Instability Exercises: Are They Beneficial?
Stability vs. Instability Exercises
Stability exercises versus instability exercises have been a debated topic in the field of sports performance for quite some time. Many questions arise from instability exercises with the most important question typically being, are they beneficial for sport? One of the main issues I consistently hear with instability exercises are that they are not applicable to sport. In terms of sport specificity, when are you ever performing a strength or power related task on an unstable surface during a sporting event? This leads many to questions the practicality of the exercises; however, what if we are asking the wrong question?
Anderson et al, conducted a study in 2005: Trunk Muscle Activity Increases with Unstable Squat Movements. This study compared three different levels of stability in the squat exercise. Squats were completed either on balance discs, free weight, or smith machine. Dyna discs were used for the unstable condition and the smith machine was used for the most stable condition. All three lifts were performed at three different loads: body weight, bar weight, and 60% of body weight. Muscle activity was measured using an EMG (which measures muscle activation), with the purpose of the study being to see how activation of the muscles involved with a squat are altered during different stability conditions. The results are as follows:
Muscles measured with EMG equipment were: Soleus (Sol), Vastus Lateralis (VL), and Bicep Femoris (BF) for leg muscles and Multifidus (MT- back stabilizer), Erector Spinae muscles (ES), and abdominal stabilizers (AS) for trunk muscles. Results of this study showed that muscle activity of the trunk muscles were greater during unstable squats compared to stable squats. Results also showed that muscle activation of the vastus lateralis (one of the muscles of the quadriceps) was highest during squats in the most stable condition. From these results some could conclude that instability squats lead to greater trunk activation and stable squats lead to greater lower limb muscle activation; however, there are several other explanations for why this may have occurred.
One of the major strengths of this experiment was that the order of the exercises and loads were completely randomized, meaning that each person experienced a different exercise order. This is important because it is often seen that muscle activation can improve as a person becomes better adapted to an exercise, but we will go over this in future posts regarding motor learning. Also, muscle activation could be altered by different levels of fatigue which is another reason why this randomization of exercise order was crucial. One major weakness to this study was that training intensity was not technically controlled for. The problem is that the same load represents a different intensity in each of the squat conditions. 60% of body weight during a squat on a balance disc represents a much higher intensity (in regards to 1 rep maximums) compared to the same load during a smith machine squat. This could be one one main reason why there was a difference in muscle activation during the three different versions of the exercise. It is theorized that as an exercise becomes more difficult, more muscles are recruited to help complete the task. From this theory it could be thought that greater activation of the trunk muscles during unstable squats was a result of a greater exercise intensity compared to the stable squats. This could also be the reason why greater activation of the VL occurred during the smith machine squat. Greater activation of a prime moving limb muscle could have occurred strictly because of a lower exercise intensity. This also brings into question two additional theories: the theory of intent and motor learning.
This study showed that there was greater muscle activation in the vastus lateralis during a smith machine squat and lowest during a free weight squat. This may have been due to the difference in the intent of the athlete. Several studies (Wulf et al., 2010; Zachry et al., 2005; Marchant et al., 2009) have looked at the difference in muscle activation during different types of focus. The results from these studies have shown that when there is an internal focus to use a muscle, greater muscle activation occurs compared to when there is an external focus such as simply moving the weight from point A to point B. Wilson, Lowery, Joy et al., 2012 conducted a pilot study comparing muscle activation during an exercise that was heavy with no internal focus compared to the same exercise at a lighter load with internal focus. Results from this study showed that the lighter load with greater internal focus led to greater EMG activity. If you incorporate these ideas with the fact that different intensities were used for this exercise, a different theory can be developed for why these results were seen. Since the smith machine squat was performed at a lower intensity, more internal focus could have occurred, which could have led to greater activation of the vastus lateralis compared to the other exercises which may have produced a greater external focus due to the increase in the difficulty of the exercise. The same could be true for the results in trunk muscle activation. Since greater external focus may have occurred during the instability squats, additional muscles (such as the trunk muscles) may have been recruited to complete the movement.
The other theory that needs to be explored is motor learning which points out an additional flaw to this study. This study was an acute study (which the researchers address), it only looked at muscle activation during a single training session. It is possible that if athletes had more experience in a certain type of exercise, they may have seen better performance in that exercise. Giboin et al., 2015 demonstrated that there is a task specific effect that happens with exercises due to motor learning. This suggests that it may be possible that the athletes weren’t able to optimally activate certain muscles during certain exercises due to their inexperience with those exercises. A better way to interpret this is let’s say one athlete strictly performs smith machine squats on a regular basis. When this athlete showed up for the study the movement was already fresh in their mind, their performance was likely better, leading to differences in muscle activation compared to a balance disc squat that maybe that athlete has never done.
There are several additional topics I would like to point out. At no point during this post am I saying that instability squat exercises are either good or bad. The researchers for this experiment bring up a point that I think is very interesting and often overlooked. This point brings us back to our original question: Are instability squats beneficial for sports? I think when considering this question, we are often only looking at it from one angle, the development of strength and power for sport. If you believe in sport specificity (exercise selection should be specific to movements made during the sport to produce a desirable transferable adaptation), then you would assume that unstable squats are not relevant to sport because you never complete movements on an unstable surface. This is where opening up our minds can bring up the importance of injury prevention. If instability exercises were able to more effectively recruit trunk stabilizer muscles, then theoretically this could be very beneficial for injury prevention in sports. These adaptations could occur due not only to stronger trunk stabilizers but also due to improvements in proprioception. These adaptations could be crucial for the success and longevity of an athlete’s career.
Currently, there is not sufficient data to make any assumptions on the practicality of instability exercises. To determine the effectiveness of instability exercises, future studies could benefit from controlling for workout intensity by measuring single rep maximums for each exercise and incorporating them into the training protocol. Also, studying the effects for a longer duration to allow for better motor learning of each exercise would lead to a more definitive answer on the topic. The concept of instability exercises will lead to much further debate and many additional studies, for now we have to interpret the results that we do have and decide whether or not to implement these practices into our or our clients training programs.
Anderson, Kenneth, and David G. Behm. "Trunk muscle activity increases with unstable squat movements." Canadian Journal of Applied Physiology 30.1 (2005): 33-45.
Giboin, Louis-Solal, Markus Gruber, and Andreas Kramer. "Task-specificity of balance training." Human movement science 44 (2015): 22-31.
Marchant, David C., Matt Greig, and Catherine Scott. "Attentional focusing instructions influence force production and muscular activity during isokinetic elbow flexions." The Journal of Strength & Conditioning Research 23.8 (2009): 2358-2366.
Wulf, Gabriele, et al. "Increased jump height and reduced EMG activity with an external focus." Human Movement Science 29.3 (2010): 440-448.
Zachry, Tiffany, et al. "Increased movement accuracy and reduced EMG activity as the result of adopting an external focus of attention." Brain Research Bulletin 67.4 (2005): 304-309.
