Objective

Aquatic shoulder exercises are widely implemented into rehabilitation programs to provide early muscle strengthening and improved joint mobility. However, since the drag – the force exerted by the water on the moving segments – is very hard to measure, an in-depth knowledge of internal joint load is lacking, which ultimately prevents healthcare professionals from understanding all clinical implications. For the first time, shoulder joint moment, work, and power were calculated through a novel, validated numerical approach coupling fluid flow simulations and inverse dynamics.

Material/patients and methods

A male participant was asked to perform slow scapular plane abductions and adductions (period T = 16 s, angular velocity 22.5 °/s) while sitting fully immersed in the pool and breathing through a diving bottle. 3D kinematics was captured by two underwater cameras (GoPro HERO4, 50 Hz) to later animate a virtual geometry of the participant’s upper limb. Instantaneous hydrodynamic forces were solved through computational fluid dynamics analysis, and further input into an inverse dynamics model to compute shoulder joint kinetics. The mechanical power delivered at the shoulder was partitioned into that related to inertia, weight, buoyancy and drag. To assess the benefit of exercising in water, the power required to perform the same movement on land was also calculated.

Results

In total, 0.21 and 0.06 W of power were respectively produced (muscles acting concentrically) and absorbed (energy dissipated by eccentric, lengthening contractions) at the shoulder, which is respectively 81.1 and 95.0% less than the required power on land. Inertia and drag relative contributions to the total power were small to negligible (0.1 and 6.0%), with most of the power expended against weight (44.2%) and buoyancy (49.7%). The shoulder extensors were dominant at all time, with net joint moments peaking at 0.6 and 1.9 Nm during shoulder abduction and adduction, respectively, and concomitant periods of power absorption and production.

Discussion–conclusion

The present results encourage the use of aquatic shoulder exercises so as to benefit the most from the upward lift of buoyancy. Surprisingly, at such a slow speed, buoyancy contribution is sufficiently high to elicit eccentric work during abduction, which is favorable to regain joint mobility at very little effort.