First study on the effects of unstable shoes with curved soles on trunk muscles and lumbar lordosis

Since the mid-nineties various companies have introduced a special kind of "unstable footwear" onto the market, which is characterized by its curved sole and its flexible outsole. This curvature of the sole provides an unstable base for the feet to simulate walking barefoot, compared to conventional shoes that contribute to the stability and thus can weaken the muscles that develop this stabilising function because they are rarely used.

The possible long-term effects of those shoes, such as easing pain or preventing injuries are of interest for biomechanics, doctors, physiotherapists and trainers.

The few existing scientific studies on unstable shoes have focused on analysing the effects on leg muscles. Now researchers of Physical Therapy at the University CEU Cardenal Herrera and the University of Valencia have carried out the first global study on the effects of this type of footwear on the lumbar region. Their research work titled "Effects of Unstable Shoes on Trunk Muscle Activity and Lumbar Spine Kinematics," has been accepted for publication in the European Journal of Physical Rehabilitation and Medicine.

The research team, led by the Vice Dean of Medicine at the University CEU Cardenal Herrera, Juan Francisco Lisón, has studied the changes that wearing this type of shoes have on the electromyographic activity of erector spinae (ES), rectus abdominis (RA) and lumbar spine sagittal plane range of motion (ROM).

According to the professor of Physiotherapy at the University CEU Cardenal Herrera, Pablo Salvador, coauthor of the paper, "increased co-contraction of the trunk muscles increases spine stability and is related to low back pain."

The study involving 48 healthy test persons, 24 men and 24 women, who had not worn this type of unstable shoes before, was carried out by comparing muscle activity when walking with a control shoe. During the tests a measurement protocol of the lumbar spine was conducted by means of electromyography. Additionally, by means of electrogoniometry, the level of lumbar lordosis was examined during walking with both types of shoes.

Main results

The results obtained in this study determine that the unstable shoe -- compared to the control shoe -- improves the activity and strength of the trunk muscles (erector spinae and rectus abdominis) in every phase of walking.

"The footwear with curved soles, which simulates an unstable surface, requires a constant muscle activity of the trunk to stabilize the lumbar area and maintain balance. This increase in co-contraction of the trunk muscles improves the stability of the spine, which can help prevent or ease low back pain," said Professor Pablo Salvador.

The researchers of the Faculty of Heath Sciences at the University CEU Cardenal Herrera, Juan Francisco Lisón and Pablo Salvador, and of the Departments of Physical Education and Anatomy at the University of Valencia, Pedro Pérez, Salvador Llana and Daniel Sánchez-Zuriaga, have also observed significant effects on the curvature of the spine or lumbar lordosis when wearing this footwear. "This natural biomechanical mechanism is considered to be the best adjustment for the spine to absorb the vertical impact when walking," added the professor of University CEU Cardenal Herrera Pablo Salvador.

New research area

The results obtained in the study indicate that the use of unstable shoes can be a regular method of training to strengthen the trunk muscles that improve the physiological posture of the spine of healthy people. Furthermore, when doing rehabilitation exercises, this kind of footwear can help people suffering from back pain by easing it.

Therefore, thanks to the positive results of this pioneering study on an international level, the research team in back pain of the Masters Course of Physiotherapy, Physical Exercise and Sports of the Faculty of Health Sciences of the University CEU Cardenal Herrera will soon begin a new study where they will compare the benefits of unstable footwear over normal footwear with patients suffering from low back pain in cooperation with the company MBT, one of the pioneers in the fabrication of this type of unstable shoes.