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Abstract

BACKGROUND:

Abdominal stabilization is an important factor during hip extension. Additionally, the prone standing position is a more effective and functional position for hip extension than the prone or supine positions.

OBJECTIVE:

To determine the concentric isokinetic strength of the hip extensors during prone standing hip extension (PSHE) using stabilization maneuvers such as abdominal resting (AR), abdominal bracing (AB), and abdominal hollowing (AH).

METHODS:

Thirty one healthy subjects were recruited for this study. In each of the 3 maneuvers the participants performed concentric isokinetic hip extensions, 4 at 60 ${}^{\circ}$ /s and 5 at 180 ${}^{\circ}$ /s. The outcome variables consisted of the peak moment (PM), total work (TW), and average power (AP).

RESULTS:

Hip extension strength, as indicated by the PM, differed significantly among the three conditions at both velocities ( $p<$ 0.05). Significantly greater strength was achieved with AB than with AH at 60 ${}^{\circ}$ /s, but no such differences were evident at 180 ${}^{\circ}$ /s.

CONCLUSIONS:

Prone standing hip extension with AB at 60 ${}^{\circ}$ /s offers an effective option for measuring hip extension strength.

Keywords

Abdominal bracing abdominal hollowing hip extension prone standing position

1. Introduction

The hip extensors provide stability to the pelvis [1] and play a particularly important role in many activities of daily living, including sit-to-stand, climbing stairs, and maintaining an upright posture during walking [2]. Inappropriate onset timing and weakness of the hip extensor muscles during gait results in decreased shock absorption at the sacroiliac joint [3, 4] and subsequent instability of the lumbopelvic region.

Active hip extension exercises in the prone position are performed primarily in a rehabilitation setting to strengthen the hip extensor muscles [5]. However, in individuals with lumbopelvic instability, prone hip extension may lead to severe back pain caused by undue stress and compression of the vertebrae due to excessive extension and rotation of the lumbar spine as well as anterior tilt and rotation of the pelvis [6, 7, 8]. In addition, individuals with low back pain (LBP) or lumbopelvic instability tend to have weaker hip extensors than healthy individuals due to over-activity of the erector spinae (ES) and hamstring muscles [9]. As a result, these individuals need to strengthen their hip extensors without causing back pain.

It has been suggested that the prone position does not allow as great a muscle contraction as the prone standing position [10, 11]. In the prone standing position, the hip is flexed and the hip extensor muscle fibers and lever arm are lengthened, allowing for a more favorable length-tension relationship and thus optimizing hip extension moment production [12]. Comerford and Mottram first proposed the prone table hip extension test as a method to co-activate the gluteus maximus and abdominal muscles, which are used to control the lumbar spine and pelvis. This posture is performed while the trunk is supported on the table, with both feet placed firmly on the floor and the lumbar spine in a neutral position [13]. Therefore, the prone standing position may be a more effective and functional position for hip extension than the prone or supine position [14].

Stabilization of the lumbopelvic region during hip extension is an important factor to prevent LBP. Kahlaee et al. reported that lumbar stabilization exercises during prone hip extension altered the delayed onset timing of muscle contractions and reduced stress on the vertebrae by decreasing activation of the ES [15]. Thus, stabilization exercises are effective maneuvers to decrease LBP and prevent its recurrence by providing spinal stability and a proper recruitment pattern of the superficial and deep muscles on the surrounding spine [16, 17, 18]. Prone stabilization exercises are often used in the initial rehabilitation of patients with LBP to minimize external load and pain [19, 20], reduce inappropriate movements of the trunk, and maintain a neutral spine while applying an internal or external force [21]. Abdominal bracing (AB) and abdominal hollowing (AH) maneuvers are commonly used to activate the abdominal muscles to increase spinal stability. AH is performed to activate the deep abdominal muscles, namely the transverse abdominus (TrA) and internal oblique, while minimizing superficial global muscle activity [22]. Meanwhile, AB focuses on activating all of the abdominal wall muscles. According to McGill, sufficient stability of the lumbar spine is achieved with modest levels of simultaneous activation of all trunk muscles [23]. Recently, it was suggested that AH is suitable for treating spinal instability with an altered abdominal muscle recruitment pattern, whereas AB might be more suitable for use in healthy participants [24].

Although there have been many comparative studies on abdominal contraction maneuvers in the supine or sitting position [25, 26], few studies have compared the dynamic postures commonly used in clinical practice [27, 28, 29]. Previous studies have measured muscle activity through surface electromyography (sEMG) during prone hip extension with abdominal contraction maneuvers [15, 30, 31, 32]. One study measured isometric hip extension strength during prone standing [26] but it did not account for dynamic strength through a full range of motion (ROM) at different velocities. However, no studies have investigated isokinetic hip extension strength with different abdominal bracing maneuvers. Thus, the purpose of this study was to determine the dynamic concentric strength of the hip extensors in the prone standing position coupled with abdominal stabilization maneuvers and movement velocity.

2. Method

2.1 Participants

G*Power software was used to estimate the sample size (ver. 3.1.9.2; Franz Faul, University of Kiel, Kiel, Germany) according to Cohen’s d. An a priori sample size calculation determined that at least 28 participants were required to achieve a power of 0.80, an alpha level of 0.05, and effect size of 0.25. A total of 31 healthy male subjects with age: 29.1 $\pm$ 4.4 years, height: 176.4 $\pm$ 5.6 cm, weight: 73.5 $\pm$ 7.4 kg were recruited from the Department of Physical Therapy at Yonsei University in Korea (Table 1). The exclusion criteria included: 1) a history of lumbar, sacroiliac, or lower limb injury within the last 6 months, 2) tightness or limited ROM with hip extension, 3) limited hip flexor flexibility as determined by the Thomas test, 4) lumbar or hip pain while performing prone standing hip extension, and 5) past or present neurological, musculoskeletal, and cardiopulmonary diseases.

Table 1
Descriptive characteristics of the participants ( $n=$ 31)

	Mean (SD)	Range
Age (years)	29.1 $\pm$ 4.4	22–40
Height (cm)	176.4 $\pm$ 5.6	170.0–191.5
Weight (kg)	73.5 $\pm$ 7.4	58.9–85.9
Gender (male)	31
Dominant side (right/left)	31/0

SD, Standard deviation.

The experimental protocols and purpose of this study were explained in detail to all participants; they subsequently provided written informed consent. This study was approved by the Yonsei University Mirae Campus Institutional Review Board (1041849-201812-BM-114-04).

2.2 Experimental apparatus

A Biodex System 4 Pro (Biodex Medical Systems Inc., Shirley, NY, USA) was used to assess isokinetic function and the strength of hip extension in the prone standing position. All exercise tests were performed in isokinetic mode. A pressure biofeedback unit (PBU, Chattanooga Group Inc., Hixson, TN, USA) was used to ensure lumbopelvic motion and abdominal contraction during exercise and facilitate retraining of the participants’ movement patterns in the clinic.

Figure 1.

Prone standing position for the intervention of IKD.

2.3 Experimental procedure

Before performing any isokinetic tests, the back of each participant was fixed to the table using a Velcro strap to minimize any unnecessary compensation in the trunk. The participants leaned forward on a table to support their trunk with the hips flexed to 90 ${}^{\circ}$ and the knees in full extension (Fig. 1). The rotational axis of the hip was aligned with the rotational axis of the dynamometer based on the position of the greater trochanter. The pad was placed 5 cm above the midpoint of the knee joint line behind the thigh [33]. The participants were trained by a professional therapist about abdominal stabilization maneuvers. A professional therapist trained the participants to correctly perform the abdominal stabilization maneuvers with the aid of a PBU for 10–40 min prior to the start of the study [34]. Then, after a 5-min warm-up on a cycle ergometer (25 W and 50 rpm), the participants performed five sub-maximal practice contractions for each mode and at each velocity. The ROM of hip extension was individually set. Both legs were tested and the participants’ dominant side was always tested first. For isokinetic testing, the participants performed 4 maximal concentric hip extension contractions at 60 ${}^{\circ}$ /s and then 5 maximal contractions at 180 ${}^{\circ}$ /s [35]. The participants were allowed a 1-min rest between the velocities and a 5-min break between each test condition. The order of the tests was allocated randomly. The three conditions were as follows:

1.
Abdominal resting (AR)

AR indicates that the abdominal muscles did not spontaneously contract [36].
2.
Abdominal hollowing (AH)

AH is performed to activate the deep abdominal muscles, namely the TrA and internal oblique, while minimizing superficial global muscle activity.
3.
Abdominal bracing (AB)

AB is performed to activate all abdominal wall muscles, including the external, internal, and transverse muscles, without an abdominal drawing-in motion.

When applying the abdominal stabilization maneuver, a blood pressure cuff was placed under the abdomen to function as a PBU, which was checked under the supervision of a professional therapist. This method was applied as a reliable standardized procedure [30, 37, 38]. It was placed under the abdomen with the navel in the center and the distal edge of the pad in line with the right and left anterior superior iliac spine. With the cuff inflated to 70 mmHg, the participants were asked to perform an isometric abdominal contraction using the two separate maneuvers. When performing AH, the pressure was expected to fall by 4–10 mmHg, whereas an increase in pressure was expected for AB [22]. The participants were alerted when the pressure gauge was reduced below 4 mmHg or increased above 10 mmHg [39], and only data with a pressure change of $\pm$ 5 mmHg or less were analyzed.
2.4 Statistical analysis

The Statistical Package for the Social Sciences (Windows ver. 23.0; SPSS Inc., Chicago, IL, USA) was used for all statistical analyses. All dependent variables are presented as the mean $\pm$ standard deviation (SD). A one-way repeated measures analysis of variance (ANOVA) was used to compare hip extensor strength in the prone standing position among the three abdominal maneuvers. The Bonferroni correction was used for post hoc analysis. A $p$ -value less than 0.05 was considered statistically significant for all analyses.

Table 2
Isokinetic hip extension at an angular velocity of 60 ${}^{\circ}$ /s (Mean $\pm$ SD)

	Abdominal resting ${}^{\text{A}}$		Abdominal bracing ${}^{\text{B}}$		Abdominal hollowing ${}^{\text{C}}$		$F$ -value	$P$ -value	Post-hoc
Right extension (dominant)
PM (Nm)	106.77	$\pm$ 33.33	122.29	$\pm$ 37.13	112.25	$\pm$ 31.67	19.37	0.000 ${}^{*}$	B $>$ A, C
TW (J)	259.25	$\pm$ 85.19	296.76	$\pm$ 103.44	256.31	$\pm$ 84.01	12.95	0.000 ${}^{*}$	B $>$ A, C
AP (W)	61.95	$\pm$ 19.28	69.83	$\pm$ 20.31	62.67	$\pm$ 19.41	11.90	0.000 ${}^{*}$	B $>$ A, C
Left extension
PM (Nm)	105.93	$\pm$ 31.59	116.59	$\pm$ 32.20	108.51	$\pm$ 33.63	14.18	0.000 ${}^{*}$	B $>$ A, C
TW (J)	248.57	$\pm$ 87.91	274.43	$\pm$ 80.08	245.92	$\pm$ 78.02	6.41	0.003 ${}^{*}$	B $>$ A, C
AP (W)	59.43	$\pm$ 20.40	65.95	$\pm$ 20.79	60.18	$\pm$ 19.13	8.19	0.001 ${}^{*}$	B $>$ A, C

SD, Standard deviation; PM, Peak Moment; TW, Total Work; AP, Average Power. Nm, Newton Meter; J, Joule; W, Watts. ${}^{*}P<$ 0.05.

Figure 2.

Peak moment of hip extension during PSHE according to three abdominal contraction maneuvers. A. Right hip extension, B. Left hip extension at an angular velocity of 60 ${}^{\circ}$ /s. C. Right hip extension, D. Left hip extension at an angular velocity of 180 ${}^{\circ}$ /s. The means and SDs are shown as bars and hatches, respectively. ${}^{*}P<$ 0.05; ${}^{**}P<$ 0.001.

3. Results

On both sides, hip extension peak moment (PM), total work (TW) in joule and average power (AP) in Watt, were significantly different between the three abdominal contraction maneuvers at 60 ${}^{\circ}$ /s ( $p<$ 0.05; Table 2). A post hoc comparison revealed that the values of all 3 variables during AB were significantly greater than during AR and AH, but there were no significant differences in the PM, TW, and AP between AR and AH ( $p<$ 0.05; Figs 2–4A and B).

Table 3
Isokinetic hip extension at an angular velocity 180 ${}^{\circ}$ /s (Mean $\pm$ SD)

	Abdominal resting ${}^{\text{A}}$		Abdominal bracing ${}^{\text{B}}$		Abdominal hollowing ${}^{\text{C}}$		$F$ -value	$P$ -value	Post-hoc
Right extension (dominant)
PM (Nm)	87.35	$\pm$ 27.74	96.03	$\pm$ 24.51	93.78	$\pm$ 23.62	6.25	0.003 ${}^{*}$	B $>$ A
TW (J)	293.81	$\pm$ 113.37	328.41	$\pm$ 120.48	303.12	$\pm$ 137.79	3.20	0.048 ${}^{*}$	B $>$ A
AP (W)	97.84	$\pm$ 37.16	113.32	$\pm$ 35.44	103.05	$\pm$ 43.89	4.01	0.023 ${}^{*}$	B $>$ A
Left extension
PM (Nm)	83.81	$\pm$ 23.78	93.16	$\pm$ 24.13	90.99	$\pm$ 28.17	5.24	0.008 ${}^{*}$	B $>$ A
TW (J)	269.37	$\pm$ 90.74	309.93	$\pm$ 101.74	302.93	$\pm$ 146.08	3.95	0.040 ${}^{*}$	B $>$ A
AP (W)	87.51	$\pm$ 31.90	104.91	$\pm$ 33.88	98.55	$\pm$ 51.13	4.71	0.024 ${}^{*}$	B $>$ A

SD, Standard deviation; PM, Peak Moment; TW, Total Work; AP, Average Power. Nm, Newton Meter; J, Joule; W, Watts. ${}^{*}P<$ 0.05.

Figure 3.

Total work of hip extension during PSHE according to three abdominal contraction maneuvers. A. Right hip extension, B. Left hip extension at an angular velocity of 60 ${}^{\circ}$ /s. C. Right hip extension, D. Left hip extension at an angular velocity of 180 ${}^{\circ}$ /s. The means and SDs are shown as bars and hatches, respectively. ${}^{*}P<$ 0.05; ${}^{**}P<$ 0.001.

Figure 4.

Average power of hip extension during PSHE according to three abdominal contraction maneuvers. A. Right hip extension, B. Left hip extension at an angular velocity of 60 ${}^{\circ}$ /s. C. Right hip extension, D. Left hip extension at an angular velocity of 180 ${}^{\circ}$ /s. The means and SDs are shown as bars and hatches, respectively. ${}^{*}P<$ 0.05; ${}^{**}P<$ 0.001.

Similarly for the 180 ${}^{\circ}$ /s condition, on both sides, hip extension PM, TW, and AP were significantly different between the three abdominal contraction maneuvers ( $p<$ 0.05; Table 3). A post hoc comparison revealed that the values of all 3 variables during AB were significantly greater than those during AR, but there were no significant differences between AB and AH or between AR and AH ( $p<$ 0.05; Figs 2–4C and D).

4. Discussion

The results of this study show that using AB during PHSE enhanced the moment generated during hip extension compared to hip extension with AH or AR. These results are similar to the findings of other studies on hip extension strength. For example, Tayashiki et al. reported that hip extensor strength in the supine position increased when the subjects performed the AB maneuver [40]. On the other hand, Kim et al. reported that hip extensor strength in the prone position increased by using the AH maneuver [41]. Thus, it is unclear whether the AB or AH maneuver is more effective at enhancing hip extension strength in a non-standing position. In addition, all of these studies only measured muscle activity and did not directly measure isokinetic muscle strength components such as moment, work, and power.

The prone standing position in this study has the advantage of allowing for more stable and increased hip extension strength than could be achieved in the prone position in previous studies [15, 32, 37, 42]. Prone hip extension exercises are often incorrectly performed with excessive compensatory movements in the lumbar spine, inducing lumbar lordosis and unwanted anterior pelvic tilt. This excessive movement of the lumbar spine and pelvis can cause compression and extension stress in the spine and surrounding soft tissues [18, 22]. Thus, Waters confirmed in his study on the correlation between hip angle and hip extension strength that hip extension with 90 ${}^{\circ}$ of hip flexion is the most mechanically advantageous position to perform hip extension [43]. Pohtilla suggested that the prone standing position allows for a favorable length-tension relationship in the gluteus muscles, enhancing moment production during hip extension. In addition, the prone standing position puts the hip in a flexed position, which lengthens the hip extensor muscle lever arm and minimizes the activity of the hamstring muscles [12].

Several authors have recommended spinal stabilization exercises during dynamic hip extension movements to control excessive movements of the lumbar spine and pelvis [6, 44]. However, previous studies [15, 31, 37] have only measured the muscle activity of the lower extremities during hip extension with abdominal stabilization maneuvers. In this study, isokinetic concentric contractions were performed to assess also the moment-velocity relationship of the hip extensors at 60 and 180 ${}^{\circ}$ /s. The results of our study show that the PM during AB was significantly greater than when the AH and AR maneuvers were used at an angular velocity of 60 ${}^{\circ}$ /s. Tayashiki et al. reported that AB increased the force generated during hip extension by bracing the deep abdominal muscles and increasing intra-abdominal pressure, which radiates force to weakened muscles in the lower extremities [40]. On the other hand, Faries and Greenwood reported that AH could enhance neuromuscular control by selectively strengthening the TrA in a static position, but it could reduce the normal activity of other muscles in a dynamic position, thereby disturbing the natural coordination of the muscles and reducing the stability of the trunk [45]. In addition, Vera-Garcia et al. reported that AB was a more stable contraction maneuver than AH for stability of the lumbar vertebrae [26]. This seems to be consistent with our results, which show that performing AB during hip extension produced greater strength than when AH was performed. Therefore, applying AB during prone standing hip extension can increase hip extensor strength while maintaining trunk stability, not only in patients with LBP but also in healthy adults.

The PM when using AH was greater than with AR, but there was no significant difference at 60 ${}^{\circ}$ /s. Oh et al. reported that the muscle activity of the gluteus maximus during prone hip extension was significantly higher in the AH group than in the control group [37]. However, the prone standing position used in this study is a more dynamic posture than the prone position used in their study [37, 41]. Thus, we concluded that hip extensor strength during AH was reduced because AH selectively contracted the TrA, which was less effective at responding to dynamic fluctuations [17]. Although AH can stabilize the trunk by activating deep muscles, when performing dynamic movements such as hip extension in the prone standing position it may not be as effective as the normal contraction method in healthy adults. Therefore, although AH may be useful for relieving LBP during less dynamic movements in a clinical setting, it is unclear whether it has a large effect on spinal stability during dynamic movements [46].

The PM while using AB was significantly greater than with AR, but there was no significant difference between AB and AH at 180 ${}^{\circ}$ /s. Thorstensson et al. reported that both slow- and fast-twitch fibers are recruited for muscle contractions at low angular velocities, whereas fast-twitch fibers are mainly recruited at fast angular velocities [47]. Smith et al. and Martin et al. reported that the PM is significantly decreased when changing from a low speed to a high speed due to the activity of slow-twitch fibers giving way to the activity of fast-twitch fibers [48, 49]. In addition, Bozic et al. reported that the concentric PM decreased as the angular velocity increased. In other words, the amount of tension generated within the muscle is determined by the number of cross-bridges formed between actin and myosin filaments during muscle contraction. At a faster muscle contraction speed, the time to form this cross-bridge is more limited, which reduces the tension generated within the muscle [50]. Therefore, the results of this study imply that the hip extensors did not produce sufficient tension because the fast-twitch fibers and formation of filamentous cross-bridges in the abdominal and hip extensor muscles was limited when the subjects contracted at a higher angular velocity using the AR and AH maneuvers. It is possible that for this reason, there were no significant differences between the AB and AH maneuvers. Therefore, this study suggests that the prone standing hip extension exercise with the AB maneuver is an effective method to strengthen the hip extensors while maintaining spinal stability.

In this study three isokinetic parameters were recorded: PM, TW and AP. Due to the characteristics of the moment curve, there is a close relationship between the PM and TW. This very strong relationship has been studied and reported before [51]. Moreover, as the power derives directly from the work, the other two are also closely related. These relationships are clearly evident from Figs 2–4, which depict very similar variations in magnitude irrespective of the parameter, relative to the experimental condition. It should also be mentioned that with individual selection of the ROM, the TW is bound to change from subject to subject while the PM, being a single point in the curve is generally stable irrespective of the former [52]. Thus, the use of the PM seems perfectly sufficient for assessing the isokinetic strength of muscles.

This study has several limitations. First, the results of this study are representative of a young, healthy male population, which may not be generalizable to other populations. Second, pressure feedback was used as the sole criterion to differentiate between AB and AH, and the sEMG activity of the abdominal muscles was not monitored. The ability to measure sEMG of the abdominal muscles was limited because the experiment was conducted in the prone standing position, which presses on the abdomen. Third, kinematic data were not collected to confirm the stability of the lumbar spine and pelvis and to describe muscle recruitment patterns.

5. Conclusion

Based on the results of our study, we recommend that the AB stabilization maneuver be used when performing dynamic motions such as hip extension in the prone standing position.

Footnotes

Conflict of interest

The authors declare no conflict of interest.

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Effects of abdominal hollowing and bracing maneuvers on hip extension strength in prone standing position

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Method

2.1 Participants

Table 1 Descriptive characteristics of the participants ( n = 31)

Table 2 Isokinetic hip extension at an angular velocity of 60 ∘ /s (Mean ± SD)

Table 3 Isokinetic hip extension at an angular velocity 180 ∘ /s (Mean ± SD)

5. Conclusion

Footnotes

Conflict of interest

References

Table 1
Descriptive characteristics of the participants ( $n=$ 31)

Table 2
Isokinetic hip extension at an angular velocity of 60 ${}^{\circ}$ /s (Mean $\pm$ SD)

Table 3
Isokinetic hip extension at an angular velocity 180 ${}^{\circ}$ /s (Mean $\pm$ SD)