Optimal function of the trunk muscles is essential for functional mobility, as they provide postural support and stability to the lumbar spine during everyday life activities (e.g., lifting, carrying). Considering that pain alters the way that we move, many studies have investigated if individuals with chronic low back pain (CLBP) present with differences in trunk control compared to people without pain. Until recently, these differences referred to muscle activity, muscle strength, muscle endurance and/or kinematic changes, with less attention given to the control of muscle force, which is another aspect of motor control that can be affected by pain.
During a voluntary movement of submaximal effort, the force produced by an individual is not constant, but it fluctuates around an average value. The ability of an individual to produce a steady force during a submaximal voluntary contraction is defined as force steadiness and is an important aspect of force control. Smooth force generation is highly dependent on our proprioceptive sense (sense of force and effort). Abundant research has shown that when people experience pain, proprioception is usually reduced, likely due to the numerous functional and structural neuroplastic changes that can occur at multiple levels of the nervous system (i.e., periphery, spinal, subcortical and/or cortical centres). Thus, proprioceptive deficits could possibly impair an individual’s ability to produce steady forces. This impaired sensorimotor control could lead to suboptimal tissue loading which can play a role in the perpetuation of symptoms in painful musculoskeletal conditions (e.g., CLBP).
To date, several studies have investigated the effect of experimental or clinical musculoskeletal pain on force steadiness but with conflicting findings. However, these findings have mainly been observed in upper or lower-limb muscles. Only a few studies have investigated the control of trunk muscle force (measured as force accuracy and/or kinematic error of trunk movements) in people with CLBP, mainly during isometric lumbar extension tasks, providing some evidence that people with CLBP present impairments in the control of trunk muscle force compared to people without pain. Nevertheless, the neuromuscular mechanisms responsible for poorer muscle force control when people have pain have not been fully established, and it is not known whether people with CLBP also present reduced force steadiness in general, and especially during dynamic (e.g., concentric and/or eccentric contractions) trunk extension and flexion tasks. Additionally, if present, it is not known how quickly these changes occur following the onset of pain.
Taking into consideration the above, this PhD thesis is divided into four studies:
The first study of this PhD (systematic review) aims to summarise the current literature to determine whether pain (clinical or experimental) influences force steadiness. The second and third studies will aim to investigate if people with CLBP present force steadiness impairments compared to asymptomatic individuals during static (isometric) and during dynamic (concentric and eccentric) trunk extension/flexion contractions respectively. Additionally, these two studies will aim to explore the neuromuscular mechanisms underlying changes in force steadiness. Finally, the last study of this PhD thesis will aim to examine the acute effects of exercise-induced muscle soreness on force steadiness to determine how rapidly changes in force steadiness occur when people experience acute soreness/pain.