Physical Therapy Korea

The gluteus maximus (GM) is a muscle that stabilizes the area of the hip joint and the hip joint through extensive ligament and attachment to the fascia [1]. The GM has the largest surface area of the hip muscle and is divided into a superficial layer and a deep layer. The superficial layer has various attachment areas on the back, such as the ilium, sacrum, and adjacent thoracolumbar fascia, stabilizing the hip joint’s anterior and posterior directions [2]. The superficial layer is attached to the tibia through the fascia latae as well as the hip joint, reinforcing the relatively continuous maintenance of the head of the femur within the acetabulum when extension of the hip joint [3]. It also stabilizes the knee joint through the tensor fasciae latae. The deep layer is attached to the coccyx, the sacrotuberous ligament, and the posterior sacroiliac ligament, strengthening and stabilizing the sacroiliac joint and the hamstring (HS) [4]. The contraction of the HS and GM causes the posterior pelvic tilt of the ilium, resulting in a nutation torque. The neutralization torque of the sacroiliac joint contributes to stability by increasing the tension in the sacrotuberous ligament and increasing the pressure and shear force of the joint surface [1].

When the muscles that provide stability to the hip joint weaken, symptoms of back pain, hip impingement, and knee pain appear. In particular, excessive HS activation due to the weakness of the GM causes pain in the hip joint [5]. Therefore, exercises have been proposed to increase the muscle activity of the GM rather than the muscle activity of the HS in people with weak GM [6]. Previous studies have shown that bridge excise and prone hip extension excise have been recommended for selective strengthening of the GM. However, Sahrmann et al. [7] argued that muscle coordination through functional movement is needed rather than selective muscle strengthening for limited functional movement. Therefore, to improve movement, it is necessary to include functional, compound exercises that appear in daily life, not single-joint exercises concentrated in a specific area. The compound exercises which include functional and appear in daily life are lunge, squat, and single-leg deadlift [8].

The single-leg deadlift is one of the highest GM activation exercises [8]. Also, it does not have a large knee flexion angle, so unlike other complex joint exercises, it can strengthen the GM without putting much pressure on the knee. In addition, exercise on one leg makes the muscle activity of joint stabilization muscles and can correct muscle imbalance [9]. Therefore, the single-leg deadlift is recommended to strengthen the trunk and posterior chain muscles in sports athletes’ rehabilitation exercises and can be applied to people with weak leg muscles [10].

Previous studies investigated the effect of leg muscle activities according to knee flexion angles during the single-leg deadlift or compared muscle activity between single-leg deadlift and other exercises [11,12]. However, changing the position of the trunk during compound exercise can affect the dynamics of the lower extremity by shifting the position of the center of mass (COM) relative to the support base [13]. That is, the leg muscle activities may be affected during the single-leg deadlift according to the forward trunk lean angle of the body changing the position of the body. Hence more research on this is needed.

Therefore, the purpose of this study was to effective muscle exercise for the GM when performing the single-leg deadlift by comparing the effect of forward trunk lean angle on muscle activity of the GM and HS during the single-leg deadlift.

1. Subjects

Twenty-one healthy female volunteers aged 19–30 entered the study (Table 1). We used G*power software (ver. 3.1.9.7; Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany) to calculate the sample size. The sample size calculated by G*power (repeated measures ANOVA; effect size f: 0.3, α err prob: 0.05, power [1-β err prob]: 0.8) number of measurement was 20. Therefore, we established that the estimated sample size was 21.

Table 1 . General characteristics of the participants.

Variable	Value
Age (y)	24.10 ± 3.25
Height (cm)	161.90 ± 4.28
Weight (kg)	54.52 ± 4.80

Values are presented as mean ± standard deviation..

Inclusion criteria were: (1) able to single-leg deadlift without pain and (2) no lumbar, knee, and hip joint pain or disorder within the last 12 months. Exclusion criteria were: (1) any history of lumbar, pelvic, and lower extremity disorders; (2) unable to single leg deadlift because of lumbar and pelvic pain; and (3) tightness of hip flexor muscles and HS [14]. The experimental protocol was detailed to all participants, and all informed consent was given to participants before participating. The study was approved by the Institutional Ethics Committee of Inje University (IRB no. INJE-2022-06-028-001).

2. Instrumentation

The activities of the GM (half the distance between the greater trochanter and the S2 spinous process) and HS (posterior part of half the distance between an ischial tuberosity and head of a fibula) [15] were recorded using 2EM instrument (4D-MT; Relive, Gimhae, Korea). The sampling rate was set at 1,000 Hz, and the band-width was set at 0–500 Hz. Electromyography (EMG) value of the muscles was analyzed after determining the root mean square (RMS). The dominant leg was selected as the testing side. The direction of the surface electrode was attached to be horizontal to the direction of the muscle fiber, and a disposable Ag/AgCl surface electrode was used.

Maximum voluntary isometric contraction (MVIC) was measured to normalize the collected muscle activities before the experiment. The MVIC measurement posture for each muscle was measured according to Hislop et al. [16]. All measurements were performed for 5 seconds and repeated three times for each muscle. The signal was analyzed by excluding 1 second each at the start and end of the measurement.

Forward trunk lean angle and HS length were recorded with an inclinometer (Acumar Digital Dual Iclinometer; Patterson Medical/Sammons Preston, Bolingbrook, IL, USA). To record the forward trunk lean angle, one side of the inclinometer is located on L3, and the other is on 2/3 from the distal femur (Figure 1). Prior to commencing the three forward trunk lean trials, the dual inclinometer was zeroed to the standing with knee flexion 20°, and the angle was measured by tracking the motion of the trunk. One measurer used the inclinometer to check whether it was maintained at forward trunk lean angles during the each task.

Figure 1. Place of dual digital inclinometer (Acumar Digital Dual Iclinometer; Patterson Medical/Sammons Preston).

To prevent injury, knee slight flexion is needed when the trunk forward flexion during the single-leg deadlift. However, the knee must be controlled in the experiment, so keep the knee flexion 20°. At this time, the other measurer used a goniometer to check whether it was maintained at knee flexion 20° during the exercise. The goniometer was placed on the lateral part of the femur and leg parallel to the line that links the great trochanter and the lateral epicondyle of the femur and lateral malleolus of the fibula. The dominant leg was selected as the testing side [17].

3. Procedures

Before subjects participated in this study, they were informed how to single-leg deadlift and perform each task. The study subjects practiced the exercise posture accurately by performing the exercise according to each forward trunk lean angle for 10 minutes to become familiar with the exercise. Single leg deadlift was performed three times for 10 seconds at 30°, 60°, and 90° in forward trunk lean angles (Figure 2). For 4 seconds out of 10 seconds was the forward tilting, for 2 seconds was maintained, and returned to the upright position for the remaining 4 seconds. When performing the exercise for each angle, the average value for 4 seconds, excluding the first and last 3 seconds after 10 seconds of measurement, was used for data analysis. A metronome was set at 60 bpm to standardize exercise time. The order of each task was randomly assigned. There was a break time to avoid muscle fatigue which 10 seconds was provided between trials and 1 minute was provided between different angles.

Figure 2. Process of the single-leg deadlift task. (A) Start position, (B) forward trunk lean angle 30°, (C) forward trunk lean angle 60°, and (D) forward trunk lean angle 90°.

With the subject standing straight, the legs maintained a 20° knee flexion. Zero adjustments are standing with 20° knee flexion. To prevent a fall, the non-dominant hand was placed on the table and instructed the subjects provided light touch which is not push off or lean on the table so that their bodies did not fall during exercise. When the signal was heard, the non-dominant leg was taken off the ground, and the forward trunk lean was slowly tilted forward. When the tone indicating the end was heard, it slowly returned to the standing position.

4. Statistical Analysis

The data were analyzed using IBM SPSS (ver. 29.0; IBM Co., Armonk, NY, USA), and the Shapiro-Wilk test was performed to determine whether the data were normally distributed. In addition, one-way repeated measures ANOVA was used to compare the muscle activities between the GM and HS during single leg deadlift according to forward trunk lean angles. If a significant effect was found, the Bonferroni post-hoc test was performed. The level of statistical significance for all analyses was set at p < 0.05.

The muscle activities of the GM were significantly different among three conditions (30°, 52.60 ± 9.42 %MVIC; 60°, 55.15 ± 8.85 %MVIC; 90°, 57.86 ± 8.71 %MVIC; p < 0.01). The muscle activities of the HS were significantly different among three conditions (30°, 50.39 ± 9.45 %MVIC; 60°, 54.33 ± 9.19 %MVIC; 90°, 59.17 ± 9.02 %MVIC; p < 0.01). The larger the angle, the higher the muscle activity of the GM and HS, and they were significantly highest at 90° of forward trunk lean (Table 2).

Table 2 . Comparison of muscle activities among the three forward trunk lean angles during single-leg deadlift (N = 21).

Muscle	%MVIC			F	p-value
	30°	60°	90°
GM	52.60 ± 9.42b	55.15 ± 8.85a,b	57.86 ± 8.71a	70.78	< 0.01**
HS	50.39 ± 9.45b	54.33 ± 9.19a,b	59.17 ± 9.02a	193.40	< 0.01**

Values are presented as mean ± standard deviation. GM, gluteus maximus; HS, hamstrings; %MVIC, %maximum voluntary isometric contraction. ^aSignificant difference from 30°. ^bSignificant difference from 90°. **p < 0.01..

GM/HS ratio was significantly higher at 30° and 60° of forward trunk lean compared to 90° (GM/HS: 30°, 1.03 ± 0.01 %MVIC; 60°, 1.03 ± 0.02 %MVIC; 90°, 0.98 ± 0.03 %MVIC; p < 0.01). GM/HS ratio was not significantly different between 30° and 60° of forward trunk lean, but GM activity was significantly higher at 60° of forward trunk lean than at 30° (Table 3).

Table 3 . Comparison of %MVIC ratio among the three forward trunk lean angles during single-leg deadlift (N = 21).

Muscle	%MVIC ratio			F	p-value
	30°	60°	90°
GM/HS	1.03 ± 0.01b	1.03 ± 0.02b	0.98 ± 0.03a	22.78	< 0.01**

Values are presented as mean ± standard deviation. GM/HS, gluteus maximus vs hamstrings; %MVIC, %maximum voluntary isometric contraction. ^aSignificant difference from 30°. ^bSignificant difference from 90°. **p < 0.01..

This study aimed to determine whether changes in the forward trunk lean angle can activate muscle activity of the GM more than the HS and suggest an effective single-leg deadlift method for the GM.

We found the increasing activity of the GM as the forward trunk lean angle increased and was significantly highest at 90° of forward trunk lean. This result is considered to have increased muscle activity due to the lever type. It was reported that when the single leg deadlift, the GM is a class 1 lever. The larger the forward trunk lean angle, the longer the length of the resistance arm, which requires more activity of the GM [18,19]. The gluteus medius at a maintained pelvic level belongs to the representative class 1 lever. Stastny et al. [20] compared the effects of dumbbell-carrying position on the muscle activity of the gluteus medius between a split squat and walking lunge exercise and showed that a split squat and walking lunge exercise could increase muscle activity of the gluteus medius by loading the opposite arm with a dumbbell. It is because the lever arm to the system’s COM is increased. Atkins et al. [21] also investigated that as the forward trunk lean increases when climbing the stairs, the COM moves forward, and the GM of muscle activity increases because of the increasing length of the resistance arm. Therefore, the muscle activity of the GM increases statistically significantly as the forward trunk lean angle increases due to the class 1 lever.

We also found that the muscle activity of the HS increased as the forward trunk lean angle increased and was significantly highest at 90° of forward trunk lean. We estimate the result to be the difference in the length of the HS. In the length-tension relationship, optimal length produces a great force by active force. However, with the muscle relaxed, there will be a force called the ‘passive’ force required to extend the muscle. So when stretching muscles, passive force is added along with active tension, creating the greatest force [1,15]. The greatest contraction of a muscle occurs when it stretches beyond its length. The HS is attached to the ischial tuberosity of the ischium, so the larger the forward trunk lean angle, the longer the length of the HS [22]. Previous studies have reported that the greater the forward trunk lean angle, the longer the muscle length, increasing the HS’s muscle activity [23,24]. This study’s result and basis correspond with previous studies. Therefore, the reason why the muscle activity of the HS increases statistically significantly as the forward trunk lean angle increases are considered to be due to the increasing HS length.

We found GM/HS ratio was significantly higher at 30° and 60° of forward trunk lean compared to 90° and 30° and 60° of forward trunk lean and greater than 1 %MVIC. It means the muscle activity of the GM is greater than the HS at 30° and 60° of forward trunk lean angle.

In this study, the muscle activity of the hip extensors muscles at 30°, 60°, and 90° in forward trunk lean angle showed a statistically significant increase. Furthermore, in the GM/HS ratio, the 30° and 60° of forward trunk lean angles were greater than 1 %MVIC and significantly higher than 90°. These results indicate that single-leg deadlift performance at 60° of forward trunk lean angle with a GM/HS ratio greater than 1 %MVIC and high muscle activity of the GM can be proposed as an effective GM muscle exercise.

There were a few limitations to this study. First, our results cannot be generalized because all the subjects were young and healthy females. Second, we recorded the data using surface EMG, which may be possible to crosstalk between recorded muscles. Third, for the safety of the subjects, the table was supported by hand, but there is a possibility of affecting the results. Fourth, the muscle activity of hip flexor muscles was not measured while performing three forward trunk lean angles. In future studies, it is necessary to obtain the muscle activity of the hip flexor muscles. Fifth, the differences in %MVIC among the three conditions did not exceed the minimal clinical differences, even though there were significant differences in the results of repeated ANOVA.

This study proposes an effective muscle exercise for the GM when performing the single-leg deadlift by comparing the effect of forward trunk lean angle on the GM and HS muscle activity during the single-leg deadlift. In the GM/HS ratio, 30° and 60° of forward trunk lean angles were greater than 1 %MVIC, and muscle activity of the GM was higher at 60° than at 30°. Therefore, we suggest that 60° of forward trunk lean angle during the single-leg deadlift is an effective muscle exercise for the GM.

None.

None to declare.

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

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