May 23, 2017
Support System
Dr. Ryan Mizner

With roughly 200,000 instances nationwide each year, ACL tears are an ever-present concern for athletes, coaches, and sports medicine professionals. Despite their frequency, nearly 85 percent of athletes who undergo ACL reconstruction surgery expect to return to their pre-injury level of sports participation. Many point to success stories as the source for these high hopes. A popular example is Minnesota Vikings running back Adrian Peterson, who won NFL MVP honors for the 2012 season that started a mere eight months after his ACL reconstruction.

However, one downside of cases like Peterson’s is that they can spark unrealistic expectations that don’t hold up in the literature. Recent evidence has revealed only half of athletes who tear their ACL will return to their sport following reconstruction surgery. Further, only two-thirds of all patients get back to their preoperative level of physical activity. Even when athletes do return, they still face a high possibility of reinjury, as athletes who have torn their ACLs are 15 times more likely to experience a subsequent tear.

These results have left many wondering: What is the cause of these shortcomings? While the answers are complex, there are two primary factors that deserve attention. The first, and perhaps most common, reason for limited return to sport is psychological, including fear of reinjury or a lack of confidence in the reconstructed knee.

Second, patients often exhibit mechanical deficits in their injured knee that persist for years after surgery. It’s common to see athletes avoid using their repaired knee to cut or control their bodyweight during jump landing. Many also limit bending and joint torque across their knee during these tasks, which inhibits their chances for successful return to sport.

Several years’ worth of biomechanical testing in the Movement Science Lab at the University of Montana suggests these two factors are interrelated. Since most ACL tears occur when an athlete places excessive stress on the ligament during jump landing or cutting, it’s not surprising that these same types of movements can evoke apprehension during recovery.

This brings us to another question: What can be done to help athletes overcome these issues and return to sport successfully? Our desire to answer this question led us to create the Bodyweight Reduction Instrument to Deliver Graded Exercise (BRIDGE) system. As its name implies, the BRIDGE was designed to facilitate the transition for athletes from ACL injury to safe return to sport. Combining unweighting technology via a suspension system and sports-specific jump training, it provides an unrestricted environment where athletes can refine their movement mechanics and build confidence in their injured joint.


Don’t get the BRIDGE confused with common rehabilitation protocols that incorporate high-intensity bodyweight jump training as part of ACL reconstruction recovery. Although both methods recognize the benefit of building proper jump training mechanics, the physical abuse that jumping at full bodyweight puts on the body can be detrimental. For instance, the loads and ground reaction forces on the operated limb are considerable even at low jump heights.

Adding to the issue is newly published research that suggests high-intensity bodyweight jump training during ACL rehabilitation could negatively impact articular cartilage health. These findings are particularly disconcerting, as nearly half of patients who undergo ACL reconstruction will exhibit early signs of post-traumatic knee osteoarthritis within 10 to 15 years of surgery.

And as mentioned, mental factors can further restrict the efficacy of bodyweight jump training. Patients recovering from ACL reconstruction will often self-limit the force borne by their operated knee due to insecurities surrounding its ability to withstand the rigors of landing.

The BRIDGE hangs from the ceiling of the Movement Science Lab on Montana’s campus. It uses 50-foot lengths of elastic tubing stretched out to 150 feet through a pulley system. The tubing ends on a sliding connection that travels across an eight-foot rail in the ceiling.

The BRIDGE system differs from traditional bodyweight jump training because its unweighting environment allows athletes to safely practice sport-specific tasks that may otherwise induce fear, such as jumping, hopping, and cutting. This encourages them to explore new movement patterns with their rehabbing knee in a safe setting. Instead of provoking feelings of apprehension and anxiety common in standard ACL reconstruction programs, athletes who have used the BRIDGE system describe it as fun and exciting.

In addition, the BRIDGE’s bodyweight support provides a natural range of motion and reduces the amount and rate of limb loading during retraining tasks, which allows us to use higher repetitions than the 20 to 120 ground contacts per session range commonly recommended for athletes recovering from ACL tear. The more reps athletes complete, the more likely they are to develop muscle memory and retain good jumping form.

The BRIDGE system is not the only treatment aimed at improving standard bodyweight jump training. Rehab specialists have tried many alternative methods that reduce loads and allow for a greater volume of training, such as upright bodyweight support systems and plyometric leg presses. Yet unlike the BRIDGE, these tools often restrict the area in which athletes can work, limit the athletes’ range of motion, prevent athletes from performing movements at normal speed, and inhibit their ability to interact with the surrounding environment.


So how does the BRIDGE work? Built as a suspension system, it hangs from the ceiling of the Movement Science Lab on Montana’s campus. It uses 50-foot lengths of elastic tubing stretched out to 150 feet through a pulley system. The tubing ends on a sliding connection that travels across an eight-foot rail in the ceiling.

Athletes using the BRIDGE stand under the rail, where the tubing attaches to a custom harness of neoprene shorts. Two nylon straps connect the harness to an aluminum yoke just above the athlete’s head. The straps enable the athlete to smoothly slide back and forth on the yoke during exercise. In total, the BRIDGE allows for movement over 50 square feet on the floor and a vertical space capable of accommodating everything from a lunge to a three-foot vertical leap.

Each tubing element in the BRIDGE carries a different weight load based on its thickness and amount of stretch. The level of unweighting is adjusted by adding or removing tubes to the harness. We can check the amount of bodyweight support provided during training with a miniature load cell placed in the rigging.

Between five to 90 pounds of bodyweight support can be supplied by the BRIDGE. Once the desired load is set, it will not vary more than five percent during training. For example, if a rehabilitation specialist is targeting 20 percent bodyweight reduction for a 200-pound athlete (40 pounds), the system will stay within 38 to 42 pounds for all movements. Such smooth and consistent bodyweight support creates a feeling of reduced gravity.


A typical training session with the BRIDGE lasts about an hour. It begins with an active warm-up on a treadmill and preparatory tasks like high knees, cariocas, etc. Then, athletes strap into the harness, and the rigging is adjusted to the desired amount of unweighting load.

After a brief accommodation period to the day’s bodyweight reduction, athletes will complete 25 to 40 minutes of jump training exercises. Some common movements include stationary or forward triple hops, split jumps, 180-degree repetitive jump turns, broad jumps, hops for distance, and three-step cuts.

When athletes first start training with the BRIDGE, we often see them adopt a stiff leg posture with limited knee flexion during landing because they are unsure of their knee. This increases the amount and rate of limb loading, which places greater stress on the ACL. Our rehabilitation specialists use positive feedback, extrinsic cueing, mirrors, and demonstrations of desired techniques to change these habits.

Typical verbal instructions to athletes include using greater flexion with their operated knee to soften and quiet their jump landings. A flexed leg posture reduces the rate and amount of landing forces, helping to protect against subsequent ACL injuries. Further, greater knee flexion improves the line of pull of the quadriceps and hamstrings to work synergistically with the ACL to limit anterior translation of the tibia on the femur.

We also emphasize vertical alignment of the hip, knee, and ankle during landing while keeping the head up and bottom down. This prepares athletes for the posture needed in a sports environment. Combined, these tips result in softer landing patterns, greater peak knee bend during landing, and a desirable pattern of thigh motor control.

As athletes progress in BRIDGE training, we provide less instruction and feedback. Instead, we promote intrinsic cues so they become aware of what it feels like to complete the desired techniques. We’ll encourage them to focus on the way impact affects their feet and legs or judge their body position during landing.

At this point in rehab, we typically add external elements and distractions — like catching or heading a ball — while instructing the athlete to maintain desired landing techniques. For instance, we had a volleyball player practice passes, hitting, and serving while set up at 20 percent bodyweight support. These sport-specific activities can make training a fun challenge and help promote lasting movement patterns.

Sessions generally conclude with a five-minute walking cool-down off of the BRIDGE so athletes can acclimatize to normal bodyweight load. To end the treatment, we complete static stretching of the major lower-extremity muscle groups.

We tend to prescribe the BRIDGE for eight weeks and start with no more than 30 percent unweighting. Each week, we use two treatments, with at least a 48-hour break in between to ensure proper recovery. The amount of unweighting is reduced by about 10 percent every two weeks.


From our testing and experience using the BRIDGE in patient care, we believe the ideal time to start training with it is at three to four months after ACL reconstruction surgery. This is typically when athletes are cleared to start running, jumping, and cutting drills in preparation for return to sport. We have some confidence in this recommendation from the success found in our initial proof of concept trial completed this year.

We started the trial by screening more than 30 athletes who were between six and 48 months post-ACL reconstruction and had been cleared for return to normal activities. We invited 19 athletes to participate in the study who had either performed below average in their clinical outcome scores or showed poor biomechanical performance during jump landing, as we felt they would benefit most from intervention. On average, they were 18 months out from surgery.

The two arms of the study were jump training with or without bodyweight support. We used a double-blind randomized trial design of two training sessions each week for eight weeks. The trial started with 30 percent bodyweight support, and the level of support was reduced every two weeks — from 30 percent to 20 to 10 to none. Both groups focused on sports-specific training with high intensity.

The bodyweight support participants performed more repetitions throughout their training than the non-bodyweight support group to help encourage improved motor learning. For instance, the first two weeks had a target of 120 to 200 contacts per treatment session. The training volume peaked in weeks three and four when the target was 250 to 500 contacts per session, and it decreased to 200 to 350 contacts in weeks five and six. Both groups had the same training volume of 120 to 200 jumps over the final two weeks.

Many jump training programs used in healthy, uninjured athletes last for six weeks, but an additional two weeks was needed in our trial so the athletes could transition to jumping under different bodyweight conditions. Also, since we chose athletes who had difficulties in landing, adding two more weeks to ensure success seemed a reasonable adjustment to the treatment length.

Overall, the BRIDGE training group saw comparable results to the group that didn’t use the device in areas like self-reported knee functional ability scores, altered landing styles, increased knee flexion, and reduced peak ground reaction forces. However, the BRIDGE participants achieved a greater safety advantage, as the relative risk of swelling within the knee joint from training was four times greater in the standard group than the BRIDGE group.

These benefits were sustained at the retention test completed two months after training. From this, we concluded that the results would last for at least a season’s worth of sports activity.

The enhanced safety factor seen with the BRIDGE athletes in the clinical trial suggests that we should be comfortable taking the next step with testing. This would include implementing the system four months post-ACL reconstruction as a means to smoothly transition athletes into practice.

We are confident that the BRIDGE training protocol will continue to provide a safe opportunity for additional training and early implementation of jumping practice for the recovering knee. The changes induced with the BRIDGE should help address current shortcomings in outcomes in ACL rehabilitation. We also hope to reduce risk of second ACL injury and improve rate of return to preinjury levels of sports participation.

This article first appeared in the April 2017 issue of Training & Conditioning.

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Development and prototyping for the University of Montana’s Bodyweight Reduction Instrument to Deliver Graded Exercise (BRIDGE) has been ongoing for five years. Montana’s Office of Technology Transfer recently applied for a patent for the current setup.

Although still in prototype phase, the university is also seeking commercial partners to consider licensing agreements and clinical development of the BRIDGE system. In a few years, it may be packaged for distribution to outside entities.

We are optimistic about the BRIDGE’s commercial potential. The closest comparable system on the market now utilizes a motorized device mounted on a track in the ceiling that shadows the patient as they move. It costs hundreds of thousands of dollars, and the speed of movements used in jump retraining protocols exceeds the limits proposed by its manufacturers. We foresee the BRIDGE system being a less expensive, more effective alternative.

Ryan Mizner, PhD, PT, is an Associate Professor in the School of Physical Therapy and Rehabilitation Science at the University of Montana. He is a past recipient of the Margaret L. Moore Outstanding New Academic Faculty Award from the American Physical Therapy Association. He can be reached at: [email protected].

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