Surface electromyography signal characteristics of lower limb muscles in children with unilateral developmental dysplasia of the hip

Abstract

BACKGROUND

Closed reduction and Salter osteotomy are commonly used to treat children with developmental dysplasia of the hip (DDH). However, it remains unclear whether the muscle activity of the lower extremities returns to normal or shows compensation and imbalance after surgery.

AIM

To evaluate changes in surface electromyography (EMG) signals of the main muscles in both lower limbs of children with unilateral DDH during standing and walking after closed reduction surgery or Salter osteotomy. We also aimed to quantify the differences in bilateral muscle activation and explore the characteristics of surface EMG signals of the muscles around the hip joint and the postoperative rehabilitation effects.

METHODS

The retrospective analysis was conducted on 59 children with unilateral DDH who received treatment in the pediatric orthopedics department of a single tertiary grade A hospital. Twenty-eight children underwent closed reduction and 31 children underwent Salter osteotomy combined with femoral shortening and rotational correction osteotomy. Surface EMG was used to compare the average root mean square (RMS) values, coordination ratio, and the ratio of the RMS values between the affected and healthy sides in the bilateral tensor fasciae latae, rectus femoris, medial hamstring, anterior tibialis, medial gastrocnemius, and gluteus maximus muscles during standing and walking at the final follow-up. Outcomes were further evaluated using the Harris hip score and the modified Severin classification.

RESULTS

The average follow-up period was 51.64 ± 34.85 months for the closed reduction group and 51.0 ± 34.76 months for the Salter osteotomy group. The average Harris score was 95.91 ± 1.01 in the Salter osteotomy group, which was lower than that of the closed reduction group (98.84 ± 0.82; P < 0.05). During walking, compared with the closed reduction group, the Salter osteotomy group showed significantly higher mean RMS values in the tensor fasciae latae on both the affected and healthy sides and in the rectus femoris on the affected side (P < 0.05). In addition, the synergistic contraction between the rectus femoris muscle and hamstring muscles was significantly increased (P < 0.05).

CONCLUSION

At the mid-term follow-up after surgery, the closed reduction and Salter osteotomy groups both achieved good functional scores and imaging results. However, during walking, muscle electrical activity of the tensor fasciae latae and rectus femoris on the affected side was increased in the Salter osteotomy group, suggesting that targeted postoperative rehabilitation exercises are warranted.

Key Words: Developmental hip dysplasia; Closed reduction; Salter osteotomy; Surface electromyography; Standing; Walking

Core Tip: Closed reduction and Salter osteotomy are commonly used to treat children with developmental dysplasia of the hip. However, it remains unclear whether the muscle activity of the lower extremities returns to normal or shows compensation and imbalance after surgery. The aim of this study was to evaluate the changes in surface electromyography signals of the main muscles in both lower limbs of children with unilateral developmental dysplasia of the hip during standing and walking after closed reduction surgery or Salter osteotomy. We also aimed to quantify the differences in bilateral muscle activation and explore the characteristics of surface electromyography signals of the muscles around the hip joint and the postoperative rehabilitation effects.

INTRODUCTION

Developmental dysplasia of the hip (DDH) is a common hip disorder in children, characterized by abnormal positions or shapes of the acetabulum and femoral head, which can occur at birth or during the developmental process. DDH is a significant cause of lower limb disability in children and hip osteoarthritis in adulthood, affecting 1%-3% of newborns, with 80% of cases being unilateral[1]. The prognosis of this disease is generally related to the age of intervention, with older age resulting in poorer prognosis. The pathological anatomy of DDH involves dual changes in bones and soft tissues. Bone changes include morphological alterations of the acetabulum, femoral head, femoral neck, and femoral shaft, while soft tissue changes manifest as morphological alterations of the acetabular cartilage, femoral head cartilage, joint capsule, labrum, and surrounding muscles[2]. The most common treatment for infants with DDH is closed reduction, which is performed under anesthesia through manual manipulation. It is fixed with a plaster cast in a flexed external position for 3-6 months. For infants diagnosed at > 6 months, closed reduction can minimize postoperative hip joint movement disorders and reduce iatrogenic ischemic necrosis[3,4]. When closed reduction fails or is diagnosed at a later stage, such as > 18 months, or in cases of severe dislocation, open reduction surgery is required. Salter osteotomy is a type of pelvic osteotomy that changes the direction of the acetabulum to increase coverage, correct the acetabular index, and restore the biomechanics of the hip joint. Its safety and effectiveness have been widely verified[5]. Salter osteotomy combined with femoral rotational shortening osteotomy can reduce the pressure on the hip joint after reduction, increase joint stability, promote acetabulum maturation, and reduce the risk of ischemic necrosis and hip re-dislocation[6].

Most current research on DDH focuses on imaging and functional scoring. Bone pathological changes related to DDH have been reported[7-9]. The complex muscle structure around the hip joint plays a crucial role in maintaining joint stability and enabling functional movements. However, current research on the muscles surrounding the hip joint in children with DDH has focused on the morphological aspect, and there is little research on the assessment and quantitative analysis of the muscles surrounding the hip joint. Before treatment, the muscles surrounding the hip joint of pediatric DDH patients show varying degrees of muscle atrophy and fat infiltration[2]. Tomaru et al[10] found that at 2 years after closed reduction surgery in pediatric DDH, the volume of the gluteal muscle on the healthy side was significantly larger than that on the affected side. Similarly, Yüksel et al[11] found that at an average follow-up of 112.6 months after Salter osteotomy in pediatric DDH patients, the areas of the iliopsoas, tensor fasciae latae, rectus femoris and gluteus maximus muscles on the affected side were significantly smaller than those on the healthy side. As pediatric DDH disease progresses, specific adaptive changes may occur in the muscle tissues surrounding the hip joint, which will reduce joint stability and thereby aggravate dislocation. Surgical trauma may also change the biomechanical characteristics of the hip joint. The following have not been fully elucidated: Whether the functional activity of the main muscles in the lower limbs of pediatric DDH after surgery is normal; whether there is a change in the direction of the muscle pull force line or torque on the affected side muscle, leading to lower muscle strength or compensatory increase; and whether the muscle electrical activity of the healthy side lower limb muscles changes accordingly.

Surface electromyography (EMG) is a safe and non-invasive detection technique that can quantitatively and qualitatively assess the functional status of nerves and muscles. It enables direct, real-time, and objective measurement of muscle activity during functional tasks[12,13]. As an important component of gait analysis, the root mean square (RMS) is used to describe the average change amplitude of muscle and nerve discharge activities within a specific period, which is the sum of the square roots of all amplitude values during that period. It is the most reliable time-domain indicator of surface EMG. This indicator is also associated with the recruitment of motor units and the synchrony of excitation rhythms. In time-domain analysis, RMS is considered the most reliable parameter for evaluating the magnitude of force generation[14]. The ratio of active muscle-to-antagonistic muscle synergy can reflect the control of joint movement and stability. Artificial intelligence and data-driven analytics are playing an emerging role in enhancing diagnostic accuracy and rehabilitation assessment, particularly in the context of multimodal physiological data such as EMG signals during postoperative follow-up[15]. Currently, surface EMG has been applied to assess hip fracture patients and the activation of hip-related muscles after total hip arthroplasty[16,17].

The aim of this study was to evaluate the changes in surface EMG signals of the main muscles in both lower limbs of children with unilateral DDH during standing and walking after closed reduction surgery or Salter osteotomy. It also aimed to quantify the differences in bilateral muscle activation and explore the characteristics of surface EMG signals of the muscles around the hip joint and the postoperative rehabilitation effects.

MATERIALS AND METHODS

General information

A total of 59 children with unilateral DDH who underwent closed reduction or Salter osteotomy in the pediatric orthopedics department of a single tertiary grade A hospital from June 2006 to February 2025 were included. Twenty-eight children received closed reduction treatment (24 females, 4 males), with 20 on the left side and 8 on the right. The treatment age ranged from 5 months to 36 months, with an average treatment age of 14.96 ± 6.94 months and an average follow-up period of 51.64 ± 34.85 months. The last follow-up age was 5.58 ± 2.88 years, the average height at the last follow-up was 112.89 ± 18.09 cm, and the average weight was 21.71 ± 11.15 kg.

Thirty-one children with DDH who underwent Salter osteotomy combined with femoral rotational shortening osteotomy (Salter group) were included. There were 24 females and 7 males, with 19 on the left side and 12 on the right. The treatment age ranged from 18 months to 90 months, with an average treatment age of 35.52 ± 16.20 months and an average follow-up period of 51.0 ± 34.76 months. The last follow-up age was 7.11 ± 3.23 years, the average height at the last follow-up was 121.71 ± 20.32 cm, and the average weight was 25.06 ± 11.83 kg. Both groups followed the standard postoperative rehabilitation protocol for DDH surgery. There were significant differences in the surgical treatment age and the last follow-up age between the two groups (P < 0.05), but no differences in the average follow-up time, height, or weight (P > 0.05), making them comparable (Table 1). This study was approved by the Ethics Committee of Hunan Provincial People’s Hospital (The First Affiliated Hospital of Hunan Normal University), approval No.[2025]-298.

Table 1 General information of the two groups of children, mean ± SD.

	Gender		Affected side		Treatment age (months)	Average follow-up (months)	Last follow-up age (months)	Last follow-up height (cm)	Last follow-up weight (kg)
	Male	Female	Left	Right
CR group (n = 28)	4	24	20	8	14.96 ± 6.94	51.64 ± 34.85	66.86 ± 34.56	112.89 ± 18.09	21.71 ± 11.15
Salter group (n = 31)	7	24	19	12	35.52 ± 16.20a	51.0 ± 34.76	85.35 ± 38.78a	121.71 ± 20.32	25.06 ± 11.83

^aP < 0.05 vs closed recovery group.

CR group: Closed reduction treatment.

Inclusion criteria: (1) Children with unilateral DDH; (2) Undergoing closed reduction or Salter osteotomy combined with femoral rotational retraction and shortening osteotomy; (3) No metabolic diseases; (4) Complete follow-up data, including imaging and surface EMG tests at last follow-up; and (5) No other surgical history for the lower extremity other than the aforementioned surgeries and removal of internal fixation.

Exclusion criteria: (1) Children with bilateral DDH; (2) Hip dislocation due to other causes; (3) Surgical procedures during treatment other than closed reduction, Salter osteotomy with femoral rotational retraction and shortening osteotomy; and (4) Children who did not cooperate or had poor cooperation with surface EMG tests.

Evaluation methods and indicators

Harris score: At each patient’s follow-up visit, a dedicated practitioner determined the Harris Hip Joint Score[18], evaluating aspects such as pain, function, deformity, and range of motion, and recorded the total score.

Severin rating: At each patient’s follow-up visit, a standing pelvic anteroposterior X-ray was acquired, and the hip joint Severin grade assessment was conducted by a dedicated pediatric orthopedic doctor, with a title of associate chief physician or above[19].

Measurement of double lower limb length: The length of both lower limbs of each child was measured at the follow-up visit. The child lay in the supine position on a hard examination table, the pelvis was aligned properly (the line connecting the bilateral iliac anterior superior spines was perpendicular to the trunk), and both lower limbs were straightened and placed together or symmetrically. The pelvis was stabilized, and the iliac anterior superior spines were identified as proximal landmark points, with the inner malleolus tip as the distal landmark point. A soft ruler was used to measure the distance from the iliac anterior superior spine to the inner malleolus tip on the same side, and the length of the lower limb was recorded. If the length of the affected limb was longer than that of the healthy limb, the difference was recorded as a positive value; if the length of the affected limb was shorter than that of the healthy limb, the difference was recorded as a negative value.

Surface EMG test

During postoperative follow-up, lower limb surface EMG tests were conducted. The test results of the last follow-up were analyzed.

A professional technician performed the surface EMG test on all patients. The EMG room was maintained at a constant temperature of 26°C and the Flexcomp surface EMG system (Thought Technology, Canada) was used. The system has a preamplifier gain of 1000, an input impedance > 100 MΩ, a common-mode rejection ratio > 100 dB, a channel sampling bandwidth of 100-500 Hz, and a noise level < 0.1 μV. The EMG signal data were collected at a sampling frequency of 2048 Hz and analyzed using BioGraph Infiniti software (Thought Technology, Canada). The test electrodes were circular disposable triode dry electrodes with an outer diameter of 5.6 cm and an electrode diameter of 1.0 cm. The distance between the recording and reference electrodes was 2 cm. The subjects were fully exposed on both lower limbs, and 75% alcohol was used to decontaminate the skin. The electrodes were placed in the fleshiest areas of the bilateral tensor fasciae latae, rectus femoris muscles, the medial heads of the hamstring muscles (referred to as hamstring muscles), gluteus maximus, anterior tibialis muscles, and medial heads of the gastrocnemius muscles (referred to as gastrocnemius muscles hereafter), parallel to the long axis of the muscle fibers. Surface EMG signals during standing and walking were collected.

Standing requirements: Ability to stand independently, relax the entire body, keep the body upright, barefoot, with both feet positioned at the same width as the shoulders, with the patella facing forward, and maintain stability for 30 seconds. The 20-second period with the most stable signal was selected for analysis. Walking requirements: 8-m long walkway. The child walked freely at a normal walking speed for 2 minutes, and their gait was observed.

The built-in signal processing software BioNeuro Infiniti was used to perform RMS processing on the collected surface EMG recordings and to conduct statistical analysis on the average RMS values of the measured muscles. The processing of relevant data is based on previous studies[20].

The mean RMS values and the ratios of the corresponding contralateral and ipsilateral muscles during standing and walking were compared in the closed reduction treatment and Salter group. The synergistic contraction ratios of the three groups of agonist and antagonist muscles (such as the tensor fasciae latae and gluteus maximus, rectus femoris and hamstring, and anterior tibialis and gastrocnemius) of the flexor and extensor muscles on the affected and healthy sides during standing and walking were calculated, represented by co-contraction ratio (CCR) 1, CCR2, and CCR3, respectively. The synergistic contraction ratios between the healthy and affected sides were compared within and between the groups. CCR1 = tensor fasciae latae RMS/(tensor fasciae latae RMS + gluteus maximus RMS); CCR2 = rectus femoris RMS/(rectus femoris RMS + hamstring RMS); CCR3 = tibialis anterior RMS/(tibialis anterior RMS + gastrocnemius RMS).

Statistical analysis

All data were analyzed using SPSS version 19.0. Normality tests were conducted for continuous variables: Data that followed a normal distribution were expressed as mean ± SD, and independent sample t tests were used for comparisons between groups. For data that did not follow a normal distribution, median and percentiles were used, Mann-Whitney U tests were used for independent sample non-parametric tests, and Wilcoxon signed-rank tests were used for paired sample non-parametric tests. Only the mean RMS values of the affected and healthy sides of the rectus femoris and hamstring muscles on the affected side in the closed reduction group, and the mean RMS values of the tensor fasciae latae on the healthy and affected sides in the Salter osteotomy group, had a normal distribution. However, the results of the paired sample t tests showed no significant difference (P < 0.05). Due to the small sample size and to ensure the consistency of the data, non-parametric tests were uniformly adopted in the statistical analysis of the surface EMG indicators, and the results were uniformly expressed as median (25^th-75^th percentile). Categorical data were presented as frequencies and percentages and analyzed using the χ² or Fisher’s exact test. P < 0.05 was considered statistically significant.

RESULTS

Comparing the difference in Harris scores at last follow-up, the average Harris score of the Salter group was 95.91 ± 1.01, and the average closed reduction Harris score was 98.84 ± 0.82 (Z = -3.090, P = 0.002). In the closed reduction group, three children had unequal lengths of both lower limbs, with the affected side shorter than the healthy side in all cases (shorter by ≤ 2 cm, average = 1.5 cm). The gait manifestations were normal in 13 children, slightly limping in 1, inward-angled gait in 8, and ascending-lowering gait in 1. In the Salter group, 11 children had unequal lengths of the lower limbs, with the affected side shorter than the healthy side in nine patients (shorter by 0.5-2.5 cm, average = 1.24 cm), and the affected side longer than the healthy side in two patients (all 1 cm longer). The gait manifestations were normal in 20 children, slightly limping in 7, inward-angled gait in 2, and ascending-lowering gait in 2.

The Severin classification showed that in the Salter group, 21 cases were excellent, 8 were good, and 2 were poor, with an excellent and good rate of 93.55%. In the closed reduction group, 21 cases were excellent, 6 were good, and 1 was poor, with an excellent and good rate of 96.42%. There was no significant difference between the two groups (χ² = 0.253, P = 0.615).

Comparison of surface EMG signals of the lower limb muscles between the affected and healthy sides in the standing position

We compared mean RMS values of the contralateral and affected sides of the same muscles while standing, as well as the muscle synergy ratios (CCR1, CCR2, CCR3) and found no significant differences (P > 0.05) in either the Salter osteotomy group or the closed reduction group. Surface EMG activity indicators of the affected and healthy sides of muscles within each group were not significantly different (P > 0.05). The mean RMS values of the healthy/affected side of the same muscle and the muscle synergy ratio of each of the three groups were not significantly different (P > 0.05) between the closed reduction and Salter osteotomy groups (Table 2).

Table 2 Comparison of surface electromyography signals of the affected and healthy lower limbs of the 2 groups of children when standing, median (25^th percentile-75^th percentile).

	Closed reduction group					Salter osteotomy group
	Affected side	Healthy side	The ratio of the affected side to the healthy side	Affected side CCR	Healthy side CCR	Affected side	Healthy side	The ratio of the affected side to the healthy side	Affected side CCR	Healthy side CCR
Tensor fasciae latae	3.62 (1.93-6.02)	3.92 (2.11-5.78)	0.91 (0.64-1.23)	0.61 (0.49-0.76)	0.64 (0.52-0.76)	4.66 (2.91-7.51)	3.85 (2.42-6.98)	1.18 (0.80-1.81)	0.68 (0.57-0.81)	0.62 (0.48-0.76)
Gluteus maximus	1.49 (1.21-2.61)	1.75 (1.22-2.52)	0.97 (0.84-1.24)			2.00 (1.47-2.24)	1.96 (1.32-3.81)	1.04 (0.59-1.47)
Rectus femoris	1.92 (1.09-4.74)	2.64 (1.18-3.97)	0.91 (0.80-1.26)	0.25 (0.11-0.44)	0.22 (0.18-0.42)	2.18 (1.60-6.55)	2.31 (1.43-6.85)	0.95 (0.68-1.32)	0.33 (0.15-0.60)	0.36 (0.16-0.54)
Hamstring muscles	6.55 (3.17-13.82)	8.94 (3.37-12.51)	0.94 (0.73-1.37)			5.39 (3.69-12.29)	7.08 (2.25-13.54)	0.99 (0.60-1.31)
Tibialis anterior muscle	4.97 (2.03-10.40)	4.71 (2.01-10.78)	0.99 (0.81-1.23)	0.40 (0.30-0.56)	0.44 (0.32-0.55)	4.98 (3.53-7.32)	4.58 (2.15-8.93)	0.99 (0.81-1.12)	0.47 (0.37-0.60)	0.49 (0.28-0.58)
Musculus gastrocnemius	5.75 (3.20-9.28)	5.60 (3.99-8.47)	0.99 (0.86-1.23)			4.44 (3.53-7.32)	6.14 (3.86-8.75)	0.93 (0.85-1.05)

CCR: Co-contraction ratio.

Comparison of surface EMG signals of the lower limb muscles between the affected and healthy sides during walking

There were no significant differences between the mean RMS values of the contralateral and affected sides of the same muscles or in the muscle coordination ratios (CCR1, CCR2, CCR3) for either treatment group(P > 0.05). There was no significant difference in the above surface EMG activity indicators of the affected and healthy sides of the muscles within the same group (P > 0.05).

Compared with the closed reduction group, the mean RMS values during walking of the tensor fasciae latae on the affected side and healthy side, and rectus femoris muscle on the affected side were significantly increased in the Salter osteotomy group (P < 0.05). Compared with the closed reduction group, the ratio of synergistic contraction of the rectus femoris muscle and the hamstring muscles on the affected side was significantly increased in the Salter osteotomy group (P < 0.05). The other surface EMG indicators of were comparable between the two groups (P > 0.05; Table 3).

Table 3 Comparison of surface electromyography signals of the affected and healthy lower limb muscles during walking in the 2 groups of children, median (25^th percentile-75^th percentile).

	Closed reduction group					Salter osteotomy group
	Affected side RMS	Healthy side RMS	RMS the ratio of the affected side to the healthy side	Affected side CCR	Healthy side CCR	Affected side RMS	Healthy side RMS	RMS the ratio of the affected side to the healthy side	Affected side CCR	Healthy side CCR
Tensor fasciae latae	35.71 (28.15-54.12)	31.60 (24.05-49.63)	1.06 (0.91-1.32)	0.78 (0.67-0.85)	0.78 (0.69-0.83)	49.31 (32.33-73.58)a	44.17 (28.94-66.38)a	1.21 (0.83-1.43)	0.82 (0.75-0.87)	0.78 (0.75-0.83)
Gluteus maximus	12.57 (6.61-17.96)	11.01 (6.73-16.73)	0.97 (0.86-1.11)			11.04 (6.52-15.75)	14.64 (7.44-18.98)	0.88 (0.75-1.14)
Rectus femoris	18.33 (14.44-27.31)	21.75 (15.57-28.71)	1.01 (0.77-1.14)	0.46 (0.37-0.52)	0.46 (0.37-0.56)	27.36 (19.24-37.04)a	25.20 (17.81-28.91)	1.06 (0.82-1.59)	0.54 (0.46-0.65)a	0.51 (0.38-0.56)
Hamstring muscles	23.77 (15.42-33.71)	23.57 (17.73-36.53)	0.95 (0.79-1.12)			22.81 (16.25-30.26)	22.64 (19.13-34.34)	0.87 (0.73-1.12)
Tibialis anterior muscle	38.44 (31.50-50.36)	37.43 (28.64-45.24)	1.02 (0.89-1.18)	0.48 (0.43-0.55)	0.47 (0.41-0.56)	39.05 (17.71-55.28)	39.07 (19.24-50.23)	0.99 (0.86-1.16)	0.53 (0.43-0.58)	0.54 (0.43-0.59)
Musculus gastrocnemius	37.43 (30.52-52.19)	39.00 (29.82-58.29)	0.99 (0.88-1.12)			34.57 (23.70-47.55)	36.65 (25.49-47.08)	1.05 (0.92-1.15)

^aP < 0.05 vs closed recovery group.

RMS: Root mean square; CCR: Co-contraction ratio.

DISCUSSION

The study revealed a gender imbalance in DDH cases, with female incidence being significantly higher than male, though this ratio aligns with the epidemiological profile of DDH. The children in the closed reduction and the Salter osteotomy groups had excellent overall Harris scores, suggesting that both groups had good recovery in static assessment, subjective feelings, and basic functions, with good pain control and the ability to meet daily life needs. However, the Harris score of the Salter osteotomy group was lower than that of the closed reduction group, primarily because scores of gaits, deformity, and range of motion were lower than those of the closed reduction group, suggesting that the surgical method affects hip joint function. There was no significant difference in the Severin scoring criteria classification between the two groups, suggesting that both groups achieved good outcomes by imaging. This is similar to the findings of Bakarman et al[21], where there was no significant difference in acetabular reshaping between closed and open reduction techniques.

Our study is consistent with previous studies. Terjesen[22] conducted a long-term efficacy study on 60 children with late-diagnosed DDH (aged 6-36 months) who underwent closed reduction. The follow-up lasted until an average age of 58 years. The results showed that more than half of the hip joints achieved good clinical and radiological outcomes[22]. Cha et al[23] conducted a 7-year follow-up study on pediatric DDH with an average age of 6.11 months and 15.29 months at the time of closed reduction, respectively. The results revealed that the overall clinical outcomes of both groups were satisfactory, but 15.29-month group, one child underwent surgical treatment at 7-years-old. In our study, the hip joint function of the children in the closed reduction group was satisfactory, but one child underwent Salter osteotomy at 6-years-old.

Salter osteotomy is widely used and effective for treating DDH[24]. Schmidutz et al[25] followed up 49 children with DDH who underwent Salter osteotomy at an average age of 27.6 months for 6.7 years. The Harris hip joint score was 85.0 ± 11.8 points, and the radiological results showed good correction of the acetabulum. However, 10 patients (20%) developed complications, among which 7 (14%) required revision surgery. In our study, the most common complication in the Salter osteotomy group was unequal length of the lower limbs, manifested as mild limping or uneven gait. There were no cases of avascular necrosis of the femoral head or secondary revision surgery.

The hip is the only joint in the human body that can support the body both at rest and during movement, and its stability is of utmost importance. For children with DDH, stability of the hip joint is primarily achieved through the hip extensor and flexor muscles during standing and walking. Additionally, the lower limb joints form a movement chain, and the position adjustment of the hip joint requires the coordinated contraction of the quadriceps femoris and hamstring muscles. The position adjustment of the knee joint requires the simultaneous contraction of the ankle dorsiflexors (anterior tibialis) and ankle plantar flexors (triceps surae) at the distal end. Therefore, in this study, we observed the EMG activities of the gluteus maximus, tensor fasciae latae, rectus femoris, medial head of the hamstring, medial head of the gastrocnemius, and anterior tibialis.

As the main extensor of the hip joint, the gluteus maximus is responsible for the extension, external rotation, and abduction of the hip joint. It is one of the main muscles that maintain hip joint stability. Its main antagonist muscle is the iliacus and psoas muscle located deep in the pelvis. Due to the difficulty of conventional surface EMG monitoring, we chose the superficially located tensor fasciae latae as the antagonist muscle of the gluteus maximus for research. The main function of the tensor fasciae latae is internal hip rotation, and it assists the iliacus and psoas muscles in flexing the hip joint. It cooperates with the gluteus medius and gluteus minimums for hip abduction and is a powerful external abductor muscle of the hip joint during standing, providing stability to the pelvis. We primarily observed hip joint stability during standing and walking, which depends on the extension function of the gluteus maximus. Therefore, we calculated the synergistic contraction instead of using the gluteus maximus as the active muscle and the tensor fasciae latae as the antagonist muscle.

This study showed that there were no significant differences in the mean RMS values or their ratios of the gluteus maximus and tensor fasciae latae during standing on the healthy and affected sides and the ratios of synergistic contractions between the two groups, suggesting that the active and antagonist muscles on the affected and healthy sides were activated equally after surgery and there was no muscle strength imbalance.

However, this study showed significantly higher mean RMS values of the tensor fasciae latae on the healthy and affected sides during walking in the Salter osteotomy group compared with the closed reduction group. Salter osteotomy changed the direction of the acetabulum, increased anterior-lateral coverage, directly altering the alignment between the acetabulum and the femoral head. At the same time, femoral shortening and rotation osteotomy corrected the excessive femoral anteversion angle, and these structural changes could directly lead to a change in rotational force line, affecting the biomechanical properties of the hip muscles, such as the lever arm and tension. The tensor fasciae latae may have a more effective lever arm and be more active during the swing phase of gait to maintain hip joint stability and control pelvic rotation. This is manifested as an increase in EMG activity of the affected side of the tensor fasciae latae. Although the EMG activity of the gluteus maximus muscle was normal, the EMG activity of the tensor fasciae latae significantly increased on the healthy and affected sides. This may be due to the insufficient or inefficient function of the gluteus medius muscle, requiring the tensor fasciae latae to compensate and increase its EMG activity. Salter osteotomy requires dissection of the tensor fasciae latae and sartorius; therefore, some children developed postoperative scars. The tensor fasciae latae on the affected side needed to recruit more muscle fibers to participate in the activity to stabilize the hip joint and compensate for the increased EMG activity. The mean RMS value of the tensor fasciae latae on the healthy side increased. We speculate that this may be related to the increased EMG activity of the tensor fasciae latae on the affected side. To maintain pelvic stability or gait coordination and symmetry, the tensor fasciae latae on the healthy side strategically adjusted its activity pattern and amplitude of EMG activity. Some studies have shown that pelvic osteotomy is not simply a reshaping and directional adjustment as traditionally believed, but that the entire pelvic ring collaboratively bears external forces[26]. From a biomechanical perspective, this explains the increased EMG activity of the tensor fasciae latae on the healthy side.

Studies have shown that in the treatment of lower limb injuries, enhancing gluteus maximus and gluteus medius muscle activation while minimizing tensor fasciae latae involvement is an important aspect[27]. Therefore, for children with DDH, the EMG activity of the tensor fasciae latae is abnormally increased compared to that of the gluteus maximus muscle, which requires the attention of pediatric orthopedic doctors, and emphasis on the rehabilitation training of the gluteus maximus muscle after surgery, such as clam exercises.

As the only quadriceps muscle that crosses the hip joint, the rectus femoris muscle is the prime mover for hip flexion and knee extension. Its knee extension function is stronger than its hip flexion function. As the extensors of the hip joint and the flexors of the knee joint, the hamstring muscles have direct functional opposition in both the hip and knee joints and are directly related to the hip extension mechanism. Children with DDH often exhibit abnormal gait, such as insufficient hip extension and knee compensation after surgery. To study hip joint stability related to extension, we selected the hamstring muscles as the active muscles and the rectus femoris as the antagonist muscle to calculate the synergistic contraction ratio.

This study showed that there were no significant differences in the mean RMS values of the rectus femoris and hamstring muscles on the healthy and affected sides, as well as the ratio of RMS values between the healthy and affected sides and the synergy ratio between the two groups when standing. This suggests that during standing, muscle electrical activity maintains balance. During walking, the mean RMS value of the rectus femoris on the affected side and the synergy ratio of the affected side rectus femoris and hamstring muscles in the Salter osteotomy group were significantly higher than those in the closed reduction group. However, there was no significant difference in the mean RMS value of the hamstring muscles. This suggests that during walking, the activation degree of the affected side rectus femoris in the Salter osteotomy group was higher than that of the hamstring muscles. The reason for this may be related to the abnormal, limping gait of the children. To increase the stability of the knee joint during walking and prevent knee flexion instability or collapse, the children may adopt the strategy of simultaneous contraction of the knee flexor and extensor muscles or excessive activation of the extensor muscles. However, this strategy may affect hip joint stability, and excessive activation of the bilateral tensor fasciae lataes also supports this speculation. At the same time, the contraction of the rectus femoris muscle may affect the iliotibial band and all broad ligaments[28]. The excessive activation of the tensor fasciae latae may also lead to corresponding changes in rectus femoris muscle electrical activity. The femoral rotational removal osteotomy changed the force line and torque of the hamstring muscles (semitendinosus and semimembranosus), which may affect their efficiency in controlling knee joint rotation and anterior-posterior stability. Thus, the rectus femoris muscle needs to have stronger activity to compensate, manifested as a stiff gait in muscle electrical activity.

The gastrocnemius muscle maintains upright posture during standing and dominates the propulsion and stability during walking. The anterior tibialis muscle primarily regulates ankle joint movement. In this study, during standing and walking, there were no significant differences in the surface EMG-related indicators of the anterior tibialis and gastrocnemius muscles between the two groups. This was speculated to be related to the indirect influence of the muscle activities of the lower leg on the hip and knee joints and the fact that the surgery did not directly involve the ankle joint.

In this study, the measured EMG activities of the healthy and affected lower limbs of the children with closed reduction were balanced. This may have been related to their main recovery of the hip joint concentric reduction through non-surgical methods (traction, manual reduction, or plaster fixation) without changing the bone shape, and having a small impact on the EMG activities of the hip peripheral muscles. However, the deeper reasons need to be explained by biomechanical modeling[29].

We found that the hip joint function scores and the impact assessment scores of the two groups were high, although there were abnormalities in gait and lower limb muscle EMG activities. This study suggests that in clinical practice, we cannot solely rely on scales and imaging results to assess prognosis. Dynamic functional assessment should also be included. After Salter osteotomy, even if the anatomical reduction is good and the hip joint function score is high, potential abnormal neuromuscular activities should be monitored. Long-term rehabilitation after surgery should focus on the function of muscle groups related to pelvic stability, especially the compensated gluteus medius muscle, and gait coordination training of the affected knee joint should also be included. For children with closed reduction, monitoring and treatment of rotational deformity should be monitored.

The limitations of this study include the incomplete surface EMG test data and the small number of subjects, which may reduce the representativeness of the results. At the same time, considering factors such as follow-up time and muscle strength development, the surface EMG results before and after surgery were not compared. This was a retrospective study and did not include healthy children as controls, lacking normal data for comparison. Based on this, our future research will be prospectively designed to measure the preoperative surface EMG of the children and include age-matched healthy controls to clarify the trajectory of neuromuscular recovery and verify the functional implications of the results in this study. Meanwhile, we will also conduct a multi-dimensional comprehensive assessment of the lower limb muscle function and gait coordination of children with DDH in various activity patterns[30]. In addition, we will also leverage artificial intelligence and machine learning-based frameworks to enhance the interpretation of physiological signals, such as surface EMG. This will assist our pediatric orthopedics department in making surgical treatment decisions for children with DDH and formulating personalized rehabilitation plans after surgery.

CONCLUSION

In summary, both closed reduction and Salter osteotomy achieved good functional scores and imaging results during mid-term follow-up. However, when walking, there was an increase in the muscle electrical activity of the fascia latae and rectus femoris in the Salter osteotomy group, suggesting that targeted rehabilitation exercises should be implemented postoperatively.