Effects of short foot exercises with ultrasound bio-feedback on motor learning and foot alignment: A double blinded randomized control trial

Abstract

BACKGROUND:

Short foot exercises (SFE) take a long time to master and require a feedback tool to improve motor learning.

OBJECTIVE:

This study aimed to investigate the effect of bio-feedback of talonavicular joint movements in learning SFE with ultrasound (US) imaging.

METHODS:

This study included thirty-one healthy volunteers and was designed as a double-blind randomized control trial. Subjects were randomly assigned to one of two groups: the control group, which performed SFE under verbal instruction, and the US bio-feedback (USBF) group, which performed SFE with real-time bio-feedback of the talonavicular joint alignment. All subjects underwent two sessions of 5 minutes each, and SFE was performed as a self-exercise, between sessions, for one week. The difference in foot length and navicular height were assessed at baseline, after Session 1, before Session 2, and one week after Session 2. These differences were compared between the two groups using the Mann-Whitney U test.

RESULTS:

In terms of navicular height change, the USBF group (7.5 $\pm$ 4.3 mm) was significantly higher than the control group (4.2 $\pm$ 3.3 mm) one week after session 2 ( $p=$ 0.04, effect size $=$ 0.86).

CONCLUSION:

SFE with USBF is an effective intervention for performing SFE.

Keywords

Foot exercise ultrasonography motor learning bio-feedback

1. Introduction

The function of intrinsic foot muscles is critical in the maintenance of the foot arch [1]. Short foot exercises (SFE) [2], which raise the arch and shorten the length of the foot without toe flexion, are commonly used in physiotherapy settings to strengthen intrinsic muscles and to improve the navicular drop [3, 4], foot position sense [5] and dynamic balance [6]. In especially, the SFE need more than 5weeks to demonstrate the significantly effects on the foot alignment [7]. Since SFE is a motor control task involving selective contraction intrinsic foot muscles, it is difficult to perform SFE and therefore it is important to instruct appropriate exercises. However, in many cases, SFE is difficult to perform without compensation. Therefore, it may have a possibility to improve the foot function such as an alignment in shortly with accurate performing the SFE.

Feedback is important for improving motor skills, and methods of feedback involving bio-signals such as electromyography (EMG) have been well established. Okamura [8] used EMG bio-feedback to perform SFE, while Kim et al. [9] gave feedback using a special air cushion. However, their methods are complicated, time-consuming, and inaccessible in clinical practice. Additionally, instructing SFE with EMG bio-feedback presents an additional challenge in terms of determining which muscle activity to feedback as well as in coordinating muscle activity. This is because SFE shows variable activity in the intrinsic foot muscles between subjects [10]. Therefore, the effective way of SFE feedback in the clinical setting remind unclear.

Recently, ultrasound imaging was used to demonstrate bio-feedback of the pelvic floor and multifidus muscles and has been reported to be effective for the same [11]. Additionally, ultrasound imaging can provide feedback to both bones and muscles. When the arch of the foot collapses due to weight bearing, the abduction movement of the talonavicular joint has been reported to increase in flat feet [12]. The muscle activity during SFE has also been found to vary [10], however, the navicular is always adducted relative to the talus during SFE. Therefore, we have instructed SFE with biofeedback focused on talonavicular joint alignment. We hypothesized that ultrasound bio-feedback of the bony movements of the talonavicular joint would improve SFE at an early stage and be useful for motor learning. This study aimed to investigate the effect of bio-feedback of the talonavicular joint movements on learning SFE and foot alignment using ultrasound imaging.

2. Methods

2.1 Design

This double-blind randomized control trial was conducted between October and December 2022, after review by the Research Ethics Review Subcommittee of the Academic Research Committee of Morinomiya University of Medical Sciences (2022–20). The study protocol was registered in advance with the Clinical Trials Registry System of the Medical Information Centre of the University Hospital (UMIN000049153). Allocation ratio set 1(control):1(intervention). There was no change of the trial information after trial commencement.

2.2 Subjects

This study included thirty-one healthy volunteers who gave consent and participated. Exclusion criteria included patients with a history of surgery due to trauma to the ankle and/or foot, those with any pain in the lower limb at the time of testing, and those who had difficulty with ultrasound imaging due to os tibiale externum or deformity. Moreover, subjects who suffered from the diabetic neuropathy and other peripheral nerve neuropathy were also excluded. Subjects were enrolled by two physical therapy students.

A sample size calculation (G*Power 3.1.9.2) based on the change of the foot length during SFE at the retention session (using $\alpha$ : .05, power: 95%) was performed in a pilot study (the change of foot length; with mean (standard deviation (SD)) $=$ 8.8 $\pm$ 3.4 mm in SFE group and 4.2 $\pm$ 2.6 mm in the control group). Based on the results, a total of 22 subjects were required to adequately power the study for variables of interest. Therefore, to account for potential dropouts (estimated at 25%) 31 participated were recruited for the study. All test had been performed at the motor function laboratory in the Morinomiya university of medical sciences.

Figure 1.

CONSORT 2010 flow diagram.

2.3 Procedure

Subjects performed SFE while seated on a bench, with the goal of pulling the head of the metatarsal towards the calcaneus to produce maximal contraction of the intrinsic foot muscles without bending the distal phalanx (Fig. 1). The forefoot or plantar surface was lifted while carefully controlled, held for 5 seconds, and then relaxed for 5 seconds [9].

Subjects were randomly assigned into one of two groups. The control group was defined as the group that performed SFE under verbal instruction after viewing an instruction movie on SFE. The US bio-feedback group (USBF group) was defined as the group that performed SFE with real-time bio-feedback of talonavicular joint alignment via ultrasound images. A random number table was prepared in advance for randomization, and the tester used “the box-substitution method.” The intervention and evaluation were carried out by different people, while randomization is performed by the interventionist. The subjects did not know which group they were assigned to, and measurements were taken by the tester who did not know which group the subjects were in either. Therapist who performed both interventions could not be blinded.

Figure 2.

Ultrasound-guided short foot exercise and ultrasound imaging of the talonavicular joint. The line transducer was placed on the medial side of the foot (a). The navicular (Nav), talus head, and sustentaculum tali (ST) were drawn (b). The talus was moved to the lateral side (c).

SFE was performed for 5 minutes (Session 1) in both groups [13], and changes in foot length and navicular height during SFE were measured at 0, 2, and 5 minutes after the session. After the end of Session 1, self-SFE was performed during the week, and one week after Session 1, the same intervention as Session 1 was performed (Session 2), moreover, the next week was designed for the duration all subjects had not performed the SFE.

Main outcome is change of both foot length and navicular height during SFE, and secondly outcome is foot alignment. Foot alignment was assessed after the end of Session 1, prior to the start of Session 2, and one week after the end of the sessions. Foot posture index-6item version (FPI-6) [14], arch height index (AHI) [15], and arch height flexibility (AHF) [16] were measured as the outcome of foot alignment. Photographs of the medial foot in the 10% and 50% loading positions were captured with a digital camera (Canon IOS X7, canon co, Japan) at 1 m from the subject’s foot. Markers were fixed to the first metatarsal head. The distance from the hallux to the heel was measured as foot length, and the dorsum height (DH) at 50% of the foot length was divided by the truncated foot length. The AHF was calculated by dividing the DH at the 10% and 50% loading positions by 40% of body weight. Measurements were taken using ImageJ (NIH). The foot length and navicular height during the SFE were also measured in the same way. FPI-6 was measured by one physiotherapist in accordance with the manual of the previous study [14]. Outcomes were no change after the trial commenced.

2.4 Analysis

Baseline data were compared between the two groups using the Shapiro-Wilk test, along with the unpaired $t$ -test for items that can be assumed to be normally distributed, and the Mann-Whitney U test for items that cannot be assumed to be normally distributed.

The percent of changes in navicular height and foot length divided own foot length (%Change-navicular height, %Change foot length) during SFE were defined as the difference between the respective values at rest, at the start of exercise (0 minutes), 2 minutes after, and 5 min after SFE. A two-way analysis of variance was performed with the intervention method and time as factors, respectively. Moreover, difference the Change of navicular and foot length during SFE and %Change navicular height and foot length between two groups assessed using unpaired $t$ -test at before intervention, after Session 1, before Session 2, and retention phase.

Changes in alignment due to SFE were compared between the two groups for each parameter at the end of Session 1, before the start of Session 2, and one week later. The results of the Shapiro-Wilk test were used as the statistical method for the comparisons, with an uncorrelated $t$ -test for items that could be assumed to be normally distributed, and a Mann-Whitney U test for items that could not be assumed to be normally distributed. The effect size was interpreted using the scheme proposed by Cohen: $<$ 0.2 equates to a trivial ES; 0.2–0.49, small; 0.5–0.79, moderate; and $>$ 0.8, large.

Table 1
General characteristic data

	Control	USBF	$p$ -value	E.S.
Age	20.5 $\pm$ 0.5	20.1 $\pm$ 0.5	0.06	0.76
Height (cm)	168.4 $\pm$ 7.9	165.2 $\pm$ 6.9	0.28	0.44
Weight (kg)	58.5 $\pm$ 7.4	54.5 $\pm$ 6.2	0.16	0.58
Foot length (cm)	23.9 $\pm$ 1.7	23.6 $\pm$ 1.7	0.67	0.17
Dorsum height (cm)	6.8 $\pm$ 0.3	6.4 $\pm$ 0.4	0.03	0.93
Foot Posture Index-6 (points)	2.4 $\pm$ 1.9	2.3 $\pm$ 1.8	0.92	0.04
Arch height index (10%)	0.38 $\pm$ 0.03	0.36 $\pm$ 0.02	0.08	0.73
(50%)	0.36 $\pm$ 0.02	0.34 $\pm$ 0.02	0.03	0.91
Arch height flexibility	1.10 $\pm$ 0.36	1.32 $\pm$ 0.72	0.34	0.39

USBF: ultrasound bio-feedback group, E.S.: effect size.

Figure 3.

Changes in foot length and navicular height during short foot exercise in Session 1. Changes in the foot length and navicular height during short foot exercises (SFE) were significantly higher in the ultrasound bio-feedback group (solid line) than that of the control group (dash line). The phase and interaction had no significant difference in both parameters.

3. Results

Five subjects who had difficulty with ultrasound imaging due to os tibiale externum were excluded, while twenty-six subjects completed the study (Fig. 2). General characteristic data for the control and USBF groups are listed in Table 1. Thirteen subjects were allocated in each group. And subject recruitment is finished, because filling the number of subjects. There were no significant differences in age, height, and weight between the two groups. As for parameters indicating the structure of the medial longitudinal arch of the foot, there were significant differences in DH and AHI while standing, but no significant differences in other parameters. And all subject did not have any important harms or unintended effects.

In the control group, the %change foot length of 1.9 $\pm$ 1.5%, 1.9 $\pm$ 2.3%, and 2.1 $\pm$ 1.2% at 0, 2, and 5 minutes, respectively. In contrast, the USBF group demonstrated the % change foot length of 1.9 $\pm$ 2.3%, 3.2 $\pm$ 2.0%, and 3.6 $\pm$ 2.2% at 0, 2, and 5 minutes, respectively. A main effect ( $p=$ 0.02, ES $=$ 0.69), no phase ( $p=$ 0.23, ES $=$ 0.31), and interaction ( $p=$ 0.24, ES $=$ 0.30) were observed between groups (Fig. 3).

The control group demonstrated the % change navicular height of 1.3 $\pm$ 1.0%, 1.1 $\pm$ 1.1%, and 1.5 $\pm$ 1.5% at 0, 2, and 5 minutes, respectively. In contrast, the USBF group demonstrated the % change navicular height of 1.8 $\pm$ 1.0%, 2.8 $\pm$ 1.4%, and 2.9 $\pm$ 1.8 mm at 0, 2, and 5 minutes. A main effect ( $p<$ 0.001, ES $=$ 0.97), no phase ( $p=$ 0.23, ES $=$ 0.31), and interaction ( $p=$ 0.23, ES $=$ 0.31) were observed between groups (Fig. 3).

Table 2
Change of navicular height and foot length during the short foot exercise

	Control	USBF	$p$ -value	E.S.
Change of navicular height (mm)
Before	3.3 $\pm$ 2.5	4.3 $\pm$ 2.4	0.31	0.41
After session1	3.7 $\pm$ 3.7	6.7 $\pm$ 4.2	0.06	0.76
Before session2	3.2 $\pm$ 3.8	7.1 $\pm$ 3.5	0.01	1.07
Retention	4.2 $\pm$ 3.3	7.5 $\pm$ 4.3	0.04	0.86
% Change of navicular height (%)
Before	1.3 $\pm$ 1.0	1.8 $\pm$ 1.0	0.24	0.48
After session1	1.5 $\pm$ 1.5	2.9 $\pm$ 1.8	0.04	0.84
Before session2	13 $\pm$ 15	3.0 $\pm$ 16	$<$ 0.01	1.11
Retention	1.8 $\pm$ 1.3	3.2 $\pm$ 1.8	0.03	0.89
Change of foot length (mm)
Before	4.6 $\pm$ 3.6	4.5 $\pm$ 5.9	0.96	0.02
After session1	5.0 $\pm$ 3.1	8.6 $\pm$ 5.4	0.05	0.83
Before session2	6.2 $\pm$ 4.7	9.1 $\pm$ 3.7	0.10	0.67
Retention	7.0 $\pm$ 5.0	8.9 $\pm$ 4.9	0.33	0.39
% Change of foot length (%)
Before	1.9 $\pm$ 1.5	1.9 $\pm$ 2.3	0.97	0.02
After session1	2.1 $\pm$ 1.2	3.7 $\pm$ 2.3	0.03	0.83
Before session2	2.5 $\pm$ 1.8	3.9 $\pm$ 17	0.07	0.74
Retention	2.9 $\pm$ 2.0	3.8 $\pm$ 2.1	0.26	0.39

USBF: ultrasound bio-feedback group, E.S.: effect size.

Table 3

Outcome of the foot alignment

	Control	USBF	$p$ -value	E.S.
After session1
AHI sitting	0.38 $\pm$ 0.02	0.37 $\pm$ 0.02	0.08	0.73
Change ratio-AHI (10%)	0.82 $\pm$ 1.80	0.94 $\pm$ 2.71	0.89	0.05
AHI standing	0.36 $\pm$ 0.02	0.35 $\pm$ 0.02	0.10	0.67
Change ratio-AHI (50%)	$-$ 0.51 $\pm$ 1.71	0.64 $\pm$ 2.52	0.19	0.53
AHF	1.45 $\pm$ 0.42	1.31 $\pm$ 0.45	0.41	0.33
Before session2
AHI sitting	0.38 $\pm$ 0.03	0.36 $\pm$ 0.02	0.04	0.87
Change ratio-AHI (10%)	$-$ 0.23 $\pm$ 2.67	$-$ 0.81 $\pm$ 2.20	0.55	0.24
AHI standing	0.60 $\pm$ 0.86	0.34 $\pm$ 0.02	0.29	0.43
Change ratio-AHI (50%)	0.49 $\pm$ 2.70	0.21 $\pm$ 3.33	0.33	0.39
AHF	1.02 $\pm$ 0.68	1.16 $\pm$ 0.61	0.32	0.40
Retention
AHI sitting	0.38 $\pm$ 0.02	0.36 $\pm$ 0.02	0.05	0.83
Change ratio-AHI (10%)	0.32 $\pm$ 3.89	0.20 $\pm$ 2.47	0.92	0.04
AHI standing	0.36 $\pm$ 0.02	0.35 $\pm$ 0.02	0.05	0.83
Change ratio-AHI (50%)	0.35 $\pm$ 3.00	0.84 $\pm$ 3.72	0.72	0.14
AHF	1.15 $\pm$ 0.38	1.21 $\pm$ 0.80	0.80	0.10

USBF: ultrasound bio-feedback group, E.S.: effect size, AHI; arch height index, AHF; Arch height flexibility.

The USBF group had a significantly higher navicular height change and the % change navicular height as compared to the control group after Session 1, before Session 2, and one week later. This group also had a significantly larger change in foot length and the %change foot length than the control group after Session 1 (Table 2). On the other hand, parameters indicative of foot alignments, such as AHI, AHF, and FPI-6, showed no significant differences at any time (Table 3).

4. Discussion

This study was designed to evaluate an ultrasound imaging bio-feedback of talonavicular joint movements on motor learning of SFEs and foot alignment. The findings showed a significant change, as compared to the control group, in foot length and arch height in the USBF group after Session 1, and a larger change both before Session 2 and one week after Session 2. Measurements after Session 1, before Session 2, and one week after the sessions were defined as the immediate effect, self-training effect, and retention effect, respectively. These results suggest that ultrasound imaging bio-feedback of the talonavicular joint is beneficial for the acquisition of SFE movements. Conversely, no significant effect on alignment during loading was observed. In other words, ultrasound imaging bio-feedback of the talonavicular joint was effective to accurately perform SFE, but did not affect foot alignment.

Many previous studies have shown the time to acquire motor control in SFE was more than 20 minutes of the intervention time [6, 8, 17]. Okamura et al. also reported that the change in foot length after 5 min of SFE was 1.5 $\pm$ 2.6 mm with EMG bio-feedback and 1.6 $\pm$ 3.1 mm with a combination of electrical stimulation and EMG bio-feedback [13]. The SFE are not easy and are motor tasks that are the result of acquired learning. Thus, biofeedback is effective for the accurate practice of SFE, but there are very few reports on this. In other words, the ultrasound bio-feedback used in this study could result in large foot length changes in a short time. Therefore, this is the first study that the ultrasound bio-feedback of the talonavicular joint alignment is effective methods to accurately perform SFE in short time. In addition, Hara et al. reported that the alignment changes require more than 5 weeks of intervention [7]. However, this 5 week result is based on the inclusion of the possibility that SFE is not being performed properly. With appropriate intervention, this 5 week period could be reduced. In the present study, we hypothesized that correct intervention could shorten the intervention period, although, unfortunately, it did not lead to changes in alignment in the loading position at 2 weeks.

The use of ultrasound imaging as a visual feedback tool during motor control exercises may be more effective than verbal or tactile feedback with increasing muscle thickness, muscle activity, and target exercise success [11, 18, 19]. However muscular activity during SFE is intricate and highly variable [10]. As a result, while ultrasonography has been employed to provide feedback on muscle activity and muscle thickness during SFE, it has been unclear which muscle activity to feedback. In this study, we used the talonavicular joint movement, the key joint in the foot when the arch collapses, as biofeedback. Talonavicular joint can be demonstrated complex movement on the plantar flexion, inversion and adduction when increase the medial longitudinal arch. The plantar intrinsic foot muscle has a role of plantar flexion, and tibialis posterior play a role of the inversion and adduction in talonavicular joint. However, these motions take place on composite axes of motion and never result in a single plane motion [20]. Therefore, the ultrasound biofeedback of the talonavicular joint is help to accurately performed SFE in short time, because motor task of the adducted the talonavicular joint in this study led to recruit the many of muscle around foot and ankle.

There are some limitations in this study. SFE did not influence the arch structure with loading conditions. It has also necessary to increase the load of the exercise or training duration. Second, the subjects who participated in this study had a normal arch structure. Subjects with flatfoot deformities may not show the same results as in the current study. Third, it was unclear which intrinsic foot muscle had increased activity. Further research is required to determine the muscle activity of both intrinsic and extrinsic foot muscles during SFE with ultrasound bio-feedback.

5. Conclusion

Ultrasound bio-feedback of the talonavicular joint motion during SFE has the potential to shorten the practice session while increasing retention effects. These findings could have implications for the use of intrinsic foot muscle exercises in physiotherapy for foot and ankle disorders. However, the subjects who performed SFE well did not have any changes in the foot alignment. Therefore, it is necessary to develop exercises that can increase intrinsic foot muscle activities and improve foot alignment.

Author contributions

Organizing the study: SK; Experiment execution: MH, KH; Statistical processing: SK; Writing of the paper: SK, SH; Reviewing: SK, SH.

Data availability statement

Data are available from the corresponding author on reasonable request due to privacy/ethical restrictions.

Ethics statement

The study was approved by the Research Ethics Review Subcommittee of the Academic Research Committee of Morinomiya University of Medical Sciences (2022–20). The study protocol was registered in advance with the Clinical Trials Registry System of the Medical Information Centre of the University Hospital. (UMIN000049153).

Founding

The authors report no funding.

Informed consent

The subjects were informed of the study in paper form and their consent was obtained.

Footnotes

Acknowledgments

We would like to thank the subjects for their cooperation in the study.

Conflict of interest

The authors declare that they have no conflict of interest.

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