SENSORY AND PROPRIOCEPTIVE

TRAINING MODALITIES FOR

THE FOOT AND ANKLE

JACOB WASILEWSKI NSCA - Certified Strength and Conditioning Specialist SMA - Level 1 Sports Trainer Master of Physiotherapy Studies Candidate - University of Queensland Bachelor of Science of Kinesiology - Queen's University

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Introduction

Ankle sprains are the most common lower extremity injury in athletic populations, representing

up to 30% of all sports-related injuries 1-3. A sports medicine centre in Melbourne found that

ankle and foot injuries represented 11.4% and 5.4% of all injuries respectively between 1994-

1995 4. These injuries can result in long-term disability, economic costs resulting from time off

work, psycho-social problems coping with the injury and can seriously affect sporting careers

in competitive athletes, particularly those who develop chronic functional ankle instability

(FAI) 5-9.

Rehabilitation includes proprioceptive training of the foot/ankle. Proprioception is defined as

a complex neuromuscular process concerned with internal kinaesthetic awareness of body

position and movement 10, 11. It is dependent on respective afferent (i.e. sensory receptors) and

efferent (i.e. motor output) signalling pathways and plays an integral role in joint stability and

prevention of injury10, 12 Proprioceptive training involves exercises that challenge the capacity

of the sensorimotor system to detect and react to afferent input signalling joint position 10, 12, 13.

Proprioceptive outcome measures include postural sway/balance, which the criterion measure

is centre-of-pressure (COP), joint movement perception threshold (JMPT) and joint position

sense (JPS).

Proprioceptive training includes balance exercises performed on an unstable surface (e.g. Dura

disc, wobble board) and are fundamental in ankle/foot rehabilitation. Although unstable surface

training has been thoroughly studied and implemented clinically 10, 12, 13-17, less is known about

the proprioceptive effects of tactile and vibratory stimulation. Perhaps these novel interventions

can be an adjunct to unstable surface training in clinical settings. The purpose of this review is

to present the literature examining the effects of conventional and novel proprioceptive training

targeting the foot/ankle in young, healthy adults as well as young adults with FAI or an ankle

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sprain injury. Studies that combine conventional and novel interventions will be included to

determine if the combination of interventions further improves proprioception/balance.

Proprioception Training Modalities

Unstable Surfaces

A recent meta-analysis examined the effects of unstable surface training on proprioception

and balance in active adults with FAI. Three studies matching the inclusion criteria examined

JPS and COP which was quantified using a force plate. The earliest study found a significant

improvement in anterior-posterior (A/P) COP but there was no significant effect on JPS

assessed using a custom-built device following 6 weeks of unstable surface training in

subjects with FAI 15. A supervised 6-wk unstable surface training program significantly

improved medial-lateral (M/L) COP and JPS which was quantified using the KinCom II

ankle inversion/eversion footplate in subjects with FAI 16. On the contrary, a prospective

study found no significant difference in total COP length after a 4 month follow-up in

subjects with an acute ankle sprain in comparison to control whom received standard

emergency-room education 17. This disagreement may be due to different parameters of COP

assessed, studying a population that did not present with FAI and the use of a prospective

study in contrast to the robustness of a randomized controlled trial (RCT). It is important to

note that prospective studies do have better external validity than RCTs which is important to

consider when prescribing exercise programs. Nonetheless, it appears that 6 weeks of

unstable surface training significantly improves select parameters of COP. The effects on JPS

are in disagreement although the latter may be due to differences in measurement techniques.

More RCTs need to be done on subjects with ankle sprains, without FAI to clarify if unstable

surface training is less effective in this population.

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Sensory Training Modalities

Tactile Stimulation

The effect of tactile stimulation on proprioception was first suggested in athletic taping

studies18. It was proposed that ankle taping increases afferent input from cutaneous

mechanoreceptors, stimulated by traction of the tape on the skin18. The basis for this is that

the benefit of ankle taping remains even following prolonged activity, when the tape’s

mechanical support is compromised 19-21. The first study examining the effects of taping on

ankle joint proprioception in healthy adult males placed two strips of tape on the anterior and

posterior borders of the foot/ankle 22. Ankle joint plantar/dorsi-flexion JPS and JMPT were

assessed with and without tape, in double-limbed weight bearing and non-weight bearing

conditions. A device consisting of two movable foot platforms was specially designed for the

purpose of objectively measuring JPS and JMPT. Non-weight bearing plantar-flexion JPS

was significantly improved with tape while the other conditions were non-significant. This

finding is important since most injuries occur during the landing phase of jumping.

Therefore, improved non-weight bearing JPS may increase awareness of foot position prior to

landing and subsequently reduce the risk of injury. Limitations to this study include that the

subjects were not blindfolded during testing, a small homogenous sample size (n=20)

consisting of males and no blinding of the assessor. Furthermore, in a follow-up

questionnaire 16/20 subjects stated that the tape helped with JPS. It is important to note that

improvement in JPS may be attributed to mechanical or proprioceptive effects of tape. The

instantaneous beneficial effect of taping is also worthy of noting. A strength in this study is

that the subjects were tested and trained without shoes which may have controlled for the

confounding effects of footwear whilst maximizing cutaneous sensory input at the foot.

Nonetheless, future studies need to determine whether better awareness of foot and ankle

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joint position is maintained when participants are shoed, as this is relevant outside the clinic

and specific to the environment in which foot/ankle injuries are often sustained.

A subsequent 10-week study compared the effects of unstable surface training on single-leg

stance A/P and M/L COP area quantified using a force plate, with and without strips of tape

bordering the malleoli in adults with FAI 23. A healthy group was included as a comparative

measure of normal postural sway and had significantly better postural sway in comparison to

both experimental groups at baseline. The tape-group significantly improved postural sway 2

weeks sooner than the no-tape group. Additionally, by the end of the intervention the tape-

group showed comparable levels (i.e. not significantly different) of postural sway to the

healthy controls, 2 weeks sooner than the no-tape group. These findings suggest that the

addition of tape to unstable surface training significantly accelerates rehabilitation in subjects

with FAI. Clinically, this suggests patients could be discharged sooner and athletes could to

return to sport more promptly. The main limitation in this study is the low number of subjects

(n=22).

A novel study examined the effects of perceptual learning on hardness discrimination of the

plantar foot, COP area and COP length in adults without FAI 24. The training involved

perceiving and attempting to identify the thickness of varied rubber sponges placed under the

sole of the foot during double-legged standing while blindfolded. Pre and post training

postural sway was assessed using a force platform in 4 conditions: standing with eyes open or

closed and standing on a balance board with eyes open or closed. Ten days of perceptual

training significantly improved thickness discrimination of randomly presented sponges as

well as postural sway in standing with eyes closed and standing on a balance board with eyes

open or closed. Although these findings look promising, it cannot be determined to what

degree perceptual learning had improved perception because the authors couldn’t clearly

identify to what degree the improvement in perception per se was improved by perceptual

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learning 24. Additionally, a follow-up assessment to determine if improvements in postural

sway persisted was not performed. Further studies are needed to determine an optimal dosage

and should include subjects with FAI or acute ankle sprain. Follow-ups should also be

included to determine long-term residual effects of this form of training.

Vibratory Stimulation

Vibratory stimulation is electrical or mechanical in nature. Vibratory stimulation has been

reported to act directly on muscle spindle mechanoreceptors or indirectly through cutaneous

fusimotor reflexes to enhance sensory signal detection25. Essentially, the introduction of low-

level noise summates with sub-threshold sensorimotor signals, which together may enable

weak somatosensory signals regarding joint position/movement to be detected and integrated

by the nervous system26-29.

An early study demonstrated that sub-threshold mechanical noise applied to the sole of the

feet through a perforated platform with built-in actuators significantly improved postural

sway (i.e. COP A/P range, COP mean radius, and COP area) in comparison to control (i.e. no

stimulation) during quiet, barefoot standing in healthy adults 30. Postural sway was assessed

using a motion analysis system which tracked an infrared marker on the right shoulder. Major

limitations in this study include the use of a non-criterion measure of postural sway and

relatively small sample size (n=30). Additionally, exclusion criteria was not listed thus it is

unknown whether subjects had FAI or other conditions that may have affected

proprioception. Nonetheless, the instantaneous benefit of mechanical noise on postural sway

is worthy of nothing. Future studies should examine subjects with FAI or ankle sprain and

use a force plate when assessing postural sway. The effects of long-term exposure should also

be addressed.

A subsequent study found that 6 weeks of barefoot unstable surface training with sub-

threshold electrical stimulation applied to the foot/ankle significantly improved single-leg

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landing, dynamic postural sway (quantified using a force plate) in comparison to unstable

surface training in active adults with FAI 31. Specifically, the electrical stimulation group

improved M/L time-to-stabilization (TTS) after 2 weeks of training and improved both A/P

and M/L TTS to a greater extent after 4 weeks of training than unstable surface training

alone. In contrast, the stable-ankle group and control (i.e. no intervention) did not

significantly improve postural sway. A 6-week follow-up study found that static single-leg

postural sway (i.e. A/P and M/L COP velocity, M/L COP max and COP area) significantly

improved unstable surface training with sub-threshold electrical stimulation in comparison to

unstable surface training alone in adults with FAI 32. In both of these studies, subjects

performed the training shoeless while the testing was performed shoed. These findings

suggest that performing these exercises in a clinical setting shoeless will transfer to balance

improvements outside of the clinic where subjects are generally shoed. Therefore these

findings are more generalizable to the environment in which foot/ankle injuries are often

sustained.

A recent study found that subthreshold vibration applied using light vibrating elements to the

tendons of tibialis anterior and gastrocnemius significantly improved plantar/dorsi-flexion

JMPT in young healthy adults without FAI33. However, vibration exceeding 60% of a

subject’s pre-determined sensory threshold, significantly deteriorated JMPT. Therefore, the

evidence suggests there is a specific threshold at which vibratory stimulation is effective for

enhancing JMPT. Although this intervention resulted in instantaneous improvements in ankle

JMPT, future studies with larger sample sizes should examine COP and whether the

respective effects on JMPT persist in subjects with FAI.

Conclusion

In conclusion, the literature suggests that novel and conventional proprioceptive interventions

can improve postural sway, particularly in subjects with FAI. The effect of unstable surface

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training on JPS in subjects with FAI was examined in two studies and findings were

inconsistent, although this may have been due to differences in measurement techniques. JPS

and JMPT significantly improved in novel interventions but studies were limited to adults

without ankle/foot injury or FAI. The combination of unstable surface training with tactile or

vibratory stimulation resulted in significantly greater and accelerated improvements in

postural sway than unstable surface training alone in adults with FAI. Currently, no studies

have explored the balance/proprioceptive effects with combination of all 3 interventions in

adults with FAI. These findings are clinically important with respect to designing effective

ankle/foot rehabilitation programs that enable return to work, sport or activity sooner.

Additionally, these findings could direct rehabilitative footwear design with built-in tactile

and vibratory stimulators implemented in combination with an unstable surface training

program. Therefore the aim of this study would be to investigate the combination of all 3

interventions on postural sway and ankle/foot proprioception in subjects with FAI. We

hypothesize that the combination of all 3 interventions will lead to significantly enhanced and

accelerated improvements in postural sway and ankle/foot proprioception in comparison to

unstable surface training in combination with tactile, or vibratory stimulation in subjects with

FAI.

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