Home training device for the deep spinal muscles

In response to my own back pain, I built a prototype intended to train the lumbar extensor muscles with the pelvis stabilized. The background to why I did that is explained below.
The background combines scientific literature, anatomy, mechanics and my personal experience. The scientific evidence and my individual observations are identified separately below.
The journey to building the device began for me with back pain in the lumbar region. I was in my mid-30s, had trained for about 15 years, had never been overweight or smoked, and drank little alcohol. Those factors can reduce some health risks, but they do not rule out low back pain. So why do I have back pain?

It seems as if everyone knows someone who has back pain. A quick search in the literature explains the reasons. Low back pain (LBP) is a leading cause of disability worldwide [1] [21]. Historical estimates illustrate its economic burden: a 1991 US review estimated annual costs of at least $50 billion [2], while a study using 2005 Swiss data estimated costs of 1.6% of GDP [3]. These figures should not be read as current cost estimates, but the global health burden remains substantial. Why can't we get back pain under control?

In most cases, LBP cannot be attributed to one clearly identifiable structure and is described as non-specific or chronic primary low back pain [21] [22]. Its course can be influenced by interacting biological, psychological, social, lifestyle and occupational factors. Age, smoking, high body weight and physical workload are associated with risk at population level [4] [5] [6] [7], but none determines whether an individual will have pain.
The complexity is also visible in spinal imaging. A disc herniation occurs when disc material is displaced through a weakened or torn annulus. It may cause pain when nerve irritation, inflammation or mechanical compression is clinically relevant, but disc abnormalities are also common in people without symptoms [8]. Imaging findings therefore need to be interpreted together with a person's symptoms and examination.

Figure 1: Herniated disc.

In the next few sections, I would like to give the reader the basic knowledge needed to clearly identify the problem of back pain. I will concentrate on the essentials in order to keep the format of this text as compact as possible.

Let's imagine we are standing behind a person and looking at their back. Under the skin there is a layer of muscle called the "superficial muscle group". Muscles that lie under this layer are called the "intermediate muscle group". Another layer "deeper" is the "deep muscle group". Having reached the "deepest" level, we can now see our target. The (lumbar) spine and some smaller muscle groups that are directly connected to the spine. There we find the so-called "lumbar extensor muscles". The so-called multifidus is one important part of this muscle group. Its fascicles run along the spine and span several vertebral segments (Fig. 3).

Figure 2: Different layers of the back muscles.

Figure 3: Multifidus muscle.

Muscles produce force by contracting. Depending on the task, that force can create movement, resist movement or stabilize a joint. A short physics problem will enable us to better estimate the forces that muscles normally have to exert in order to be able to carry out our everyday tasks. Let's assume someone is lifting a 4 kg object in their hand (Fig. 4). Now we want to calculate how much force the biceps has to exert.

Figure 4: Lever law on the human body.

Figure 5: Simplified lever concept related to spinal motion.

Figure 6: Proprioception principle at the multifidus muscle.

Studies frequently report differences in multifidus morphology between groups with and without LBP. The most commonly studied features are reduced cross-sectional area, asymmetry and myosteatosis, meaning increased fat within and between muscle tissue. Reduced size does not automatically mean reduced strength, and these changes can have several causes, including pain-related inhibition, reduced activity, ageing, nerve injury, metabolic factors and spinal pathology.
Most studies in this area are observational. They can show an association, but usually cannot determine whether a muscle change preceded the pain, resulted from it, or is influenced by a third factor [23]. Examples include:

• Study[11] examined multifidus muscle size in 80 people (15-18 years old).
  Result: Adolescents with LBP had smaller multifidus measurements on ultrasound. The study does not establish that smaller muscles caused the pain.
• Study[12] examined multifidus muscles in 78 people (17-72 years old).
  Result: Multifidus atrophy was observed in 80% of this clinical LBP sample and was associated with leg pain. The authors noted that cause and effect were unknown.
• Study[13] examined multifidus muscles in 90 people.
  Result: Group differences in multifidus size and symmetry were observed, particularly at lower lumbar levels.
• Study[14] examined multifidus muscles in 132 people with a herniated disc.
  Result: Compared with 132 controls, the disc-herniation group showed differences in multifidus size and fatty infiltration. This retrospective study demonstrates correlation, not causation.
• Study[15] examined multifidus muscles in 23 professional football players with LBP.
  Result: Players with LBP showed differences in trunk muscle size or function and hip strength. The sample was small and sport-specific.

Overall conclusion: people with LBP have, on average, somewhat smaller multifidus muscles and more intramuscular fat than pain-free controls [23]. This supports studying the muscle as one potentially relevant factor. It does not show that multifidus dysfunction is present in everyone with LBP or that strengthening it will prevent or resolve every case. Exercise is recommended for many people with persistent LBP, with the programme matched to the individual rather than focused on one muscle by default [21] [24].

For the lower back area, fitness equipment such as that shown in Figure 7 is usually available.

Figure 7: Typical training equipment for training the lower back.

In 1986, Arthur Jones founded the company MedX in the USA. In collaboration with the University of Florida, after years of development, they released medical training and measuring devices. The MedX LE Lumbar Extension is one device used to measure and train lumbar-extension torque. Its development is described in the book "The Lumbar Spine, The Cervical Spine And the Knee; Testing and Rehabilitation". Research using this type of device supports an important mechanical point:
Stabilizing the pelvis reduces pelvic rotation and contribution from the hip extensors, allowing lumbar-extension torque to be tested and trained more specifically [20]. The exercise still involves a group of lumbar extensor muscles; it does not isolate or measure the multifidus alone. The stabilization also reduces the available movement from combined trunk and hip motion to a smaller lumbar range, commonly about 72 degrees on this device.

Figure 8: MedX lumbar extrension schematics.

Figure 9: Original MedX lumbar extension.

Figure 10: Measurements with a MedX LE.

After a few months of tinkering, I thought I had built a usable prototype for training my lumbar extensor muscles with the pelvis stabilized. I trained with it once a week for 12 weeks and then went back to the MedX LE to take measurements. The result is the orange line in Figure 11.

Figure 11: Measurements with a MedX LE after training with the prototype.

Figure 12: Working principle for a home training device.

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