Balance and the foot.

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How do humans stay balanced on their feet or foot?

As most will know we take and integrate sensory information from our eyes, the vestibular apparatus of the ear, and proprioceptive input from joints and muscles.

Interestingly, the intrinsic muscles of the foot are closely linked to the vestibular apparatus of the inner ear and also generate afferents to the brain that closely track movements in the COM.

A very interesting paper has just been published on proprioceptive afferents generated by spindles in the intrinsic foot muscles and these appear to important in balance just as cutaneous receptors on the sole of the foot are.

. 2025 Apr 10.
doi: 10.1113/EP092348. Online ahead of print.
Firing properties of muscle spindle afferents in the intrinsic foot muscles and tactile afferents from the sole of the foot during upright stance

Thomas P Knellwolf 1, Alex Burton 2, Elie Hamman 1, Vaughan G Macefield 1 3
Affiliations Expand

• PMID: 40211475
  
• DOI: 10.1113/EP092348

Abstract

We review our approach for undertaking microelectrode recordings from the posterior tibial nerve at the ankle, which has allowed us to identify, for the first time, the firing properties of muscle spindle endings in the intrinsic muscles of the foot and of cutaneous mechanoreceptors in the sole during unsupported standing. The responsiveness of muscle spindles in the short muscles of the foot to stretch and related joint movements was similar to that of spindles located in the intrinsic muscles of the hand. Only 27% were spontaneously active in the unloaded condition, whereas 50% were active during unsupported free standing. Moreover, in the latter condition firing rates of 67% of the endings were correlated with changes of the centre of pressure (CoP), primarily (88%) along the anteroposterior axis. The firing of cutaneous afferents supplying the sole of the foot in unsupported free standing depended on the receptor type and location of the receptive field: fast-adapting type I and slowly adapting type I afferents responded transiently during contact with the substrate on standing and to spontaneous postural adjustments, whereas the tonic firing of slowly adapting type II endings encoded fluctuations in the CoP. We conclude that muscle spindle endings in the intrinsic muscles of the foot are recruited or increase their spontaneous discharge on standing and can faithfully encode changes in CoP during spontaneous or evoked postural sway, a function shared by slowly adapting type II afferents in the sole. These data emphasize the important contributions of sensory sources in the foot to maintaining and responding to perturbations in upright posture.

 

Below is figure from a Knellwolf et al 2018.



Fig. 4.Spontaneous fluctuations in mean frequency of 3 muscle spindles (thick lines) superimposed onto corresponding changes in center of pressure (anteroposterior); center of pressure smoothed with a Bartlett window of 2 s (thin lines). A and B correspond to the same recording shown in Fig. 3, A and B. Data from another 2 units recorded from flexor digitorum brevis muscle (C) and flexor hallucis brevis muscle (D) are also shown.




Quote from Knellwolf et al 2018
"NEW & NOTEWORTHY We have characterized the firing properties of muscle spindles in the intrinsic muscles of the human foot for the first time. The majority of the spindle endings are silent in seated subjects, and most fire tonically during standing, their discharge covarying with center of pressure during postural sway. We conclude that spindle endings in the intrinsic muscles of the foot provide useful proprioceptive information during free standing."

 

From Knellwolf et al 2025

"Moreover, selective anaesthesia of cutaneous afferents of the sole of the foot increases postural sway by only ∼11% (Meyer et al., 2004), whereas increases of ∼40%–60% occur in diabetic neuropathy, in which both muscle and cutaneous afferents are affected (Boucher et al., 1995; Simoneau et al., 1994). Accordingly, it is likely that muscle afferents from the foot contribute more to the control of postural sway than do cutaneous afferents, but in the absence of direct data this is speculation."

It would seem that the spindles in the intrinsic foot muscles are a vital part of the balance equation, but so what?
Muscle atrophy, such as is seen in aging, can have a negative effect on muscle spindle function whilst condition exercises for skeletal muscle has a positive effect . Perhaps balance can be improved in those with sarcopenia by improving proprioception through better muscle/spindle function achieved by foot resistance exercises.

Muscle spindles pick up and encode stretch in the intrinsic foot muscles. Medial shift of COP can cause foot pronation and stretching in the muscles of the medial longitudinal arch . But what if the arch has been lost in adult acquired flat foot ? Less stretch and less defined afferent input ?

We know that flat feet can have a detrimental affect on balance and that restoring some form of medial arch through orthotic support can improve balance . Perhaps this is linked to greater changes in muscle length during pronation in orthotic supported feet, provided the arch moves through a greater range of motion than the unsupported, flat foot condition.

A bespoke orthotic ,designed to allow some arch support flex during loading may give better results than a more rigid design . Easy enough to test and probably important to everyday orthotic provision.

In a nutshell , responsive orthotics that allow changes in intrinsic muscle length during fluctuations in COP movement, may be superior to those that don't .

 

Looking at what little research is available, it seems likely that receptors in the intrinsic foot muscles provide the brain with important information with regard to balance .

A new piece of research ,below, demonstrates that the balance apparatus of the inner ear has a controlling link to the intrinsic foot muscles and that this contributes to whole body balance .

So the intrinsics have important sensory and motor roles to play in whole body balance.

Characterizing the vestibular control of balance in the intrinsic foot muscles

Megan Trotman 1, Mathew Ib Debenham 1, Phuong L Ha 1, Nicole Strachan 1, Liam Stewart 1, Evan J Lockyer 2, Jacob Coelho 1, Brian H Dalton 3
Affiliations Expand

• PMID: 39787879
  
• DOI: 10.1016/j.gaitpost.2024.12.026

"Significance: Our findings demonstrate that whole-body vestibular-evoked balance responses were adjusted in response to altered mediolateral stability and head posture, in part, via modification of intrinsic foot muscle activity."

Abstract

Background: To maintain standing balance, vestibular cues are processed and integrated with other sensorimotor signals to produce appropriate motor adjustments. Whole-body vestibular-driven postural responses are context-dependent and transformed based upon head and foot posture. Previous reports indicate the importance of intrinsic foot muscles during standing, but it is unclear how vestibular-driven responses of these muscles are modulated by alterations in stability and head posture.
Research question: The purpose was to investigate the effect of altered mediolateral stability on the modulation of intrinsic foot muscle postural adjustments when driven by vestibular perturbations in anteroposterior and mediolateral directions.
Methods: For experiment 1 (n = 17) and 2 (n = 12), time-domain, vestibular-evoked balance responses to continuous, stochastic electrical vestibular stimulation (EVS) during various foot (narrow, wide, and unipedal) and head yaw postures were assessed for the abductor hallucis (AH), abductor digiti minimi (ADM), medial gastrocnemius (MG), and soleus, as well as ground reaction forces.
Results: With increased mediolateral stance width, AH, ADM, MG, soleus, and whole-body vestibular-evoked balance responses decreased. The AH vestibular-evoked balance response increased for unipedal compared to narrow bipedal stance. The AH vestibular-evoked balance response exhibited an opposite polarity than the ADM when the head was positioned anatomically, indicating that these intrinsic foot muscles function antagonistically towards the summation of whole-body postural adjustments.
Significance: Our findings demonstrate that whole-body vestibular-evoked balance responses were adjusted in response to altered mediolateral stability and head posture, in part, via modification of intrinsic foot muscle activity.

 

In general ,we know that you can improve proprioceptive feedback from muscles by strengthening and or stretching them . Any evidence that stretching the intrinsic foot muscles can improve balance, gait etc ?

Well yes : Even in stroke victims these parameters are improved by stretching the intrinsics . Pretty low hanging fruit , if you ask me !

"Conclusions: This study demonstrated that foot intrinsic muscle dynamic stretching training improved dynamic balance, velocity, step lengths, stride length, and DGI in patients with chronic stroke."
Effects of foot intrinsic muscle dynamic stretching on balance, gait parameters, and dynamic gait index in patients with chronic stroke: A randomized controlled study (CONSORT)

Younghwan Kwag 1 2, Donghwan Park 2
Affiliations Expand

• PMID: 39993133
  
• PMCID: PMC11857030
  
• DOI: 10.1097/MD.0000000000041507

Abstract

Background: Foot intrinsic muscle dynamic stretching intervention can correct balance ability and induce a change in spatiotemporal parameters gait ability. Our objective was to compare the effects of a 4-week program of foot intrinsic muscle dynamic stretching with those of lunge exercise on static balance, dynamic balance, gait parameters (velocity, cadence, step length, and stride length), and the dynamic gait index (DGI) in chronic stroke patients.
Methods: The participants were randomized to either the foot intrinsic muscle dynamic stretching (n = 10) or standard lunge exercise (n = 10) groups. Both groups performed 3 sets of lunge exercises 5 times per week for 4 weeks. Each set comprised 10 repetitions. Static and dynamic balance, gait parameters, and the DGI were measured after 4 weeks of training.
Results: After 4 weeks of training, the foot intrinsic muscle dynamic stretching group showed significant improvement in all outcome measures compared with the baseline (P < .05). Furthermore, timed up and go, velocity, step length, stride length, and DGI showed greater improvement in the foot intrinsic muscle dynamic stretching group than in the standard lunge exercise group (P < .05).
Conclusions: This study demonstrated that foot intrinsic muscle dynamic stretching training improved dynamic balance, velocity, step lengths, stride length, and DGI in patients with chronic stroke.

 

You are right, I agree with you.

 

My research and clinical studies have elucidated the importance of the plantar sensoreceptors in maintaining upright posture during gait and stance phase.

Specifically, Type 1 FA Meissner corpuscules are activated during movement and velocity (gait), sending coded sensory feedback to the cerebellum, where it is decoded. This provides information on the contour of the ground surfaces underneath the feet, facilitating the maintenance of an upright posture during ambulation.

Standing, Type 1 SA Merkels disks are recruited, again providing information on the ground surface (level, slanted), which allows the individual to maintain an upright standing posture, without falling.

Within the past few years, I have unraveled the role gravity-driven pronation plays in the development of postural distortions. Basically, gravity-driven pronation distorts the foot´s sensory feedback to the cerebellum, providing false information (regarding ground surface contour). This results in the cerebellum shifting the body´s position from an erect posture to a bio-imploded posture. This shift in posture frequently leads the patient into chronic, debilitating muscle and joint pain.

Later this year, I will present a series of podcast seminars with Dr. Stephen Barrett, discussing my research and clinical findings.

 

"Moreover, selective anaesthesia of cutaneous afferents of the sole of the foot increases postural sway by only ∼11% (Meyer et al., 2004), whereas increases of ∼40%–60% occur in diabetic neuropathy, in which both muscle and cutaneous afferents are affected (Boucher et al., 1995; Simoneau et al., 1994). Accordingly, it is likely that muscle afferents from the foot contribute more to the control of postural sway than do cutaneous afferents, but in the absence of direct data this is speculation."

 

Knellwolf et al, in their research, conclude that when postural sway occurs, it activates the SA Type II Ruffini corpuscules (which are sensitive to stretch). This sensory information is relayed to the cerebellum.

The control/attenuation of this postural sway is via the recruitment of the intrinsic (and most likely) foot and leg muscles.

But where does all this occur. The cerebellum. This is the control center that regulates posture.
Sensory feedback from the plantar afferents, the muscle spindles (as well as afferents from the eyes, inner ears and occlusion) all provide information about the upright orientation of the body, that is processed in the cerebellum. The cerebellum acting on this information, regulates posture via a global spatial postural coding. However, IMO, the foot´s sensory feedback is the primary source of postural orientation.

Having said all that, this is not my area of research. Specifically, I have linked postural perturbations to the skewing of the foot´s sensory feedback to the cerebellum. Normalizing this sensory feedback reverses postural distortions. Reversing postural distortion, eliminates chronic muscle and joint pain.

 

selective anaesthesia of cutaneous afferents of the sole of the foot increases postural sway by only ∼11% (Meyer et al., 2004),

 

That paper assumes that the foot´s sensory feedback is transmitted to the cerebellum via A-beta fibers. In that case, anesthesia would prevent the transmission of the sensory feedback.

However, what if that was not the case? Brush up on your understanding of quantum entanglement.

 

You have had no involvement in a randomized controlled trial ever ? Never ,ever?

 

I direct your attention to Frederic Viseux publication in Clinical Neurophysiology - The sensory role of the sole of the foot. He emphasizes "the important relationship between the foot sole as a sensory structure and balance maintenance". This paper goes into detail on this subject.

My 40+ years of clinical research replicate Viseux´s detail discussion.

Point of fact: Anaesthetizing the plantar surface of the foot and observing postural sway is not the same as investigating the role of foot´s sensory feedback in dynamic postural maintenance. Entirely two different investigative protocols. That is postural sway (static) is not a barometer of dynamic postural adjustments occurring during ambulation. Different plantar sensory organs are recruited in standing relative to walking.

To date, there have been no double blinded published studies (when the plantar surface has been anesthesized) negating the role the plantar sensory feedback exerts during gait.

I would encourage any readers who have done research in this area, to add their voice to this discussion. Otherwise, my involvement in this discussion has come to an end.