The Effectiveness of Isometric Exercise on the Management of Chronic Shoulder Pain

The Influence of the Intensity of Isometric Exercise on Pain Intensity, Muscle Pain Sensitivity and Function in Patients With Chronic Shoulder Pain: A Parallel Single-blind Randomized Study

The aim of this study was to evaluate the influence of the intensity of Isometric Contraction of shoulder external rotators, comparing the effect of a high-level IC (80% maximal voluntary isometric contraction) versus a low-level Isometric Contraction (20% maximal voluntary isometric contraction) on pain intensity, pain thresholds, and function in chronic pain shoulder patients.

This was a single-blinded, randomized parallel study. Eighty-two patients presenting chronic musculoskeletal shoulder pain were allocated to two groups. Patients in group 1 (n= 41) received a five-series Isometric Contraction of shoulder external rotators at 20% maximal voluntary isometric contraction, while patients in group 2 (n= 41) received similar protocol but the intensity of contraction was set at 80% maximal voluntary isometric contraction. In both cases, each series of IC was done until exhaustion or up to a maximum of 5 minutes. Patients received five sessions of treatment during a two-week period. Outcome measures included muscle pain sensitivity (pressure pain thresholds) and pain intensity (pain intensity numerical rating score), and shoulder function (Constant Score).

Participants were randomly assigned to groups by using a random number table. This procedure was conducted by an external assessor. The allocation concealment strategy included the use of sealed-opaque and consecutively numbered envelopes.

The assessor responsible for the outcome measures was blinded to the subjetc´s treatment assignment. The assessor was not present in the room while the treatment was delivered. He returned once the treatment was completed to perform the measurements.

Patients in group 1 received a five-series Isometric Contraction of shoulder external rotators at 20% of maximal voluntary isometric contraction. Each series of Isometric Contraction was done until exhaustion or up to a maximum of 5 minutes. Patients received five sessions of treatment during a two-week period.

Patients in group 2 received a five-series Isometric Contraction of shoulder external rotators at 80% of maximal voluntary isometric contraction. Each series of Isometric Contraction was done until exhaustion or up to a maximum of 5 minutes. Patients received five sessions of treatment during a two-week period.

The intervention included the application of a series of isometric contraction for the external rotators shoulder muscles. The exercise protocol included two intensities; high-level isometric contraction (80% maximal voluntary isometric contraction) versus a low-level isometric contraction (20% maximal voluntary isometric contraction).

The Pain intensity Numerical Rating Scale is a self-reporting measure of pain intensity. This involves asking patients to rate their pain intensity by selecting a number on a horizontally depicted 11-point scale from 0 (no pain) to 10 (worst possible pain). Based on its several advantages (i.e. more responsive, sensitive and easy to administer) over other pain measuring scales, the PI-NRS has been recommended as a core outcome measure in clinical trials of chronic pain treatments. The minimal clinically important change for this scale has been reported to range from 1.5 to 3.2 points. Other authors have determined a meaningful clinical change of 2 points from baseline pain scores.

The shoulder function was assessed by the Constant Score scale. This is a multi-item functional scale assessing pain, activities of daily living, range of motion and strength of the affected shoulder. Its score ranges from 0 to 100 points, representing worst and best shoulder function respectively. Pain and activities of daily living (subjective aspects receiving 35 points) are answered by the patient ; and range of motion and strength (objective aspects receiving 65 points) require a physical evaluation and are answered by the orthopaedic surgeon or the physiotherapist.

Muscle pain sensitivity was evaluated through the pressure pain threshold, or the minimum pressure that induces pain or discomfort. Pressure pain thresholds measurements have been shown to have good or excellent inter-rater intraclass correlation coeficient values ranging from 0.74 to 0.90, and intra-rater reliability interclass correlation coefficient values ranging from 0.75 to 0.99. The minimal clinically important change calculated for this outcome has been reported to be ≥ 1.10 Kg/cm2/s

Inclusion Criteria: Male or female having a chronic shoulder soft-tissue injury of at least 3 month duration resulting in a mild to moderate level of disability (Constant score scale), pain intensity score between 3 and 8 points (pain intensity numerical rating scale), and age between 18 and 65 years. Exclusion Criteria: Participants were excluded if they had any contraindications related to the application of isometric exercise, neurological problems (central or peripheral), concomitant physiotherapy or chiropractic treatment, fibromyalgia or general systemic disease conditions, had participated in any moderate or vigorous physical activity in the las 72 hours previous the outcome assessment.