by Eric Cressey
Here's what you need to know...
• In one study, balance board training reduced the rate of ankle sprains in volleyball players, but this beneficial reduction was limited to those players with a previous history of ankle sprains.
• In elite female soccer players, balance board training actually increased the incidence of anterior cruciate ligament tears.
• In testing two groups of athletes on sprinting time, the unstable training group was, on average, 0.04 seconds faster than the stable group during pre-testing, but was actually 0.06 seconds slower at post-testing. Do you think a tenth of a second might make a huge difference in high-level athletics?
• Unstable surface training (UST) appears to chronically impair the stretch shortening cycle in healthy athletes and, if you train slowly - as you do with UST - you'll be slow.
• You can train balance just fine using stable surfaces - and without attenuations of power-related athleticism. Second, balance and proprioception are both skill-specific, and would therefore seemingly be better trained on the same surface present in competition.
• There may be some merit to utilizing upper body UST in scenarios where one's goal is to maintain muscle activation, but reduce stress on the joints.
About 15 years ago, unstable surface training (UST) exploded in popularity - and it's stayed pretty popular since that debut. That's not to say, though, that its lifespan hasn't been characterized by controversy about whether it actually has merit. On one hand, you have polarized powerlifters and bodybuilders who crap on this style of training because it doesn't get you bigger and stronger. On the other hand, many trainers resorted to using these implements extensively - to the point that one could argue that excessive UST gave the term "functional training" a bad rap.
As luck would have it, the BOSU ball was invented in 1999, the same year some little whippersnapper named Eric Cressey started college and got serious about training. It worked out quite nicely that this controversy was "peaking" in 2003, just when I began graduate school and had to select a research topic for my Master's thesis. This was a perfect opportunity to research something that would directly impact how I programmed for athletes and clients thereafter.
To that end, the subsequent two years of my life were devoted to designing a study, programming and training our subjects (high level soccer players with significant resistance training experience), and collecting and analyzing data. The end result was a publication in the August 2007 Journal of Strength and Conditioning Research. The Effects of Ten Weeks of Lower-Body Unstable Surface Training on Markers of Athletic Performance. Eric M. Cressy, Chris A. West, David P. Tiberio, William J. Kraemer, and Carl M. Maresh. Human Performance Laboratory, Department of Kinesiology, University of Connecticut, Storrs, Connecticut 06269.
We were the first researchers in history to look at the effect of a chronic (10-week) UST intervention on performance in healthy, trained athletes. Before we get to the results, though, I should give you a quick background on the origins of UST.
Where It All Began
Unstable surface training's original application was in clinical rehabilitation settings, particularly with respect to functional ankle instability. Following an ankle sprain, many patients develop functional ankle instability. Effectively, the peroneals (muscles on the outside of the lower leg that resist inversion) fire slower, meaning you don't have as much protection against re-sprains.
Fortunately, subsequent research demonstrated that this proprioceptive deficit could be addressed by adding UST. Effectively, these initiatives improved afferent (sensory) function, meaning that the central nervous system received better feedback distally to improve the messages (efferent, or motor function) it sent out. Awesome benefits, right?
Here's where things got hairy. With the success they saw using UST with functional ankle instability, clinicians began applying it to other populations. Most of the studies were poorly designed, with issues ranging from untrained study subjects, to no control group, to short training intervention duration, to not accounting for previous history of injury. There were some favorable outcomes, but there were also some huge red flags. At risk of seeming self-consumed, I'll quote myself here:
It's possible to even question the injury prevention benefits of unstable surface training interventions in healthy athletes in light of two recent studies. Verhagen et al. (2004) found that a balance board training program reduced the rate of ankle sprains in volleyball players, but this beneficial reduction was limited to those players with a previous history of ankle sprains. The researchers noted no preventative effect of unstable surface training in healthy athletes, but did find that the incidence of overuse knee injuries actually increased in the experimental (UST) group players (1).
Likewise, in elite female soccer players, balance board training did not decrease the rate of traumatic lower extremity injuries. The frequency of major injuries - including four of five anterior cruciate ligament tears - was actually higher in the intervention than the control group (2).
Taking all this into consideration, it's a tough sell to say that UST will reduce the risk of injury in most athletes more than comparable exercises on stable surfaces would. But what about making athletes bigger, stronger, faster, and more agile? Surely, since training has historically focused almost exclusively on the efferent (motor) side of things, there had to be some merit to emphasizing the afferent (sensory) component. Was UST an effective way to achieve those benefits in healthy, trained athletes? Well, this is where we came in.
The Study
Our subjects were 19 members of an elite collegiate soccer team. Players were matched for both age and position to account for different activity levels during training and game play, and then randomly assigned to either a control (stable surface) or experimental (unstable surface) group. All had a minimum of six months of resistance training, but none with any experience with unstable surface training. Athletes were excluded from the study if they'd had ankle sprains in the previous six months. We wanted to make sure that we were training people, not just "accidentally" rehabbing them.
We pre-tested all players on the bounce drop jump (BDJ) and countermovement jump (CMJ). BDJ assesses proficiency with the shortstretch-shortening cycle (SSC) and CMJ covers the long SSC. We also measured their 40-yard sprint time, with a 10-yard split, and then assessed them on a T-test (which includes forward, lateral, and backward running) for agility.
After pre-testing, they started a 10-week strength and conditioning program during their spring season. The training program for the groups was approximately 98% identical. However, the experimental group performed the unstable surface training intervention on one of the lower-body exercises in each of the 27 resistance-training sessions that took place over ten weeks - including a week off at spring break after week 4. The unstable surface training was performed on 1-2 Dyna-Discs, inflatable rubber discs that measure 14 inches in diameter. The unstable exercises were performed for 2-5 sets of 5-15 reps (or for a certain duration, in the case of balance exercises) and consisted of variations of exercises such as squats, deadlifts, lunges, single-leg squats, and single-leg balances.
The experimental group performed the UST exercises with body weight or body weight with an additional load prescribed as a percentage of estimated one repetition-maximum (1RM) for that unstable surface exercise. The control group simply performed the same exercises on stable surfaces; the same percentage of 1RM was prescribed for loading, but it was based on the estimated 1RM for stable conditions. The reduction in the load used parallels what happens in the "real world" with UST. Additionally, we integrated these exercises toward the end of the sessions, as they are generally treated as assistance exercises. Effectively, we replicated exactly what happens in everyday strength and conditioning and personal training scenarios.
One important last note: all of these athletes - in both groups - still lifted challenging loads on stable surfaces. They Olympic lifted, squatted, deadlifted, and did challenging single-leg exercises just as they would have done if no intervention were taking place. We simply plugged UST in for roughly 1/50 of the overall training volume. The guys busted their butts for us, and eleven weeks later, we post-tested them.
The Results
While there were no pre-intervention differences between groups for predicted power on the BDJ (bounce drop jump) or CMJ (countermovement jump), the post-intervention differences were readily apparent. The stable group demonstrated significant improvements in both BDJ and CMJ, but the unstable group didn't demonstrate significant improvements in either measure.
There were no pre-intervention differences between groups for 40-yard sprint time or 10-yard split time, and both groups saw significant improvements on both sprinting measures. However, the stable group improved significantly more than the unstable group on the 40-yard sprint time, and there was a trend (p=0.06) toward greater improvement in the 10-yard split time as well.Interestingly, while it wasn't significant, the unstable group was, on average, 0.04 seconds faster than the stable group during pre-testing, but was actually 0.06 seconds slower at post-testing. Do you think a tenth of a second might make a huge difference in high-level athletics?
There were no pre-intervention differences between groups for T-Test time, and while both groups improved significantly from baseline to post-testing, there wasn't a significant difference between the two groups. It should be noted, however, that while the stable group had a slower average time by 0.09 seconds at pre-testing, it was actually 0.03s faster than the unstable group at post-testing.
Interestingly, the T-Test was the assessment we used with the longest duration (almost twice as long as the 40-yard sprint), meaning that it was likely the one that would be least impacted by alterations to SSC function. Had this been an agility measure with a shorter duration, I suspect the results might have been a bit different. What does this all mean? Well, 2% adjustments to a training program doesn't seem like much, but it can have a tremendous impact on the success (or lack thereof) with a training program, especially at the highest levels.
The Problem
When all is said and done, all the possible explanations for the differential training effects between groups come back to the fact that UST effectively ignores the principle of specificity of training, at least in the lower body. Most athletic endeavors take place on stable surfaces, with instability applied further up the kinetic chain. While the lower extremity functions in predominantly closed-chain motion, the torso and arms often encounter instability in open-chain motion while the base is stable. Generally, in athletics, this "base" is the feet, and in a standing position.
As such, it's an easy sell to say that UST probably has more merit with training the "core" and upper body than with lower body training approaches. This more appropriate application would consist of movements where an individual is seated, prone, or supine atop a stability ball, and in some cases with added resistance.
Conversely, lower body instability training should be imposed in much more sport-specific ways via several training modalities on stable surfaces. Remember, this is all about specificity; as an example, a running back's legs and body get hit by opposing players, but the ground never moves underneath him.
Building on this theme of specificity, most athletic movements take place at high velocities and involve the stretch-shortening cycle (SSC) to a great degree. Unstable surfaces lengthen the amortization (pause between eccentric and concentric) phase of SSC movements, so the desired force production that follows eccentric preloading is considerably less. This is on par with trying to jump out of sand, as the stored elastic energy is lost as heat instead of going to concentric force production. Sure, UST can positively impact function in those with previous injuries and related proprioceptive deficits; unfortunately, whether it does so via mechanical (lost elastic energy) or psychological (tentative movements) reasons (or both), UST appears to chronically impair SSC function in healthy athletes. As we've always known, if you train slowly - as with UST - you'll be slow.
To make things a bit geekier, activation of antagonist musculature increased with UST as a mechanism to increase joint stability. This acute modification works contrary to one long-term benefit of resistance training that's conducive to strength gains: reduced antagonist activity. To illustrate this point, we'll use a basic example: If you want to be strong on leg extensions, your hamstrings (knee flexors) need to be able to relax to allow that movement. When you train with UST, the antagonists fire like crazy to help you stabilize the joint, not create strong, powerful muscle actions. Granted, high-level performance isn't all about strength and power, but it's still logical to recognize that such an adaptation could (and seems to have, according to our research) interfere with athletic performance when applied for an extended period of time.
It goes without saying that training for athletic performance often involves a variety of competing physiological demands, so selecting the most effective and efficient acute program variables is essential. When training athletes, there's always something that the athletes could be doing instead of the selected program. Examining the results from our study, it becomes clear that the experimental (unstable) group would have responded better to stable surface training focusing on dynamic muscle actions. Had this been a clinical (injured) population, they might have benefited more from the subtle addition of UST.
This, of course, leads to the observation that I always seem to get from folks who swear by UST: "But we're using these devices to train balance, not strength and power!"
It's a fair point, but there are two counterpoints I'd make. First, you can train balance just fine using stable surfaces - and without attenuations of power-related athleticism. Second, and more importantly, balance and proprioception are both skill-specific, and would therefore seemingly be better trained on the same surface present in competition (or real-life, for that matter). In fact, we've known this for 48 years! In 1966, Drowatzky and Zuccato found little carryover from static (like UST) to dynamic balance skills (3). Tsigilis, et al., confirmed this finding 35 years later (4). Therefore, one must question whether unstable surface training, which necessitates a significant amount of static balance, transfers to sporting movements, which typically are more dependent on proficiency with dynamic balance (5).
In short, if you're telling me that you're going to improve balance, you better tell me exactly what kind of balance you're improving. Otherwise, football players would just be magical athletes the first time you put them in hockey skates. You can't tell me that balancing on an unstable surface will teach one of my pitchers to accept and control force on his landing leg like a drill like this does:
Practical Applications
One of the most commonly cited arguments against UST is that it doesn't allow for adequate resistance to provide for strength gains, and our findings seem to verify that point of contention. Further, we demonstrated that UST actually interferes with the power increases athletes should experience with concurrent stable surface training. In other words, as coaches and trainers implement UST with clients in an attempt to improve proprioception, they're actually negatively affecting other athletic qualities.
However, in addition to rehabilitation contexts for the lower body, there may be some merit to utilizing upper body UST in scenarios where one's goal is to maintain muscle activation, but reduce stress on the joints. Anderson and Behm noted that muscle activation (as measured by EMG) is maintained with UST, but there were lower joint torques (6). Such use could be perfect for deloading periods when athletes need a chance to recuperate with lower forces and velocities. I'd recommend only incorporating this approach with upper body exercises. One such example would be the stability ball dumbbell bench press as a "deload" exercise in place of barbell bench press variations.
As an aside, Louie Simmons has discussed the value of employing this dumbbell stability ball bench press strategy for quite some time, which is yet another example of how research is sometimes years behind what's going on "in the trenches." You won't find him squatting guys on unstable surfaces, though, and rightfully, so, as the lower extremity operates in an almost exclusively closed-chain fashion.
The value of UST in rehabilitation is well known, particularly following lateral ankle sprains. After rehabilitation, there may also be a role for preventing re-sprains in those athletes with an ankle sprain history, but we can't say for sure without more research.
As an extension of the utilization of UST in deloading the upper body, there may be some merit to incorporating it in upper body rehabilitation programs when muscle activation is desired, but injured athletes are not yet prepared for full-on stable surface loading. Loading an athlete up with, say, weighted push-ups, would require considerable joint torques, whereas push-ups on a stability ball or BOSU ball could prove useful in delivering a training effect without the same potentially harmful stress.
There are clearly some negative consequences to the utilization of lower body UST with healthy athletes, so what do we do instead? Very simply, we look to specificity. For example, a destabilizing torque might be applied further up the kinetic chain with the athlete's feet fixed on the ground, essentially simulating what happens when a football players is tackled. Anti-rotation drills in half-kneeling, tall kneeling, and standing positions are good examples.
Unilateral lower-extremity exercises are, by nature, instability training because of the smaller base of support, which is also constantly changing with lunge variations. And, when you use variations where the weight is positioned up at the shoulders, you actually increase the amount of instability, as the center of mass is moved farther up from the base of support.
Uneven loading also has merit, as the positioning of this weight moves the center of gravity closer to the edge of the base of support, so the athlete must counteract it with muscular activation. This applies to the cable exercises noted above (as well as rotational medicine ball drills), but also asymmetrically loaded drills like one-arm farmer's walks and one-arm lateral lunges, to name a few.
Additionally, classic agility or change-of-direction drills constitute sport-specific instability training, as they shift an athlete's center of gravity within his base of support. To increase the challenge of the drill, movement speed can increase or the base of support can become smaller (e.g., single-leg rather than double-leg deceleration).
A Balancing Act
As is the case with almost all components of the fitness industry, the answer with respect to unstable surface training appears to be "it depends." When used appropriately, it can afford considerable benefits for health and high performance. When applied indiscriminately, it's a disaster waiting to happen.
References
1. Verhagen, E. et al. The effect of a proprioceptive balance board training program for the prevention of ankle sprains: a prospective controlled trial. Am J Sports Med. 32(6):1385-93. 2004.
2. Soderman, K. et al. Balance board training: prevention of traumatic injuries of the lower extremities in female soccer players? A prospective randomized intervention study. Knee Surg Sports Traumatol Arthrosc. 8(6):356-63. 2000.
3. Drowatzky, JN and Zuccato, FC. Interrelationships between selected measures of static and dynamic balance. Res. Q. 38:(3) 509-510. 1966.
4. Tsigilis, N. et al. Evaluation of the specificity of selected dynamic balance tests. Percept Mot Skills. 92(3 Pt 1):827-33. 2001.
5. Cote, KP. et al. Effects of Pronated and Supinated Foot Postures on Static and Dynamic Postural Stability. J Athl Train. 40(1):41-46. 2005.
6. Anderson, KG and Behm, DG. Maintenance of EMG activity and loss of force output with instability. J Strength Cond Res. 18(3):637-40. 2004.
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