Somatosensory Electrical Stimulation to Improve Motor Control in Patients Suffering From Knee Osteoarthritis

Worldwide, 9.6% of men and 18% of women aged over 60 years suffer from osteoarthritis (OA), most of which involve the knee. Within the OA patient population, 80% of the OA patients have limitations of movement, and 25% cannot perform the majority of their daily activities (WHO). OA is a progressive articular cartilage disorder involving motor and sensory dysfunctions, including a decreased range of motion, muscle weakness, impaired proprioception, and pain. Some of these symptoms contribute to arthrogenic muscle inhibition (AMI), a reflexive decrease in motor output to the muscles surrounding the affected joint that is most likely mediated by GABAergic inhibitory mechanisms.

AMI is characterized by abnormal afferent information transmitted to the central nervous system, resulting in altered afferent feedback to the quadriceps motoneurons (MN) which in turn results in reduced excitability of that particular pool. The altered afferent input is suggested to stem from stimulation of mechanoreceptors, via joint effusion or excessive movements, nociceptors as a response to pain, or loss of joint receptors. Although the evidence concerning the role of the central nervous system is scarce, pre- and postsynaptic spinal mechanisms directly affecting alpha-MNs seem affected. Dysfunction of γ-loops also seems to be involved. These mechanisms together result in AMI that manifests through aberrations in voluntary quadriceps torque, force control, and reflex excitability often measured by the H-reflex.

Motor dysfunction thus is an important limiting factor for OA patients and involves motor and sensory factors. Besides the evident role of motor efferents, sensory afferents also influence motor control and poor proprioceptive function is predictive of poor chair-stand performance. Reduction of the sensory deficits could potentially increase motor function in knee OA. The present study aims to evaluate whether low-intensity peripheral electrical nerve stimulation, a form of increasing afferent input, could potentially improve OA patients' motor function. The most limiting factor in OA patients, however, is pain, experienced at rest and during movement. High-frequencies stimulation paradigms are normally used to reduce pain, but 40-50 minutes of low-frequency electrical stimulation (4 Hz, < MT intensity) can also be effective in reducing pain at rest, pain during movement, and pain sensitivity, although a recent systematic review questions consistency of these effects considering the limited evidence. Although, to the best of our knowledge, there is no clear physiological explanation concerning pain reductions after peripheral electrical nerve stimulation, it is possible that reductions in experienced pain are mediated by reduced analgesia, i.e., decreased excitability of nociceptive neurons.

Electrical stimulation of peripheral nerves and muscles has been applied in various forms to modulate muscle strength, motor skill, spinal- and cortical excitability. Neuromuscular electrical stimulation (NMES) evoked forces can generate torques up to 112% of maximal voluntary contraction force and increases with increasing stimulation frequency up to 70-80 Hz.

Whereas NMES is primarily focused on increasing muscle strength, low-intensity somatosensory electrical stimulation (SES) has been used to increase motor skill acquisition in neurologic patients and healthy participants. SES excites group Ia, Ib, and II afferents, as well as secondary muscle afferent fibers. SES increases motor performance the most when applied to a peripheral nerve at high sensory intensities (2-3 times perceptual threshold) at 10 Hz (SES applied in 1 Hz trains consisting of 5 pulses at 10 Hz). High-frequency transcutaneous electrical nerve stimulation (TENS) and sensorimotor training can both repositioning error reflecting impaired knee proprioception in patients with knee OA. Both TENS and SES thus excite afferent fibers involved in knee proprioception. Impaired knee proprioception has been associated with impaired ability to accurately and steadily control force and impaired eccentric strength. In contrast with NMES, in which induced increases in muscle force most likely involve changes in metabolism, hypertrophy, and possibly spinal mechanisms, and TENS, where large diameter primary afferents are pivotal in pain reductions, SES targets sensory pathways, both cutaneous and proprioceptive, which seem an appropriate target to reduce sensory deficits involved in OA.