The bilateral squat is one of the most prevalent exercises used in lower limb full weight-bearing exercises during the early rehabilitation of hip and knee injuries [1, 2]. In addition, individuals perform various patterns of squats to accomplish the tasks associated with the activities of daily living [3, 4].

Bilateral squats are used widely to strengthen the muscles around the knee, particularly in the presence of musculoskeletal conditions such as patellofemoral pain syndrome (PFPS) and osteoarthritis. PFPS is characterized by pain around the patella (kneecap) because of imbalances in the forces acting on the knee joint, which bilateral squats can help remedy by strengthening the quadriceps and surrounding musculature [5]. Osteoarthritis, which involves the degeneration of joint cartilage and the underlying bone, particularly in the knees and hips, can also benefit from the muscle-strengthening effects of bilateral squats to improve joint stability and function [6].

Maintaining physiological posture is essential for preventing musculoskeletal injuries. Frequently performing deep flexion postures, such as cross-legged sitting, squatting, and kneeling during activities of daily living, might lead to knee joint problems [7]. Poor posture and poor postural control are major contributors to pain conditions. They can affect a wide range of structures and lead to functional imbalances [8, 9]. Furthermore, poor posture can lead to reduced mobility, impaired social participation, and pain, which can degrade an individual’s quality of life and impair their physical and mental health [10]. Hence, analyzing movement patterns relative to physiological postures is fundamental for proper posture analysis.

The bilateral squat is a fundamental movement pattern that requires stability at the foot, knee, and lumbar spine and mobility at the ankle, hip, and thoracic spine [11]. Stability requires that the knees maintain a position aligned with the hips and feet when performing a bilateral squat [12, 13]. A bilateral lower limb full weight-bearing squat can be described as having descent and ascent phases. During the descent phase of the squat, flexion occurs at the hip and knee joints, while the ankle joint dorsiflexes. Conversely, during the ascent phase of the squat, the knee and hip joints extend while the ankle joint plantarflexes to return to the starting position when standing up [14]. On the other hand, performing a bilateral squat with an improper posture alters the forces acting on the knee and hip joints, altering the kinematic dynamics of the tibiofemoral area. In addition, incorrect movement patterns can lead to muscle imbalances in the torso and hip joints or motor changes in the feet and ankles, increasing the risk of injury [15].

Knee valgus motion is characterized by excessive medial displacement of the knee. Hewett et al. [16] reported that medial knee displacement arises from a combination of femoral adduction and internal rotation, as well as tibial abduction and external rotation. Abnormal motion of the tibia and femur in the transverse and frontal planes can be detected by observing if the center of the patella moves medially and crosses over a line extending upward from the great toe during a bilateral squat [17]. Excessive knee valgus causes knee injuries and contributes to the specific strength and flexibility deficits in hip and ankle joint musculature [18]. Individuals with medial knee displacement often exhibit restricted foot and ankle motion, leading to compensation at the knee joint [19]. Padua et al. [20] reported that medial knee displacement can be addressed using heel lifts to reduce gastrocnemius restriction. Nevertheless, there has been limited research on the kinematic analysis of the lower extremities in dorsiflexion or plantar flexion positions of the ankle during bilateral squat performance in individuals with a high Q-angle.

This study hypothesizes that individuals with a high Q-angle will exhibit significant differences in lower limb joint kinetic parameters compared to those with normal Q-angles. Thus, this study examined the biomechanics of the lower extremities during bilateral squats under different ankle joint positions in groups with normal and high Q-angles.

1. Participants

The participants were recruited from Youngsan University in Yangsan-si, South Korea. Fifty-two university students aged over 18 years who were capable of a normal gait and volunteered to participate in the study were collected using an advertisement on the notice board at the university. The inclusion criteria were as follows: no pathological abnormality in the knee joint, no history of lower extremity trauma or surgery, no pathological development in the lower limb bones, and no muscle convulsion or stiffness. The exclusion criteria were as follows: discomfort due to lower extremity pain during daily activity, moderate pain with a visual analog scale score > 5 after performing squats ≥20 times, inability to complete a 100 m walk, presence of a tumor, and pregnancy [21].

For group classification, 20 participants with a Q-angle of ≤20° were set as the normal Q-angle group (N), and 20 participants with a Q-angle > 20° were set as the high Q-angle group (HQ) were recruited on a first-come, first-served basis. The Q-angle became the primary endpoint in this study when determining the sample size. The significance level (α) = .05, power (1 − β) = .85, effect size = 1, and a two-tailed test were applied. At least 19 participants were required for each group; thus, 40 participants were enrolled to cope with potential losses [21]. The Institutional Review Board of Youngsan University approved this study (YSUIRB-201506-HR-001-02). Informed consent was obtained from all participants.

2. Procedure

The Q-angle was measured in 52 volunteers and is defined as the angle between the line connecting the center points of the anterior superior iliac spine and the patella and the line connecting the patella and tibial tuberosity [22]. All landmarks were marked using an aqueous pen. A trained physiotherapist guided the participant to take a supine position. After setting a parallel pelvic alignment, the Q-angle at the right foot was measured using a long goniometer (Lafayette extendable goniometer, Lafayette, IN USA) in triplicate. A long-arm goniometer has good reliability with an intra-observer ICC of .92 and an interobserver ICC of .88 [23]. When the mean Q-angle was ≤20° and > 20°, the participant was assigned to the normal Q-angle group (N) and high Q-angle group (HQ), respectively, based on previous research [21, 24, 25]. The high Q-angle of the dominant leg should be considered to be greater than 17 and 20 for males and females, respectively [26]. A Q-angle > 20° was considered excessive regardless of gender [21, 25].

Twenty eligible volunteers in the normal Q-angle group were recruited on a first-come, first-served basis; 12 subjects with a Q-angle of 20° or below were excluded. Each participant practiced the squat posture. For the squat posture, the participant in a standing position with their arms folded and feet shoulder distance apart was guided to bend the knees to the maximum while verbally guided by the therapist, so the trunk was not bent forward or backward to avoid the compensatory effect through trunk flexion. In a pilot study, the squat movement allowing trunk flexion showed various compensation patterns. Therefore, trunk flexion was limited to focus on the hip and knee joint in the experiment. This was controlled by the research assistant checking the trunk bending based on the long stick on the side of the trunk. The squat was performed as much as possible because of the large differences in the end range of motion for everyone.

The participant performed the squat on the bare floor, a tilt table for dorsiflexion fixation, and a tilt table for plantar flexion fixation to assess the biomechanics of the lower extremity during the squat in the three ankle joint conditions. The squat performed on the bare floor was called the bare floor squat (B). The squat performed on the tilted plane at 15° with the support in the dorsiflexion direction was called the dorsiflexion squat (D). The one performed on the same plane with the support in the plantar flexion direction was called the plantar flexion squat (P). The squat was performed up to the moment the participant could achieve the maximum knee joint bend with their trunk straight, and the measurements were taken during the last 10 seconds as the participant held the posture (Fig. 2).

Fig. 2. Squat posture on the bare floor, 15° dorsiflexion tilt table, and 15° plantar flexion tilt table, respectively.

Before the experiment, the participants were given a 10-minute orientation, followed by a 10-minute rest. The dorsiflexion squat could be difficult and painful for individuals with muscle tightness and weakness. The subjects who had difficulty holding this pose for 10 seconds three times were excluded. The subjects were excluded from this study if they felt discomfort, wanted to quit, or could not maintain a constant motion. Thirty out of 52 participants remained for the study (Fig. 1).

Fig. 1. Flowchart of the recruitment and study procedure.

3. Data Analysis

Three-dimensional motion analysis was performed using six cameras with a Falcon System (Motion Analysis, Santa Rosa, CA, USA). The six cameras with a Falcon System were used to acquire motion data (Motion Analysis, Santa Rosa, CA, USA). The motion capture cameras were set to a sampling rate of 60 Hz. The system was calibrated before each data collection session according to the manufacturer’s guidelines. The Helen Hayes marker set was selected. Fifteen reflective spherical markers were placed on the left and right anterior superior iliac spines, midthighs, midshanks, lateral femoral epicondyles, lateral malleoli, second metatarsals, calcanei, and the sacrum. Four additional spherical markers were taped to the medial epicondyles of the femur and medial malleoli to calibrate only the standing position (static data) [21, 27]. The joint angle was calculated as the angular value between the parent and child segments.

The effect of the tilt table was confirmed by the angle of the sagittal plane of the ankle joint. The sagittal plane movement at the ankle joint was 17.56 ± .76, 1.26 ± .67, and 32.00 ± .68 and in a bare floor squat, dorsiflexion squat, and plantar flexion, respectively (Table 2). Approximately 15° differences in dorsiflexion and plantar flexion compared to the bare floor were observed because of the effect of the tilt table.

Table 2

Comparison of the three conditions of squat exercise between normal and abnormal alignment groups (unit: degree)

(n = 30)

Movement	Joint	Condition	N group	HQ group	t-value	p-value
			Mean ± SE	Mean ± SE
Sagittal plane movement (flexion/extension)	Hip	B	-46.56 ± 3.67	-49.54 ± 5.06	.477	.637
		D	-30.47 ± 2.08	-29.41 ± 5.77	.210	.835
		P	-41.09 ± 3.30	-43.76 ± 5.04	.452	.655
	Knee	B	31.77 ± 1.96	32.09 ± 3.37	-.088	.930
		D	18.26 ± 1.34	22.62 ± 3.60	-1.372	.182
		P	38.70 ± 1.66	35.70 ± 2.81	.969	.342
	Ankle	B	17.56 ± .76	16.11 ± .70	1.224	.232
		D	1.26 ± .67	.61 ± .52	.647	.523
		P	32.00 ± .68	31.11 ± .29	.872	.391
Frontal plane movement (abduction/adduction)	Hip	B	-7.16 ± 1.38	-5.90 ± 1.97	-.534	.598
		D	-5.80 ± 1.43	-3.01 ± 2.10	-1.120	.273
		P	-7.19 ± 1.15	-4.19 ± 2.69	-1.215	.235
	Knee	B	-1.12 ± .70	-2.03 ± .56	.863	.396
		D	-3.14 ± .88	-3.08 ± .75	-.044	.965
		P	-.04 ± .81	-3.27 ± 1.02	2.363	.026*
	Ankle	B	-12.07 ± .87	-13.73 ± 1.92	.907	.373
		D	-10.49 ± 1.09	-10.85 ± .89	.221	.826
		P	-5.71 ± .79	-5.97 ± .93	.194	.848
Transverse plane movement (internal/external rotation)	Hip	B	-3.41 ± 1.26	-5.26 ± 2.02	.819	.420
		D	-3.36 ± 1.48	-7.89 ± 2.34	1.712	.098
		P	-4.27 ± 1.24	-9.34 ± 2.14	2.187	.038*
	Knee	B	1.03 ± 1.47	4.11 ± 2.54	-1.128	.269
		D	-2.18 ± 1.63	-1.75 ± 2.49	-.148	.884
		P	-1.95 ± 1.24	-2.86 ± 1.11	.439	.664
	Ankle	B	13.96 ± 1.12	12.25 ± 1.44	.906	.373
		D	12.67 ± 1.30	12.68 ± 1.65	.003	.997
		P	16.48 ± 1.24	14.31 ± 1.82	.986	.333

The values are expressed as the means and standard error. * indicate significant differences between the N and HQ groups. Positive values represent extension, adduction, external rotation in the hip joint, and flexion, adduction, external rotation in the knee joint, and plantar flexion, inversion, and external rotation in the ankle joint. The abbreviation is as follows. N, normal group; HQ, high Q-angle group; B, squat posture on Bare floor; D, squat posture on 15° Dorsiflexion tilt table; P, squat posture on 15° Plantar flexion tilt table. *p < .05.

All subjects performed the squat, and the 15° dorsiflexion squat and 15° plantar flexion were recorded for 10 seconds. Further analysis was performed using the Cortex 3.0 software program (motion Analysis Corp.), and the three-dimensional joint angles for the hip, knee, and ankle were determined. All movements were measured in triplicate, and the statistics were calculated as the average value.

4. Statistics

The normality test was satisfied according to the Kolmogorov–Smirnov test. Therefore, an independent t-test, a parameter test, was used to compare the effect of various squat options on the change in the kinematics of the lower extremity between the N and HQ groups. Statistical analyses were performed using SPSS version 25.0 (IBM Corp., Armonk, NY, USA), and the statistical significance was set at p < .05.

1. General Characteristics of Subjects

Thirty subjects (N: 20, HQ: 10 subjects) were included, and the general characteristics of the subjects are presented in Table 1. Differences in other general characteristics besides the Q-angle were observed, but this study was not intended to examine gender differences. Q-angles ≥ 20° were targeted regardless of gender.

Table 1

General characteristics of the subjects (n = 30)

	N group	HQ group	t-value	p-value
Gender (n)	Male 11, Female 9	Male 1, Female 9
Height (cm)	169.00 ± 1.90	161.60 ± 1.96	2.438	.021*
Weight (kg)	69.59 ± 3.70	56.67 ± 3.93	2.106	.045*
BMI (kg/m2)	20.44 ± .86	15.69 ± 1.95	2.594	.015*
Age (year)	23.50 ± .46	22.80 ± .29	1.016	.318
Q-angle (degree)	13.30 ± .59	23.70 ± .91	-9.845	.000 *

The value expressed means and standard error. * indicate significant differences between the N and HQ groups. The abbreviation is as follows. N, normal group; HQ, high Q-angle group; BMI, body mass index. *p < .05.

2. Comparison of the Three Conditions of Squat Between the N and HQ Groups

In the N group, the ankle joint angles in the sagittal plane were 17.56 ± .76°, 1.26 ± .67°, and 32.00 ± .68° for the barefoot condition, dorsiflexion, and plantarflexion, respectively. In the HQ group, the corresponding values were 16.11 ± .70°, .61 ± .52°, and 31.11 ± .29° for the barefoot condition, dorsiflexion, and plantarflexion, respectively (Table 2). A comparison of the barefoot condition to dorsiflexion and plantarflexion revealed an approximate 15° difference in both groups. Minor variations in the sagittal plane angles were observed, likely due to individual differences, even with the foot fixed in the same tilt position.

Significant between-group variations were found in the frontal plane movement (abduction/adduction) of the knee joint in the plantar flexion squat (p < .05) (Table 2). A significant between-group variation was observed in the transverse plane movement (internal/external rotation) of the hip joint in the plantar flexion squat (p < .05) (Table 2).

This study examined whether there are kinematic differences between the N and HQ groups in how the lower extremity joint is controlled during bilateral squats under three ankle joint conditions. When abnormal dynamics occur in a specific lower extremity joint, the dysfunction is sequentially transferred to the subsequent joint because of the close attachment of the lower extremity segments through the kinematic chain [28, 29]. In individuals with an abnormal Q-angle, compensatory movements were observed in the hip and knee joints during the plantarflexion squat movement. A significant difference in the frontal and transverse planes was observed between the HQ and N groups when performing the plantarflexion bilateral squat. On the other hand, no significant difference was observed in the sagittal plane reflecting flexion and extension. This lack of statistical significance may be attributed to variations in the degree of flexion and extension among subjects because they were instructed to flex and extend the hip and knee joints as much as possible during the bilateral squat.

The first result of this study revealed a significant increase in knee joint abduction in the frontal plane during the plantarflexion squat in the HQ group compared to the N group. This suggests that the lateral displacement in the HQ group was greater than that in the N group, suggesting that the plantarflexion posture of the ankle joint affects the medial and lateral stability of the knee joint. Padua et al. [20] reported that the frontal plane knee displacement values of the medial knee displacement group were all directed laterally. In contrast, Bell et al. [18] showed different results from the present study, as correcting knee valgus with heel lifts relieved the restrictions on posterior flexion by the triceps to normalize knee alignment during squats. The clinicians use heel lifts to increase ankle plantarflexion to correct medial knee displacement and improve control over knee valgus and foot pronation. In the present study, no modification was observed in the alignment of knee joints in the HQ group despite heel lift bilateral squats being performed. Therefore, the improvement of the lower limb alignment caused by the application of heel lifts is effective when the cause of knee valgus is related to the muscle function, such as strength or flexibility, but the present study focused on structural problems, such as the Q angle, yielding different results from previous studies.

The second result of this study also revealed a significant difference between the two groups in the transverse plane movement value of the hip joint. This finding suggests that the hip internal rotation angle is significantly greater in the HQ group than in the N group. In the HQ group, knee valgus was more severe under the plantarflexed condition than in the N group. Hence, the barefoot squat did not induce knee valgus in the HQ group compared to the N group. On the other hand, valgus intensified when the ankle was in a plantarflexed position. Particularly noteworthy is the increase in hip internal rotation in this posture. The increased hip rotation in the HQ group during the plantarflexion squat coincides with a similar decrease in hip abduction. Although not statistically significant, hip adduction tends to increase during plantarflexion squats, likely exacerbating knee valgus. A previous study reported that a high Q-angle increased hip adduction when performing a single-leg squat, causing femoral internal rotation and placing the knee into a valgus position [30]. Willson and Davis also reported that the transverse plane rotation of the lower limb could decrease the patellofemoral contact area during a bilateral squat, increasing retro-patellar stress [31]. Schmidt et al. reported that increased hip internal rotation was associated with greater chronic hip joint pain during a single-leg squat [32]. Long-term and repeated abnormal movement patterns may lead to lower extremity joint problems because plantarflexion bilateral squats in the HQ group showed increased hip internal rotation.

In clinical practice, heel-raised bilateral squat exercise is applied to make the body stand upright and improve the strength of the legs [33]. This exercise is also applied to induce greater squat depth in subjects with a limited ankle dorsiflexion range of motion [34, 35]. In people with normal knee alignment, the heel-raised bilateral squat may be used as an effective method, but this study showed that HQ results in compensatory movements such as knee joint abduction and hip joint internal rotation. The results of this study alone would not be able to confirm that long-term performance of ankle plantarflexion posture, such as a heel-raised bilateral squat, induces lower limb disease in the valgus knee. Nevertheless, preventive strategies are needed to control the ankle angle when performing exercise in the valgus knee because abnormal alignment and incorrect posture of the lower limb joints may act as risk factors for lower limb injury.

This study had some limitations. The major limitation is the difficulty in generalizing the results because of the small sample size and the unequal distribution of males and females in each group. The subjects were instructed to perform squat movements up to their maximum range of motion for 10 minutes, but the HQ group had a high drop-out rate because of the difficulty in performing full squat movements. Controlling the drop-out rate was challenging because the participants could withdraw from the study at any time, according to the research ethics guidelines. The minor limitation is that these findings are limited to individuals with high Q angles despite various factors influencing squats. Therefore, further studies will be needed to include larger sample sizes and explore comprehensive factors affecting squat movements.

This study investigated the change in the angle of movement of the lower extremity joints in healthy men and women in their 20s to determine how the posture changes in the ankle joint affect the lower extremity joints in a normal Q-angle group and high Q-angle group. The current study showed that, when performing a plantarflexion squat, the HQ group showed further intensified knee joint abduction and hip joint internal rotation compared to the group with normal knee alignment. Therefore, this study highlights the need to control the posture of the lower extremity joint when performing the bilateral squat and an analysis of the effect of an abnormal alignment of the lower extremities on the body to develop strategies for preventing future injuries.

This work was supported by Youngsan University Research Fund of 2024.