Background
Unrestricted shoulder function is largely dependent on the stability of the trunk, which in turn, is closely linked to the stability of the lower extremities and to balance control. The shoulder girdle has to compensate a loss of core stability and/or a deficient coordination of legs, torso or scapula by increasing movement speed and/or strength. For patients with shoulder pathologies, it is unclear whether the deficits in balance ability contribute to or is rather a consequence of the shoulder pathology.
Current evidence shows that patients with shoulder pain often suffer from proprioceptive deficits in the shoulder, as well as coordination deficits in their trunk and lower extremities. As Myers et. al explain, these proprioceptive deficits lead to abnormal proprioception within the entire muscle chain, which ultimately affects central control. Thus, somatosensory deficiencies in one area of the body, such as the lower extremities or the trunk, can lead to general functional problems in the shoulder area.
Treede et. al suggest that pain processing may cause balance disorders. One explanation for this could be the fact that pain processing, the balance control circuit and the inhibition of muscles caused by pain share some pathways of the central nervous system. Not only pain in the spine, but also pain in the lower extremities might have a negative influence on motor control through inhibition of the musculature and/or changes of the proprioceptive feedback in painful structures. According to Sibley et. al, pain causes presynaptic inhibition of muscle afferents. Carpa and Ro showed that pain in the masticatory muscles can alter the central modulation, which in turn, influences the proprioceptive muscle spindles. These muscular inhibition mechanisms due to pain might have a negative effect on balance ability.
Similarly, pain could have an effect on balance by disrupting neural speed processing. Research conducted by Luoto et. al showed that the speed of central information processing is decelerated when patients suffer from pain in the lumbar spine. It remains unclear as to whether pain from shoulder pathologies could also result in a deficient ability to balance either by 1) disrupting the central information processing speed, or by 2) inhibiting the musculature or proprioception.
Other factors known to influence balance are cognitive process, such as attention and characteristics such as age, sex and Body Mass Index (BMI).
To our knowledge, no studies have examined the influence of pathological shoulder pain on the balance system. The aim of this study was to investigate 1) if there is a connection between shoulder pain and deficits in balance ability and postural stability, 2) if pain intensity is related to balance ability and postural stability, and 3) if there is a connection between body mass index and balance ability and postural stability. Patients with pathological shoulder pain were compared to a healthy control group in regards to balance ability, postural stability (stability index, sensorimotor index and symmetry index) and body symmetry using the S3-Check system (MFT - Multifunktionale Trainingsgeräte GmbH, Grosshoeflein, Germany).