Introduction

Studies on muscle alterations, specifically examining hip muscle strength, have been conducted in chronic low back pain (CLBP). Some studies found that CLBP patients had hip muscle weakness, whereas others did not. The methods used to measure muscle strength varied significantly. Though CLBP exams often focus on hip abductors, it is crucial to remember that hip abductors, extensors, and external rotators all work together to generate dynamic hip stability, and their interactions with antagonistic muscles must also be considered. An assessment of hip muscles other than the abductors is crucial for determining lumbopelvic stability in individuals with CLBP. One important synergist for lumbar extension is the gluteus maximus muscle. Knowing which muscles are weaker in these patients is critical for designing successful exercise-based treatments. Therefore, the purpose of this study was to compare the strength of hip abductors, adductors, extensors, and external and internal rotators in adult patients with nonspecific CLBP to healthy subjects. Furthermore, it was explored whether there is a link between hip muscle strength and favorable outcomes in simple clinical tests such as the Trendelenburg and Step-Down tests.

Methods

In this cross-sectional study, 40 participants with nonspecific CLBP were included. They were not meeting the minimal aerobic activity levels of 150 minutes per week and they were not doing any type of strength training. CLBP was defined as low back pain of a minimal duration of 12 weeks. The control group was composed of people without lower back, knee, and hip pain.

At baseline, the Roland Morris Disability Questionnaire was filled, pain was rated on the numeric rating scale, and isometric strength of the abductors, adductors, extensors, and internal and external hip rotators was measured using a dynamometer. The mean of 2 4-second maximal isometric contractions was analyzed.

The Trendelenburg was performed and rated visually from a posterior view as described in the picture below.

The Step-Down test was performed according to the picture below.

Results

Eighty participants were included, of which 40 had CLBP and 40 participants were healthy. Most participants were female and they were on average 32 years old. They had a normal BMI of 24 kg/m2. The CLBP group had a mean pain intensity of 6/10 on the NRS and they suffered from CLBP for about 21 months.

The strength assessment showed a statistically significant difference in strength values for the following muscle groups:

• Abductors: Mean difference between 28-30%
• Extensors: Mean difference around 21%
• Adductors: Mean difference of around 18%
• Internal rotators on the right side: Mean difference of about 8.7%

The healthy subjects had greater muscle strength for all values. They however did not differ in the Trendelenburg and Step-Down tests. There was also no association between these tests and hip muscle strength.

Questions and thoughts

Why were the Trendelenburg and Step-Down tests not related to hip muscle strength? The study of Kendall et al., in 2010 already showed that hip abductor strength linked poorly with the magnitude of pelvic drop during the static Trendelenburg test and walking in controls and people with LBP. They proposed that hip abductor strength may therefore not be the primary factor influencing pelvic stability, and stated that the static Trendelenburg test has limited utility as a measure of hip abductor function. This study by Pizol et al. seems to confirm this statement.

In a later study by the same research group, the addition of hip strengthening exercises to a motor control exercise program did not appear to improve clinical outcomes for people with non-specific low back pain. This confirms the finding that we should maybe not so much focus on stabilizing the hip, but rather our focus should be on progressively strengthening the hip.

The people suffering from CLBP had relatively high pain levels (6/10) and this was going on for nearly 2 years. They had lower hip strength compared to healthy participants who had no pain. The design of this study only leaves us to objectify this difference, but as it is measured at one moment in time, we do not know whether strength diminished due to the pain, or if pain reduced strength along the course of CLBP.

Despite that the healthy group was sedentary, they considered themselves healthy for their ages. They were able to participate in normal daily activities considered normal for their ages. Interestingly, this subset had a normal BMI, which is not always true in people with chronic (low back) pain.

Talk nerdy to me

• All participants were matched by age, sex, and body mass index. Further, they were all sedentary and this should have led to homogeneity in the baseline characteristics and presumably a comparable group of people. As such, I assume the comparison of hip muscle strength in CLBP participants and healthy participants was possible.
• The intra-rater reliability of the strength assessment was excellent, with the intraclass correlation coefficients ranging between 0.92 and 0.98.
• The differences in strength were larger than the commonly foreseen clinically relevant difference of 15%, except for the internal rotators (right). As such we can assume that these exceeded the measurement error and are true differences in strength.
• The strength assessment was normalized to body weight. As such, the values can be compared between subjects.

Take home messages

This study wanted to provide insights into whether these functional tests can serve as practical indicators of muscle strength in CLBP patients, facilitating simpler assessments in a clinical setting. Since the associations are not shown, healthcare providers can redirect their focus toward other relevant aspects in the assessment of CLBP. It should be noted that hip muscle strength in CLBP patients was lower, and as such it is recommended to objectify this in this population.

Reference

Pizol GZ, Ferro Moura Franco K, Cristiane Miyamoto G, Maria Nunes Cabral C. Is there hip muscle weakness in adults with chronic non-specific low back pain? A cross-sectional study. BMC Musculoskelet Disord. 2023 Oct 7;24(1):798. doi: 10.1186/s12891-023-06920-x. PMID: 37805476; PMCID: PMC10559475. 

Additional references

Kendall KD, Schmidt C, Ferber R. The relationship between hip-abductor strength and the magnitude of pelvic drop in patients with low back pain. J Sport Rehabil. 2010 Nov;19(4):422-35. doi: 10.1123/jsr.19.4.422. PMID: 21116011. 

Kendall KD, Emery CA, Wiley JP, Ferber R. The effect of the addition of hip strengthening exercises to a lumbopelvic exercise programme for the treatment of non-specific low back pain: A randomized controlled trial. J Sci Med Sport. 2015 Nov;18(6):626-31. doi: 10.1016/j.jsams.2014.11.006. Epub 2014 Nov 13. PMID: 25467200.