot before returning to running. Here’s one of my favorite exercises for loading a pelvic of femoral stress fracture before running.

Plyometrics prior to beginning treadmill running regimen

Plyometrics are very short duration, high-velocity muscular contractions that call on the stretch-shortening cycling. They are one of the best activities for improving bony integrity. Low volume plyometric work helps our bones get stronger, but it also helps get our tendons ready for the ballistic contractions they have to handle when someone is running. Overall, these serve as ways for us to make sure our tissues are getting stronger while also allowing us to monitor for pain that would indicate the runner is not ready to run. Here’s one plyometric variation Chris Johnson and myself implement with our athletes. This video is out of our upcoming new resource “The Cinderblock Series” which is a comprehensive guide to strength training for the endurance athlete. We put this together to help those with minimal access to equipment and to show you can get an elite-level strength program for a $2 trip to your local hardware store.

So how do we use these things appropriately?

1. You have to do a thorough investigation into the runner’s stress levels, relationship with food, menstrual status, and past medical history before we run.

2. You have to progress through weight-bearing exercises loading the affected bone and plyometrics before we run.

3. Proper monitoring of the runner is necessary to make sure a negative shift in their mechanics occurs. This means they aren’t displaying something when running on the treadmill that shows they are shying away from the fracture site.

Do all those things, and you are on the path back to consistent training.

Thanks for taking the time to read!

Nathan Carlson PT, USATF

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