EFFICACY OF HIGH-INTENSITY MECHANICAL STRETCHING DEVICES IN
TREATING ANKLE STIFFNESS: A RETROSPECTIVE REVIEW
Samantha J Beckley PhDa, Shaun K Stinton PhDa, Maha Karim BSa, Thomas P Branch MDb
a ArthroResearch LLC, 441 Armour Place NE, Atlanta, GA, 30324, United States
b Ermi LLC, 2872 Woodcock Blvd. Suite 100, Atlanta, GA, 30341, United States
Address for correspondence:
Samantha Beckley, 441 Armour Place NE, Atlanta, GA, 30324, USA
Phone: +1 404-579-1546
Email: s.beckley@arthroresearch.com.
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Abstract:
Background: Ankle joint dorsiflexory stiffness is commonly addressed with physical therapy.
When patients fail to meet clinician-set goals, surgical intervention is often considered.
Mechanical therapy using a high-intensity stretch (HIS) device offers a non-operative alternative.
This study evaluated the efficacy of an HIS device in treating ankle joint dorsiflexory stiffness.
Methods: This retrospective study analyzed 263 patients prescribed an HIS device after
plateauing in dorsiflexion recovery despite at least four weeks of standard treatment. Range of
motion (ROM) measurements, obtained via the ErmiMotion app, included dorsiflexion angles in
straight and bent leg positions at baseline and follow-up. Paired t-tests were used to compare
initial and final ROM values, with significance set at α < 0.05.
Results: In 75 patients with complete data, final dorsiflexion ROM significantly improved in
both straight (12.5° ± 10.2° vs. 4.0° ± 10.8°; p < 0.001) and bent leg positions (15.0° ± 10.8° vs.
8.1° ± 11.7°; p < 0.001). Effect sizes were large (Cohen’s d = 0.86 and 0.80, respectively).
Patients with severe motion loss showed marked improvement (straight: 7.3° ± 11.4° vs. -7.7° ±
6.5°, p < 0.001; bent: 9.6° ± 10.3° vs. -9.1° ± 10.7°, p = 0.001), with very large effect sizes
(Cohen’s d = 1.52 and 3.06, respectively).
Conclusions: The HIS device effectively improved dorsiflexion ROM in patients with ankle
stiffness, including severe cases. Restoring ROM enhances quality of life, supports timely return
to work, and reduces costs. Therefore, HIS devices should be considered for patients
unresponsive to standard treatments.
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Keywords: Ankle; Adhesive Capsulitis; Motion Loss; Stiffness, Mechanical Therapy;
Non-operative Treatment
Background
Ankle joint stiffness as a result of arthrofibrosis is most commonly caused by adhesions,
scarring and contracture of the joint capsule and ligaments as a result of trauma, surgery or
infection 1,2
. Arthrofibrosis results from excessive scar tissue formation within, as well as around
the joint, leading to severe motion loss, while in contrast, ankylosis involves the fusion or near
fusion of joint structures, resulting in essentially complete motion loss 3,4
. Although ankle
arthrofibrosis has been reported to be common by some, there are a limited number of studies
available investigating its etiology and prevalence. One study reported a frequency of
arthrofibrosis in the ankle joint after open reduction and internal fixation repair of ankle fractures
as high as 73% 5
. Others have reported lower frequencies between 22% and 40% following ankle
arthroplasty or arthroscopy, respectively 6,7
.
Past literature has defined two forms of ankle joint stiffness (or arthrofibrosis) as extra-
and intra-articular. Causes of intra-articular ankle stiffness include adhesive capsulitis and
intra-articular fractures, which affect both ankle dorsi- and plantarflexion 8
. Extra-articular ankle
stiffness occurs in the tissues surrounding the ankle due to conditions such as soft tissue
infections, and muscle and ligament injuries which can result in excessive scarring, and/or
contraction. Depending on which structures are affected, it is possible that only one motion,
either dorsi- or plantarflexion will primarily be affected 8,9
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Typically, patients with ankle joint stiffness are started in physical therapy. The focus of
therapy is to improve range of motion (ROM) through a series of stretching exercises which
increase in intensity. Dynamic splinting with low intensity and prolonged stretching is an
additional option, although there is limited research demonstrating the success of this form of
treatment 8
. Another conservative treatment option is adjunctive use of intra-articular
corticosteroid injections. Joint manipulation under anesthesia (MUA) a medical procedure that
usually requires general anesthesia, is a more aggressive option 10
. Both corticosteroid steroid
injections and MUA have reported low success rates and/or little supporting data 10–12
.
Surgical intervention is considered when a non-operative program fails to achieve patient
goals, both in terms of total motion and speed of recovery. These options include tendon
lengthening, arthroscopy for lysis of adhesions, or an open surgery for osteophyte resection.
While these procedures may increase ankle range of motion or position the joint for improved
dorsiflexion initially, inherent ankle joint stiffness may recur. Furthermore, the potential
complications of surgical intervention which include infection, articular cartilage damage and
ligament instability may change the function of the ankle resulting in significant morbidity 4,13
.
The use of at-home mechanical therapy with a high intensity stretch (HIS), is an
additional conservative treatment option. The usage of a HIS device allows for the maximization
of the total end-range time (TERT) formula allowing for a shorter duration of use in comparison
to the low load devices. The TERT formula is based on the product of intensity, duration and
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frequency 14
. Proper utilization of the TERT formula has been shown to permanently elongate
scar tissue to regain joint range of motion 15
.
To date, there are no other studies documenting the use of a HIS device to treat ankle
motion loss, but there are a number of other studies reporting the successful treatment of motion
loss in the knee and shoulder joints 16–19
.
Therefore, the purpose of this study was to evaluate the efficacy of a HIS device in
treating ankle joint motion loss due to stiffness. We hypothesized that a HIS device would
effectively address motion loss, thereby aiding patients in restoring functional ROM.
Methods
Study Population
This study was exempt from institutional review board review and a waiver of
authorization was granted prior to initiating the study. Records for 263 patients who were all
prescribed a HIS device (the Ermi Ankle Flexionater+; Figure 1) for range of motion recovery
from motion loss due to joint stiffness (ankle or achilles contracture) between October 2021 and
May 2023 were retrospectively reviewed from an internal database. Patients included for
analyses in the study had two or more dorsiflexion photo documented measurements recorded
from an app used to document patient progress (ErmiMotion). Other information available for
patients in the internal database included dates of measurements, device drop-off and pickup
dates, medical insurance information, doctor and physical therapy notes (including ICD-10
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codes). Other inclusion criteria required that the initial measurement be taken within 30 days
from the delivery of the device and the second measurement at least 14 days after the initial
measurement. Patients diagnosed with the inflammatory conditions, plantar fasciitis and Achilles
tendinopathy were excluded from the study.
HIS Device Protocol
Patients were typically prescribed a HIS device after reaching a plateau in their motion
recovery after at least 4 weeks of treatment with a standard protocol of physical therapy. These
patients were likely unable to meet their ankle dorsiflexion goals that were set by the treating
clinician based on the injury and/or surgical procedure, contralateral ankle dorsiflexion, age and
sex of the patient. Typically, the HIS device was used in conjunction with physical therapy
treatment.
Patients used the HIS device for dorsiflexion stretching of the ankle joint. The device was
prescribed to be used three times per day during which the patient would stretch the ankle into
maximum tolerable dorsiflexion using the hydraulic system for 10 minutes, followed by 10
minutes of resting and another 10 minutes of stretching. During the stretch periods the patients
were instructed to stretch the ankle to the point of discomfort just below the pain threshold.
ErmiMotion Measurements
The ROM measurements were determined from patient photographs that were captured
and uploaded to an internal application. Photographs were taken of the patient’s ankle while
stretching to the end ROM during straight and/or bent leg dorsiflexion using the HIS device. The column 2-->
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patient was required to wear no shoes and have their feet flat on the footplate of the device in the
image. Photographs were taken at the level of the ankle joint and the dorsiflexion angle was
measured using the three points: i) fibular head bony marker, ii) intersection of the lateral
malleolus to the heel and iii) the 5
th metatarsal (Figure 2).
Outcomes of interest
The outcomes used for the purpose of the study included straight and bent leg
dorsiflexion and the number of days between the first and last measurement taken.
Statistical analysis
The Shapiro-Wilk test was used to determine whether the data was normally distributed.
Paired t-tests used to compare initial and final dorsiflexion ROM measurements for straight- and
bent- leg dorsiflexion. In datasets that were normally distributed, results were presented as mean
and standard deviation, and if not normally distributed, as median and interquartile ranges. The
significance was set at α < 0.05. The GraphPad Prism software (version 10.1.1) was used for all
statistical analyses.
Results
After excluding patients that had unmeasurable photographs, fewer than two dorsiflexion
measurements within the required timeframe and inflammatory conditions, a total of 75 patients
were included in the analysis. On average, patients were 47.6 ± 11.3 years old, 55.4% were male
and 50.0% were treating the left ankle. Most of the patients used Veterans’ Affairs (VA; 51.4%)
or Workers’ Compensation (45.9%) health insurance and only 2.7% were self-paying patients.
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The most common conditions treated based on patients’ ICD-10 codes included joint stiffness
(61.1%), plantar fibromatosis (8.2%), fractures (8.2%), sprains (8.2%) and strains (2.7%).
Comparing pre-and post-treatment ROM demonstrated that on average, the patients
gained 8.5° ± 9.7° and 7.0° ± 8.7° dorsiflexion in the straight and bent leg positions, respectively,
over a period of 97 ± 69 days. The final dorsiflexion ROM measurements (straight leg 12.5° ±
10.2°; bent leg: 15.0° ± 10.8°) had significantly improved after using the HIS device compared
to the initial ROM dorsiflexion values for both the straight leg (4.0° ± 10.8°; p < 0.001; Figure
2A) as well as bent leg (8.1° ± 11.7°; p < 0.001; Figure 2B) positions. Effect sizes were
considered to be large when comparing the pre- and post-treatment measurements for straight
(Cohen’s d = 0.86) and bent leg (Cohen’s d = 0.80) positions.
In patients exhibiting severe motion loss with initial dorsiflexion less than 0°, significant
improvements in ROM were observed. The average ROM gains were substantial, with increases
of 15.0° (n = 24) for straight leg measurements and 18.7° (n = 13) for bent leg measurements
over an average time of 95 ± 63 days. Post-treatment, the final ROM measurements (straight leg:
7.3° ± 11.4°; bent leg: 9.6° ± 10.3°; p < 0.001) were markedly greater than the initial values
(straight leg: -7.7° ± 6.5°; bent leg: -9.1° ± 10.7°; p = 0.001). These improvements correspond to
large effect sizes, as indicated by Cohen’s d values of 1.52 for straight leg and 3.06 for bent leg
measurements.
Discussion
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The most important finding of this study is dorsiflexion ROM values significantly
improved after treatment using the HIS device in patients who were failing to achieve ROM
improvement goals using a standard course of physical therapy for at least four weeks.
On average, most patients achieved a final ROM of at least 12° dorsiflexion, starting
from an average of 4°. Regaining ankle dorsiflexion ROM is crucial for returning to functional
activities such as walking without pain and disability, as well as reducing the risk of ankle
reinjury 20
. While the normal amount of ankle dorsiflexion in healthy ankles is 20°, the minimum
dorsiflexion ROM required for normal gait on a flat surface is 10° 21,22
. Studies have shown that
ankle dorsiflexion ROM is vital for physical performance, especially in older adults, as it impacts
the risk of falling 23–25
. Therefore, the observed increase in ROM likely significantly enhanced
patients' quality of life. Overall, 63% of patients achieved the ROM necessary for walking on a
flat surface. Among the 18 patients who had not yet reached 10° dorsiflexion, 14 (78%) had their
final measurements taken before completing their treatment. Thus, it is likely that dorsiflexion
would continue to improve until the end of the treatment period. This study also demonstrated
substantial improvements in patients with severe motion loss (<0° dorsiflexion at the start of
treatment), with most gaining at least 15° of ROM, compared to the overall population, which
typically increased dorsiflexion ROM by at least 7°. Interestingly, the average duration of HIS
device usage was similar for patients with severe ROM loss (95 ± 63 days) and the overall
population (97 ± 69 days), suggesting that HIS device usage may accelerate improvements in
patients with severe ROM deficits.
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The effect size of this treatment method is higher than other reported conservative
treatments methods investigated for ROM recovery after an ankle sprain. Manual therapy was
reported to have a small (Cohen’s d = 0.09) to moderate (Cohen’s d = 0.45) effect size when
comparing pre- and post-treatment dorsiflexion measurements 20
. While the HIS device used in
this study was used to treat a number of different injuries, these values suggest that high intensity
mechanical stretching is an effective form of conservative treatment for ankle ROM loss.
A recent study demonstrated that Worker’s Compensation patients undergoing common
ankle and foot procedures were more likely to have postoperative complications and secondary
surgeries 26
. This study showed the HIS device was an effective form of treatment amongst
Worker’s Compensation patients to restore ankle dorsiflexion ROM. Ensuring ROM is restored
in the ankle may lower the risk of a secondary surgery and the associated costs and
complications.
A limitation of this study is that patient information, including diagnoses and potential
comorbidities, conditions or additional procedures that may affect the outcomes may not have
been available in the database. For example, osteoarthritis and the presence of osteophytes, and
other chronic inflammatory or neurological conditions and/or other injuries may affect ROM
measurements 27,28
.
This study demonstrated that a HIS device effectively improved dorsiflexion ROM in
patients with ankle stiffness, including those with severe motion loss. Restoring ROM is
essential for improving quality of life, facilitating return to work, and reducing healthcare costs.
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These findings support the use of HIS devices as an effective treatment option for patients not
responding to standard protocols.
List of abbreviations
HIS High intensity stretch
MUA Manipulation under anesthesia
ROM Range of motion
TERT Total end-range time
VA Veterans’ Association
Declarations
Ethics approval and consent to participate
This retrospective study was determined to be exempt from institutional review board oversight
and a waiver of authorization was granted.
Availability of data and materials
The datasets analyzed during the current study are not publicly available due to their ownership
by a private company but are available from the corresponding author on reasonable request.
Competing interests
TB is a paid employee of and has stock ownership in Ermi LLC.
Funding
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Ermi LLC paid expenses related to data analysis and manuscript preparation and submission.
Acknowledgements
The authors would like to thank Alicia Salamani for her assistance in data collection.
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Figure Legends
Figure 1: The Ermi Ankle Flexionater+ set up to stretch ankle dorsiflexion in the straight leg
position. The device can also ankle plantarflexion (not pictured).
Figure 2: Example images of the measurement of ankle dorsiflexion in the A) straight and B)
bent leg positions.
Figure 3: Dorsiflexion range of motion measurements for patients pre- and post-treatment in the
A) straight, and B) bent-leg positions.
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