Shoulder mechanical demands of slow underwater exercises in the scapular plane

The mechanical demands of underwater shoulder exercises have only been assessed indirectly via electromyographical measurements. Yet, this is insufficient to understand all the clinical implications. The purpose of this study was to evaluate musculoskeletal system loading during slow (30°/s) scapular plane arm elevation and lowering performed in two media (air vs water) and body positions (sitting vs supine).

Eighteen participants' upper bodies were scanned and virtually animated within unsteady numerical fluid flow simulations to compute hydrodynamic forces. Together with weight, buoyancy and segment inertial parameters, these were fed into an inverse dynamics model to obtain net shoulder moments, power and work.

Positive mechanical work done at the shoulder was 32.4% (95% CI [29.2, 35.6]) and 25.0% [22.8, 27.2] that when performing the same movement on land, supine and sitting respectively. Arm elevation was ~2.5× less demanding sitting than supine (mean 0.012 (SD 0.018) vs mean 0.027 (SD 0.012) J·kg^-1, P = 0.034). Instantaneous power was consistently positive when sitting albeit very low during elevation (0.003 W·kg^-1) whereas, when supine, it was alternately negative for short period (~1.2 s) and positive (~4.8 s), peaking at levels 3× higher (0.01 W·kg^-1).

Performing sitting elicited concentric muscle contractions at very low effort, which is advantageous during early rehabilitation to restore joint mobility. Exercising supine, by contrast, required rapid pre-stretch followed by concentric force production at an overall higher mechanical cost, and is therefore better suited to more advanced rehabilitation stages.