Effects of unstable shoes: Many questions remain

By Jacob Gardner, PhD

Current research suggests that relatively small but statistically significant changes are associated in the short term with the use of unstable shoes. However, it is unclear if those changes are clinically meaningful, or whether long-term use would yield greater or diminished results.

In the past few years the use of unstable (rocker-bottom) shoes has been a topic of hot debate. Researchers have suggested that using unstable shoes has the potential to improve static and dynamic stability during gait.^1,2 Companies promoting unstable shoes have made substantial claims of health benefits and muscle toning outcomes that in many cases appear too good to be true. Examples include claims that these “toning shoes” are burning more calories than conventional athletic shoes, toning lower limb muscles, improving posture, and reducing joint stress.³ However, to date, there is not enough evidence to confirm these shoe companies’ sizable claims. Some are now including disclaimers on their websites to indicate that certain assertions have not been clinically demonstrated.

The majority of research done on the topic has been conducted on Masai Barefoot Technology (MBT) shoes.^2,4-10 However, since the introduction of these shoes, many other companies with their own unstable shoe styles have joined the ranks, and further research has been done on some of the different shoe variations, including Skechers Shape-ups, Dockers Active Balance, New Balance True Balance, Reebok EasyTone, and several others. With so many companies interested in unstable shoes, it raises the question: Does the unstableness of the shoe really deliver an advantage over a regular shoe? After all, the shoe’s underlying concept seems to have some validity: If a surface is made more unstable, wouldn’t the leg muscles need to work harder to maintain equilibrium? And, if the leg muscles are working harder, wouldn’t more calories also be expended? The answers to these questions, however, may not be so clear.

Spatiotemporal changes

A few earlier studies of footwear with an unstable design showed the shoes did affect some spatiotemporal variables during walking, decreasing walking speed, stride length and stride time, and step length compared with a control shoe.^1,2 Interestingly, when walking speed is controlled across conditions, differences in spatiotemporal variables seem to disappear.¹¹ Thus, it is more difficult to assess shoe-related differences accurately when walking speed is not consistent.

Postural control

An important question related to shoe design is how the shoes affect the stability of the wearer. The variable most researchers use to test for stability is the trace of the center of pressure either during standing (static) or walking (dynamic) as a measurement of postural sway. During both static and dynamic conditions, studies have shown that unstable shoes either slightly increase the postural sway^6,12-18 or have no effect;^14-17 thus, postural sway appears to be greatly dependent on the specific unstable design of the individual shoes. The majority of these studies tested postural sway in an acute scenario. To test the long-term effects of the unstable shoes on participants’ balance, Ramstrand et al split 31 women older than 50 years into either a test group (unstable shoes) or a control group (their usual shoes) and instructed them to wear the shoes as much as possible for eight weeks.

The researchers used three separate tests to determine the training effect of the unstable shoe on balance. At the end of the eight weeks, both groups improved significantly in the three balance measures, but there was no significant difference between groups;⁹ indicating no advantage of the unstable shoe over the control shoe.

Joint kinematic changes

The most notable kinematic changes researchers have found to be associated with wearing unstable shoes involve the ankle. For example, research has shown that, compared with a control shoe, wearing unstable shoes is associated with an increased dorsiflexion angle at heel strike,^1,2,13,18 a decrease in plantar flexion range of motion during early stance,^13,18 an increased¹¹ or decreased² dorsiflexion angle in late stance, and an increase in ankle eversion range of motion.^13,18 Additionally, a few researchers have noted some kinematic changes in the knee and hip in participants wearing unstable shoes, but again, these results are ambiguous. For example, the earlier studies of rocker-bottom shoes showed the unstable design was associated with decreased peak knee flexion, hip extension and flexion, and knee and hip range of motion.² However, in more current studies, in which walking speed was controlled, only a decreased knee extension angle and decreased hip extension angle were evident when wearing one unstable shoe,¹¹ and differences were nonexistent in another brand of unstable shoe.¹⁸ A study of trunk kinematics has also shown a few kinematic changes associated with wearing unstable shoes, including increased midthoracic flexion and velocities of angular displacement in the low back.⁵ These changes were also accompanied by an increase in erector spinae muscle activity, which may provide some benefits related to trunk support.

Joint kinetic changes

An analysis of joint kinetics can provide researchers with a more complete picture of unstable shoes’ effects on the ankle, knee, and hip joints. In the ankle, research has shown that the use of unstable shoes is associated with decreased plantar flexor^11,18 and dorsiflexor moments,¹⁸ as well as an increased peak inversion moment, compared with a control shoe.^6,27 Results have also suggested a reduced peak knee flexion moment, as well as an increase in hip abduction moment.²⁷ It has been suggested that the net moments across each of these joints have implications for increased or decreased muscle activity to control the ankle, knee, and hip joints during unstable gait. It may be possible to detect these muscular differences by monitoring muscular activity with electromyography (EMG).

Muscle activation

Researchers studying the effects of unstable shoes are often seeking to determine whether the shoes’ effects on muscle activation result in a “toning effect.” Currently, studies show ambiguous results. One of the earlier studies on unstable shoes, published by Romkes et al in 2006, showed that wearing the unstable shoes was associated with increased muscle activity at the gastrocnemius and decreased activity at the tibialis anterior (TA) during stance, followed by increased TA activity during swing when compared with the individual’s regular shoes.² The authors concluded that wearing unstable shoes could be used as a training method to strengthen the leg muscles. Zhang et al¹⁸ in 2012 showed that, in men wearing a different brand of unstable shoe than in Romkes et al, there was a decrease in TA activity associated with unstable shoe wear that was similar to the results reported by Romkes et al; however, they noted no changes in activity for any other lower extremity muscles for unstable shoe wear compared with a neutral control shoe.

One study did find small increases in the muscle activity of the peroneus longus and soleus muscles when walking in an unstable sandal,¹⁴ and another found increases in tibialis anterior, peroneus longus, and lateral gastrocnemius activity when walking in a shoe with round surfaces embedded in the forefoot and heel of the sole of the shoe.¹⁹

Other EMG studies conducted on unstable shoes found no differences in muscular activity compared with a control shoe. For example, Branthwaite et al²⁰ in 2013 analyzed the muscular activity of women wearing an unstable shoe compared with activity in their own trainer shoes. While some variation in muscle activity did exist, the differences between shoe conditions were not statistically different. Similar results were also noted by Sacco et al²¹ in 2012 for women walking in an unstable shoe, by Santo et al in 2012 during a 10-minute walk performed by men and women in unstable shoes,²² and by Demura et al in 2012 during a 10-minute walk performed by men in two different types of unstable footwear.²³

Interestingly, the aforementioned studies focused on acute changes when donning the unstable shoes for the first time. There are few studies that have considered the long-term effects of the unstable shoes over six- and even eight-week periods. One example of such a study, published by Sousa et al in 2014, found that unstable shoes were associated with an increase in medial gastrocnemius muscle activity during an acute bout of walking compared with a conventional shoe. However, after eight weeks of use, muscular differences no longer existed.²⁴ These results tend to suggest that long-term use of the unstable shoes may not have a substantial effect on muscle strength or tone.

Plantar pressure and joint pain

It is common in clinics to use hard nonflexible rocker-soled off-loading boots to help alleviate pressure beneath the feet. Generally these boots may be used in patients with foot ulcers secondary to peripheral neuropathy or other foot injuries that might benefit from pressure redistribution. Research has suggested that perhaps a soft-soled rocker-bottom shoe such as the examples described in this review might be a good solution to alleviate foot pain and under-foot pressures, especially in the forefoot.^12,25,26 The results of studies designed to address this issue are, again, slightly ambiguous, but may be a little clearer than previously discussed variables.

Several studies have shown pressures under the forefoot during gait are reduced when wearing unstable shoes compared with a control shoe,^12,25,26 while the pressure under the heel may be increased.²⁶ This may have positive implications for patients requiring forefoot pressure alleviation. However, it has also been shown that, during standing, pressures are dramatically increased in the forefoot, while heel pressures are reduced.²⁷ Whether the increased forefoot pressures experienced during standing would negate the benefits of pressure alleviation in the forefoot during gait remains to be determined. Additionally, it has been suggested that an unstable shoe is not necessarily a safe option for patients with peripheral neuro­pathy due to its effect of reducing perception in the feet; introducing an unstable shoe may actually increase their risk for falls.²⁸ More plantar pressure research is necessary to determine if unstable rocker bottom shoes might be beneficial for patients with various foot injuries.

Considerations

As this review indicates, it is clear from the research that unstable shoes do have some effects on the body during standing and walking. It is important to note, however, that in nearly all circumstances, the changes reported in each of these studies are relatively small and typically acute. For example, consider the small change in the mediolateral center of pressure associated with one type of unstable shoe compared with a control shoe—an increase of approximately 1 cm.¹³ It is true that this change was statistically significant, but it is unclear if 1 cm translates to a clinically meaningful difference. Similar examples can be pulled from nearly all sources referenced in this article.

This makes it difficult to determine just how effective unstable shoes are at producing an unstable effect. It also makes it difficult to determine if a training effect (even a small effect) is truly present. Will these small changes add up and be intensified over long periods, or do these differences tend to diminish as the body gradually adapts to an unfamiliar shoe? These questions have yet to be answered definitively with long-term studies, but some current “long-term” research (eight weeks) might suggest the latter is true.²⁴

It is also important to consider that not all unstable shoes are created equal, as is suggested in the ambiguity of results reviewed here. Thus, the design of the shoe is an important factor when considering the effects of unstable shoes. Most shoes promote anterior-posterior instability, but newer shoes are emerging that have a greater impact in the mediolateral direction, as well. Thus, another question must be asked: How much of a change in the sole of a shoe is necessary to produce the desired results? Perhaps the current shoe designs are not unstable enough to create a long-term training effect and thus simply result in acute temporary changes. We certainly do not want to increase the risk of ankle sprains or other injuries, but the small acute changes associated with wearing unstable shoes are promising results that may be made more useful with improved shoe designs.

Conclusion

Current research suggests that relatively small changes are associated in the short term with the use of unstable shoes. However, it is unclear if long-term use would yield greater or diminished results. Initial studies seem to indicate that results are not long lasting; thus, the unstable shoe design will probably not have as large an impact as various shoe company claims might suggest.

Jacob Gardner, PhD, is an assistant professor of kinesiology at Biola University in La Mirada, CA.