Tired muscles can actually be a source of neck pain, according to a study on spinal movements.

Learning new languages, sending emails, attending a virtual class or talking to loved ones on the other side of the world are just some of the tasks that can be accomplished with the push of a button on a smartphone. Unfortunately, the simplicity and convenience of modern devices also comes with a painful stiff neck. The sedentary nature of work and prolonged use of portable devices and computers have contributed to a sharp increase in neck pain.

Although fatigue of the neck muscles has long been suspected as the cause of the pain, the actual mechanical changes in the spine and muscles that precede the weakness remain an open question.

Using high-precision X-ray imaging to track spinal movements during tasks involving neck strain, Texas A&M University researchers have found that sustained neck strain causes muscle fatigue, which then worsens the curvature of the cervical spine. This leads to neck pain.

Their results are published in the Proceedings of the National Academy of Sciences.

“We’re talking about subtle neck movements in static positions, which are difficult to capture. They’re also very complex because there are so many individual parts in the neck, or as we call it, motion segments,” said Dr. Xudong Zhang, a professor in the Department of Industrial and Systems Engineering. “With this study, we have, for the first time, provided unequivocal evidence that fatigue causes mechanical changes that increase risk.”

Zhang said this understanding can help make informed decisions about how we work and the design of products (for example, head-mounted wearables) that can potentially reduce the risk of neck pain.

Neck pain is one of the most common musculoskeletal disorders. Globally, approximately 2,500 out of every 100,000 people suffer from some form of neck pain. In fact, by 2050, the estimated number of neck pain cases worldwide is expected to increase by 32.5%. A significant risk factor for neck pain is poor posture maintained for long periods of time. Therefore, working long hours in front of a computer in a hunched position or using smart devices for a long time are significant factors for neck pain.

Neck posture is dynamically maintained by the bones of the spine being pulled into position by the muscles attached to them. Although the neck is very flexible, it is also very unstable.

“Muscle generates movements by producing force,” Zhang explains. “We hypothesized that when the force-producing capabilities of different muscles decrease, the position of the bones changes and this can be taken into account.”

To test their idea, they recruited healthy volunteers for a “sustained to exhaustion” neck strain task. Subjects held their necks in neutral, 40° extended (bent backward), and 40° forward positions for a certain duration. The researchers used electromyography (EMG) to measure muscle electrical activity. In particular, they objectively measured muscle fatigue through changes in the frequency of the EMG signal. In addition, they used high-precision dynamic X-ray technology to track small-amplitude cervical spine movements, on the order of a few degrees.

“We imagined the cervical spine as a cantilever bridge,” Zhang explains. “If the bridge is subjected to excessive and/or repeated stresses, it can collapse or buckle; similarly, if the muscles fatigue, the cervical spine can buckle.”

The researchers’ experimental paradigm validated that sustained efforts do indeed result in fatigue EMG signals. Biomechanically, muscle fatigue alters spinal mechanics, increasing the propensity for injury.

As a next step, the researchers will develop dynamic biomechanical models, a new approach that promises to provide a more realistic understanding of the muscle events that precede fatigue. Unlike the model in this study, which assumes static efforts at the neck, the dynamic model will capture subtle but consequential changes in muscle and bone over time.