The present series of studies examined neuromuscular function during knee exercise in water and training responses of an aquatic exercise program. Emphasis was placed on the quantification of water resistance, the drag. Subjects who participated in this series of studies were 25-35-year-old healthy women and men. Neuromuscular function was examined using electromyography (EMG), force and reflex sensitivity measurements in the isometric condition on land and in water. EMG and angular velocity were measured during dynamic knee extension-flexion in still and flowing water. The drag forces acting on the leg and foot model were measured in the barefoot condition and with a large resistance boot. The calculated coefficients of drag were then utilised to further calculate the drag forces for the human subjects. Finally, the effects of 10 weeks of aquatic resistance training on isometric and isokinetic torque, EMGs and lean muscle mass (LCSA) were investigated. The results showed lower values for EMG activity and for the EMG/force ratio in water than on land. The same trend was observed for the H- and tendon reflex responses. The EMG patterns in the repeated knee extension-flexion performed in the flowing water demonstrated an early decrease in the concentric activation of the agonists with simultaneous eccentric activation of the antagonists. This indicates stretch-shortening cycle type of exercise whereas the single trial exercises seemed to be purely concentric. The drag forces calculated for the human subjects in knee extension-flexion showed significantly higher values when using the resistance boot than in the barefoot condition with values reaching 210 ± 46 N in men and 146 ± 30 N in women. The forces measured on land were significantly higher than the drag forces in water. Ten weeks of aquatic training resulted in a significant improvement (8-13%) in maximal torque, and this was accompanied with significant increases of 19-27% in the EMGs and 4-6% in the LCSA of the knee extensors and flexors. The results indicated an impairment of neuromuscular function in water, which is possibly due to hydrostatic pressure and the reduced gravity conditions. The results also indicated that by considering the principles of hydrodynamics, and by using additional devices, water resistance can produce a sufficient exercise stimulus to cause positive changes in the neuromuscular system. This will add to the body of knowledge concerning the nature of aquatic exercises and the design of progressive aquatic exercise programs used in rehabilitation and conditioning in water.
hydrodynamic drag, water immersion, electromyography, reflex sensitivity, aquatic training, muscle torque, muscle mass