Research Diagnosis & Imaging January 20, 2025
Kapre and Alexander (2024)

The Relationship Between Core Strength and Hamstring Flexibility

Core strength and hamstring flexibility

Introduction

A good core is paramount for optimal function of the extremities. The hamstring muscle, via its attachment to the ischial tuberosity, can alter the position of the pelvis upon which the core muscles act. Shortened hamstrings can lead to excessive posterior tilting of the pelvis, which can reduce lumbar lordosis. A cascade of compensatory mechanisms may develop, making individuals prone to injuries. This study wanted to find out whether there is a correlation between core strength and hamstring flexibility in young people.

 

Methods

A cross-sectional observational study was conducted in India, including healthy young adults between the ages of 18 and 25 years. Both participants with and without hamstring tightness were recruited.

The Active Knee Extension Test assessed the hamstring muscles’ flexibility. This test requires the participant to extend his knee from a 90° flexed position in supine with both hips flexed to 90°. The degree of knee flexion from the vertical (terminal knee extension) was measured using a goniometer.

Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

The McGill’s core endurance test battery was used to assess core strength. This test battery consists of 3 different tests:

  • The Flexor Torso Endurance Test: The participant sits on the floor with the knees flexed to 90° and the hands across the chest. The trunk is in a 60° inclination and the participant is instructed to hold this position for as long as possible.
Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

  • The Lateral Torso Endurance Test: The participant is in a side-lying position and performs a side plank with the non-supporting arm crossed against the chest. The participant is instructed to remain in this position for as long as possible.
Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

  • The Extensor Torso Endurance Test: The participant is in prone lying position with the legs strapped to the table or with an assistant holding the legs. The trunk is unsupported with the arms crossed over the chest and in a horizontal position. The participant has to maintain this position for as long as possible.
Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

The outcome of interest was the correlation between the results from the McGill Core test battery and the hamstring flexibility measured using the Active Knee Extension test. A Pearson’s correlation coefficient was calculated. Correlation coefficients are interpreted as follows:

  • Values between ±0.50 and ±1 suggest a strong correlation.
  • Values between ±0.30 and ±0.49 indicate a moderate correlation.
  • Values below +0.29 are considered a weak correlation.

 

Results

Thirty-six healthy young adults were included. They were on average 20 years old and achieved a mean of 64° in the Active Knee Extension test.

Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

Their core endurance strength is tabulated hereunder.

Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

A significant moderate positive correlation was observed between core strength and hamstring flexibility for the Flexor Torso Endurance test. A significant but weak positive correlation was found between the Extensor Torso Endurance test and hamstring flexibility. No correlation was observed between the Lateral Torso Endurance and Active Knee Extension tests.

Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

Questions and thoughts

The moderate positive correlation between the Flexor Torso Endurance test and Active Knee Extension test means that the higher the endurance of the anterior abdominal muscles, the better the hamstring flexibility and vice versa. This can be observed by looking at the blue dashed line in the picture below. Participants who achieved only 40° of knee extension from the starting position in 90° of knee flexion (figure 2) had significantly lower endurance than those approaching the terminal knee extension position.

Core strength and hamstring flexibility
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

A positive moderate correlation between hamstring flexibility and core endurance shows that increased hamstring flexibility may be linked to improved core endurance capabilities. This link can be explained using multiple interconnected physiological and biomechanical factors.

  • First and foremost, hamstring flexibility is essential for maintaining proper pelvic posture. Tight hamstrings can cause a posterior pelvic tilt, disrupting the lumbar spine’s normal curve and affecting overall pelvic and spine alignment.
  • This imbalance can put additional strain on the core musculature, thereby reducing endurance and stability during physical activity.
  • Improved hamstring flexibility, on the other hand, allows for better pelvic alignment, which can lead to more effective core muscle engagement and hence increased core endurance.

While a positive correlation between core strength and hamstring flexibility was observed, we can not say which direction the effect should be interpreted. Also, this correlation does not simply mean that the one is causing the other.

However, this info may help us to understand the demands of young adults and athletes. For example, athletes with better hamstring flexibility may suffer less strain during dynamic movements, resulting in more efficient energy transmission and less fatigue in the core muscles. This efficiency is especially crucial in sports that require quick changes of direction or prolonged physical exertion, as core endurance is essential for performance.

Remarkably, in this sample, the participants achieved lower endurance scores in the McGill Core test battery than the normative data, except for the Flexor Torso Endurance test, where they achieved almost twofold the time of the norm. This may have implications that would be interesting to investigate further.

Comparison descriptive normative data
From: Kapre and Alexander, Bulletin of Faculty of Physical Therapy (2024)

 

While we can not specify the direction of the effects and the cause of the observations, we can still use these findings to clinically reason for every individual patient. For example: a reduction in extensor endurance of the trunk may overwork the hamstrings since they contribute to hip extension. It might be possible that, for instance, limited hamstring flexibility causes the hamstring muscles to perform less efficiently or the overload on the hamstrings created by inefficient trunk extension makes them prone to tightness. Every individual may present differently, but these findings may help you resonate with the observations in clinical practice.

 

Talk nerdy to me

A limitation of this study is that one of the outcome measures is described in another way throughout the text than described in figure 2. In the text, the authors specify to flex both hips, while the image only shows one flexed hip. Further, they refer to the study by Olivencia et al. (2020) where the test is conducted with a foam roller underneath the non-tested leg.

Although the Active Knee Extension Test has been conducted in several ways, the inconsistent use of the same name for different test executions makes comparison across studies difficult. Underneath is a picture of the Active Knee Extension test performed with the other leg extended. We have made a video on this test, conducted with both hips flexed to 90°, and plead for the use of the name 90-90 Straight Leg Raise Test, when performed using both hips flexed and using the Active Knee Extension test when the test is conducted according to the picture below.

active knee extension test
From: Kuilart, K. E., Woollam, M., Barling, E., & Lucas, N. (2005). The active knee extension test and slump test in subjects with perceived hamstring tightness. International Journal of Osteopathic Medicine, 8(3), 89-97. https://doi.org/10.1016/j.ijosm.2005.07.004

 

Therefore, a limitation of this study lies in the inconsistent description of the outcome measure used, which limits the correct interpretation of the findings.

 

Take-home messages

This study found two positive correlations between core strength and hamstring flexibility indicative of lower core endurance related to worse hamstring flexibility. This study only shows a correlation between the variables, but can not elaborate on the exact cause of the relationship.

 

Reference

Kapre, T. M. S. and Alexander, J. O. R. (2024). A correlation study of weak core muscles with hamstring muscles flexibility in young adults. Bulletin of Faculty of Physical Therapy, 29(1).

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