Physical Therapy Korea

The lumbo-pelvic-hip core complex is a musculoskeletal structure that stabilizes the spine and pelvis [1,2]. The increased lumbo-pelvic muscles increase trunk stability and muscle coordination, thereby reducing the risk of low back injury [3]. Inappropriate control of the gluteal muscles in asymptomatic individuals prevents lumbo-pelvic stability, which is an early sign of low back diseases [4-6]. Thoracolumbar stability requires lumbo-pelvic muscle activation [7]. Trunk stabilization exercises enhance the coordination of the pelvis and hip joints and the activation of the gluteal and lower extremity muscles [8,9]. However, during these movements, repeated excessive mechanical load on the lumbar region and pelvis results in compensatory movements, such as tilt and rotation, which compress the spine and cause low back pain (LBP) [10-13].

Women with gluteus maximus (GM) weakness adopt abnormal movement strategies, resulting in compensatory movements of the lower back [6]. GM stabilizes the lumbo-pelvic-hip complex and is therefore a major contributor to weight-bearing movements. Its most important role is in hip extension. Weakness of GM increases the stress on the intervertebral disc and lumbar spine ligaments [6,14,15]. Trunk and lumbo-pelvic exercises instantaneously increase the activities of erector spinae (ES) and hamstring muscles to compensate for the hip extension movement [5,9,16]. Thus, the level setting and steps of the exercise program should be determined to ensure greater GM muscle activity than ES or hamstring muscle activity during trunk stabilization exercises [17].

The four-point kneeling arm and leg lift (FKALL) exercise are the most common types of lumbo-pelvic strengthening exercises. They are frequently used in movement control tests to detect uncontrolled movements of the trunk and pelvis [18,19]. Stevens et al. [19] found high relative activity levels of the ipsilateral lumbar and contralateral thoracic parts of the iliocostalis lumborum and the contralateral lumbar multifidus (MF) during FKALL. Moreover, 120° flexion of the supporting leg during exercise induced high relative muscle activity levels of all back muscles, except for the latissimus dorsi. Therefore, the assisted hip variable position during trunk stabilization exercise is affected by lumbo-pelvic muscle activity. Also, previous studies have found significant differences in muscle activities between prone hip extension (PHE) and PHE-adduction and PHE-abduction exercises. The PHE-abduction position more increases the GM muscle activity than PHE [14]. Additionally, Jeon et al. [20] measured the activities of ES, GM, biceps femoris (BF), and semitendinosus (ST) muscles during three hip extension exercises. The GM muscle activity was the greatest during PHE with knee flexion, whereas the BF and ST amplitudes were significantly higher during PHE than during other exercises [20,21]. Based on the results of this study, the changes in the hip position affect the lumbo-pelvic muscle activity during hip exercise.

Stability of the trunk and lumbo-pelvic complex muscle activities is often associated with spine stability [19,22,23]. The co-activation of the trunk, gluteal, and lower extremity muscles decrease LBP and prevents abnormal movement patterns [24-26]. Also, reduced gluteal muscle activity and strength were found in patients with LBP [6,15,27]. Previous studies have shown that hip position can alter the activation of the abdominal muscles and the GM, which has important effects on lumbo-pelvic stability. However, no studies have measured the activation of the GM and the abdominal muscles at different hip positions during FKALL. Thus, we examined internal oblique (IO), external oblique (EO), MF, GM activations and pelvic tilt and rotation angle during three different FKALL (pure FKALL, FKALL with 120° hip flexion of the supporting hip, FKALL with 30° hip abduction of the lifted leg). We hypothesized that the GM and MF muscle activities are increased during change of the hip posture, and the pelvic angle will show no significant change during FKALL.

1. Participants

The present experiment included 28 healthy participants without diseases or disorders of the back or pelvis within the previous year, history of surgery or fracture, or limited range-of-motion. We excluded participants with low back surgery or disorders, such as disc herniation, spondylolisthesis, and stenosis, or neurological disorders, including sciatic nerve pain and current physical treatment for low back or pelvic pain. The mean age, height, and weight of participants were 30.6 ± 4.5 years (range, 20–50 years), 161.3 ± 4.16 cm (range, 157–194 cm), and 70.0 ± 2.44 kg (range, 42–90 kg) (Table 1). All participants read and signed an informed consent form approved by the Institutional Research Review Committee of Inje University (IRB no. INJE-2022-11-020-001).

Table 1 . General characteristics of the subjects (N = 28).

Variable	Value
Age (y)	30.6 ± 4.5
Height (cm)	161.3 ± 4.16
Weight (kg)	70.0 ± 2.44

Values are presented as mean ± standard deviation..

Based on the study by Kim et al. [18], a power analysis was performed using the participants’ muscle activity levels of the lower extremity and lower back during three FKALL exercise (pure FKALL, FKALL with 120° hip flexion of the supporting leg, FKALL with 30° hip abduction of the lifted leg). The sample size for the study was calculated using the G-power software (ver. 3.1.2; Franz Faul, Kiel University) a power of 0.80 and α level of 0.05. The power analysis showed that at least 15 participants were required. We enrolled 28 participants to perform FKALL to identify significant differences in muscle activity and pelvic rotation during leg flexion and abduction.

2. Surface EMG and Data Processing

Three leg posture variation exercises were performed by the study participants. The exercises were performed with the supporting leg in 120° flexion, lifted leg in 30° abduction and without it hip posture (dominant right leg). The right-side GM, IO, EO, and MF muscle activities were measured during the three FKALL positions. Electromyography (EMG) electrodes were attached to the GM, IO, EO, and MF muscles on the right side of the trunk and back in accordance with the previous studies [8,10,28,29]. EMG signals were processed using a Trigno wireless EMG system (Delsys). EMG value of the muscles was analyzed after determining the root mean square. Raw EMG signals were rectified and bandpass-filtered (20–450 Hz). The EMG activity of each muscle was normalized to that during maximal voluntary isometric contraction (MVIC), measured using manual muscle tests after linear filtering of the data for 5 seconds. The MVIC measurement posture for each muscle was measured according to previous studies [10,18,19,22]. The initial and final 1 second of data were discarded, and the average EMG signal from the middle 3 seconds was considered 100% of MVIC. Each FKALL exercise set was performed three times, and a 90-second rest was allowed after each set. Each set was repeated six to eight times, with a holding time of 6 seconds. The entire set was repeated three times, with a 90-second rest period after each set.

3. Real-time Angle Inclinometer

The WitMotion sensor BWT901CL function of the Bluetooth inclinometer (WitMotion, Shenzhen Co., Ltd.) was used to measure frontal pelvic tilt and transverse rotation. The WitMotion sensor consists of a triaxial gyroscope and an accelerometer. This motion sensor tool transmits real-time information related to the pelvic motion in sagittal, frontal, and transverse planes to the Android inclinometer app (Wit-app, WitMotion, Shenzhen Co., Ltd.). Wit-app displays the pelvic tilt and rotation angles in three planes on the screen. The positive numbers on the x-axis of the Wit-app represent the anterior tilt of the pelvis and positive numbers on the y-axis represent the rotational angle of the pelvis. In the present study, we measured the pelvic tilt and rotation angles during FKALL using the WitMotion tool attached at the level of S2. Before exercise, the motion sensor was calibrated to 0° at the starting four-point kneeling position in pelvic neutral alignment. To maintain the starting kneeling position, participants were instructed to assume a resting kneeling position with their body weight equally distributed between both knees, feet, and arm on the testing floor.

4. Experimental Procedures

Figures 1 and 2 illustrate a step of FKALL. Participants asked to perform FKALL with 120° hip flexion of supported leg and 30° abduction of lifted leg while maintaining neutral pelvic alignment and then raise their left arm position for 5 seconds, with arm and leg extended. First, the study participants performed the FKALL without control any movement to get comfortable with performing the exercise on their own. Second, they performed FKALL with the supporting leg in 120° flexion and maintained their ischial bone in contact with the target bar and right leg in extension for 5 seconds (Figure 1B). Third, they performed FKALL with 30° abduction of lifted leg while maintaining their calcaneus bone in contact with the target bar for 5 seconds (Figure 2B). These three steps were performed by the participants in a random order. Each trial was repeated three times. The resting period between the FKALL exercises was 3 minutes. The goniometer was used to measure positions of 120° hip flexion and 30° hip abduction before contact the target bar. The tape was attached to the floor to mark the position of 120° flexion and 30° abduction in relation to the target bar positioned on the floor before exercise.

Figure 1. FKALL exercise with the supporting hip in 120° flexion. (A) Starting position with 120° hip flexion, (B) final position with 120° hip flexion. FKALL, four-point kneeling arm and leg lift.

Figure 2. FKALL exercise with hip abduction in 30° extension. (A) Start position with 30° hip abduction, (B) final position with 30° hip abduction. FKALL, four-point kneeling arm and leg lift.

5. Statistical Analysis

Data were analyzed using PASW Statistics software (ver. 18.0; IBM Co.). Data were analyzed using repeated-measures one-way analysis of variance (ANOVA) to compare differences between participants in terms of the intervention factors (pure FKALL, FKALL with 120° hip flexion of the supporting leg, FKALL with 30° hip abduction of the lifted leg), including the right IO, EO, MF, and GM muscle activities and pelvic tilt and rotation angles. In analyses, p < 0.05 was considered statistically significant.

Table 2 presents the EMG data for the IO, EO, MF, and GM muscle activities during pure FKALL, FKALL with 120° hip flexion of the supporting leg and FKALL with 30° hip abduction of the lifted leg. The GM muscle activity was increased during 120° hip flexion and 30° abduction compared to the comfortable position (GM with 120° hip flexion: mean difference = –14.692, p < 0.005; GM with 30° hip abduction: mean difference = –21.944, p < 0.001). Additionally, a significant increase in MF muscle activity was observed during exercise with 120° hip flexion and 30° abduction compared to exercise without control any movement (MF with 120° hip flexion: mean difference = –13.337, p < 0.05; MF with 30° hip abduction: mean difference = –20.166, p < 0.001). By contrast, the IO muscle activity showed no significant difference between 120° hip flexion (p = 0.791) and 30° abduction (p = 0.781) and the without control any movement. The EO muscle activity was also not significantly different between exercise with and without 120° hip flexion and 30° hip abduction (Table 2).

Table 2 . Muscle activities of the IO, EO, MF, and GM during the FKALL (N = 28).

Muscle (%MVIC)	FKALL	FKALL with hip flexion	FKALL with hip abduction	F	p-value
IO	54.24 ± 15.53	60.33 ± 22.04	60.37 ± 22.42	0.851	0.431
EO	39.97 ± 20.21	43.00 ± 21.35	46.89 ± 18.29	0.843	0.434
MF	55.67 ± 20.07	69.01 ± 15.08a	75.84 ± 13.42b	10.899	< 0.001
GM	52.10 ± 13.99	66.80 ± 15.36a	74.05 ± 16.83b	14.678	< 0.001

Values are presented as mean ± standard deviation. %MVIC, percentage of maximal voluntary isometric contraction; FKALL, four-point kneeling arm and leg lift; IO, internal oblique; EO, external oblique; MF, multifidus; GM, gluteus maximus. aSignificant differences between with and without hip flexion. bSignificant differences between with and without hip abduction..

Table 3 shows the analysis of pelvic rotation angles. The pelvic tilt angles were 20.37° ± 5.63° without hip movement conditions, 19.01° ± 5.62° in hip flexion, and 18.60° ± 6.18° in hip abduction. The pelvic rotation angles were 15.35° ± 0.76° without hip movement conditions, 14.46° ± 1.10° in hip flexion, and 13.97° ± 1.97° in hip abduction. The pelvic transverse rotation angle was not significantly lower in hip flexion than in hip abduction (p > 0.05). No significant difference in pelvic tilt angle was found between with and without hip flexion and abduction (p > 0.05).

Table 3 . Comparison of the pelvic tilt and rotation angles during the FKALL (N = 28).

Variable (°)	FKALL	FKALL with hip flexion	FKALL with hip abduction	F	p-value
Pelvic tilt	20.37 ± 5.63	19.01 ± 5.62	18.60 ± 6.18	0.196	0.734
Pelvic rotation	15.35 ± 0.76	14.46 ± 1.10	13.97 ± 1.97	2.227	0.162

Values are presented as mean ± standard deviation. FKALL, four-point kneeling arm and leg lift..

Previous studies have found that altered posture and joint mechanics affect muscle contraction and tension by changing the posture of the hip joint through increased activities of the GM and MF muscles [15,30]. Our study showed increased GM activity based on the changes in the position of the hip joint; a similar increased MF activity was also found. Increased GM and MF activities were observed in the 120° hip flexion and 30° hip abduction positions. These positions require greater muscle tension to stabilize the FKALL posture for leg lift [19,31]. As a result, the GM and MF activities may be increased during 120° hip flexion, as the associated ilium-posterior tilt leads to relatively increased ilium-anterior tilt of the extended leg for reciprocal movement balance [32,33]. Therefore, a relative increase in the posterior pelvic tilt of the 120° flexed hip causes further extension of the lifted leg, leading to greater contraction of GM and MF as hip extensors. It is because the relatively more increased lower extremity extension moment has occurred during FKALL than inclinometer sensor located in the center of the pelvis in S2, is in a neutral sacrum alignment while there is no change in the pelvic tilt angle and only change muscle activities by the simultaneous contraction of the extensor muscles of the lifted leg and the flexor muscles supporting leg side for the reciprocal movement balance control.

Previous studies have shown that the GM and MF activities increase hip flexion and abduction during trunk strengthening exercises, because of greater efficiency with movements in the direction of the muscle fibers [19,34]. An anatomical study by Barker et al. [34] showed that GM muscle fibers lie in a 32°–45° diagonal direction. Kang et al. [15] showed that 30° hip abduction maximizes the stimulation position of the GM muscle fibers by producing increased GM muscle activity. In the present study, a similar increase in muscle activity was observed with 30° hip abduction. Gluteal muscles weakness is common in LBP and these muscles are the major extensors of the hip joint. Among them, GM is the most important component of the lumbo-pelvic complex during initial hip extension movement [5,35]. Our results suggest that 30° hip abduction during FKALL induces selective GM muscle contraction. The 30° hip abduction posture during trunk stabilization exercises, such as the FKALL, effectively strengthens the GM muscle in patients with LBP.

Also, our study showed increased MF activity in the 30° hip abduction lifted leg position during FKALL. Previous studies have shown that the lumbar spine controls intersegmental movements and co-activates the spine extensor and gluteal muscles [12,26]. Increased activity of MF prevents LBP and controls excessive movement of the lumbo-pelvic region [23,36]. MF muscle is a short muscle that connects the thoraco-lumbar fascia and the paraspinal muscles through the lumbar spine. It can generate sufficient tension to allow inter-segmental control of the spine vertebrae [18,37]. LBP leads to instability of the lumbo-pelvic region due to repetitive load compression and abnormal segmental movements, causing compensatory ES muscle activity. Choi et al. [38] examined that increased MF and GM muscle activities as agonists during hip extension could reduce or inhibit ES muscle activity as a synergist. Additionally, MF muscle fascia is mainly located from the thoraco-lumbar junction to the gluteal line [39]. These fascial connections are responsible for the increased MF muscle activity in 30° hip abduction during FKALL. The delayed MF muscle activity and a relative increase in ES muscle activity in individuals with LBP evoke excessive extension and pain [40]. Thus, our study finding that activation of the MF and GM muscles, which is important during hip extension movement for lumbo-pelvic stability and 120° hip flexion, 30° hip abduction posture change, affects the stabilizer muscle activity during FKALL.

However, our results indicate a lack of changes in EO and IO muscle activation during exercise. MF is a co-contracting muscle, similar to the transverse abdominis (TrA), for lumbo-pelvic control [2]. These muscles are activated during the acute stages for stabilizing the trunk and lumbo-pelvic region while maintaining minimal contraction of the other synergistic muscles, such as the EO, IO, rectus abdominis, and ES [41,42]. On the basis of the present results, MF muscle shows greater feed-forward contractions than the synergistic muscles for trunk and lumbo-pelvic control. The EO and IO muscles do not alter the trunk and lumbo-pelvic control during exercise, possibly by activating agonist muscles, such as MF. Therefore, the changes in hip posture increase the lower back, stabilizing muscle activity, and maintain the activity of the minimal global synergistic muscles (IO and EO).

Moreover, in the present study, no significant differences in pelvic tilt and rotation angles were found. Previous studies have shown that control of excessive pelvic tilt and rotation requires the recruitment of the local and global abdominal muscles for the stabilizing lumbo-pelvic complex constructure by using a breathing technique, such as the abdominal draw-in maneuver [43]. In this study, effectively breathing techniques were not applied to during FKALL exercise for the stabilize lumbo-pelvic complex, which appears to have been less effective in causing changes in pelvic tilt and rotation angle. Moreover, the local stabilizing muscles regulate the spine segmental motion within a tonic muscle function rather than not involving the stabilizing muscles only in a particular direction through motion during the FKALL [41]. Therefore, other supporting methods have been proposed to achieve pelvic tilt and rotation control.

There were a few limitations of this study. First, ES muscle activity was not measured, which could demonstrate compensatory contraction of the TrA and GM, the stabilizing muscles of the trunk and lumbo-pelvic. Second, various techniques are required to measure MF muscle thickness, including ultrasonography. Additionally, hip extension was measured only on the right side and could not be measured using the contralateral MF or GM and abdominal muscles that are simultaneously active during FKALL. Finally, future studies should include individuals with LBP and musculoskeletal disorders, to verify the effects of exercise in this population.

This study proposes an effective muscle exercise for the MF and GM when performing the FKALL by comparing the effect of hip position on the abdominal and lumbo-pelvic muscles activities during the FKALL exercise. The change in hip movement in relation to the supporting hip flexion or lifted leg abduction position was more effective than not changing the hip position during FKALL exercise. This finding suggests that the muscle activities of the MF and GM increased more with than without the supporting hip 120° flexion position. The lifted hip 30° abduction position can also be used for selective activation of the MF and GM muscles during the FKALL. In the present study demonstrated that no significant change in abdominal muscles activities and pelvic angle during hip movement in the FKALL exercise. Therefore, it is recommended to perform FKALL with hip flexion or hip abduction to facilitate thoraco-lumbar stabilizers like MF and GM muscles effectively.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: NK, MK, JO. Data curation: NK, JO. Formal analysis: NK, MK, JO. Investigation: NK, WJ. Methodology: NK. Project administration: NK, JO. Resources: NK, MK, JO. Software: NK, WJ, MK. Supervision: NK, JO. Validation: NK, JO. Visualization: NK, MK, JO. Writing - original draft: NK. Writing - review & editing: NK.