The purpose of this study was to identify the influence of difference in muscular strength expression mechanism on change of trunk range of motion (ROM), and to present the basal data for supporting the importance of muscular function in health promotion program. Participants were 30 male university students, and divided into two groups randomly: muscular strengthening group (MSG, n = 15) and muscular relaxation group (MRG, n = 15). In MSG, hydro-exercise was applied actively whereas hydro-exercise in MRG was applied passively with supine floating position in high-salinity deep sea water without buoyant aids. Hydro-exercise for cervical joint was consisted of eight movement, and hydro-exercise for lumbar joint was consisted of eleven movement. Maximal muscular strength were measured during flexion and extension in cervical and lumbar joint with manual muscle tester repeatedly. Sternocleidomastoideus muscle, upper trapezius muscle, and splenius capitis muscle activity were measured with electromyography during flexion and extension in cervical joint repeatedly. Internal oblique abdominal muscle, rectus abdominis muscle, and erector spinae muscle activity were measured with electromyography during flexion and extension in lumbar joint repeatedly. ROM were measured with goniometer during flexion and extension in cervical joint repeatedly. ROM were measured with tapeline during flexion and extension in lumbar joint repeatedly. Balance were measured with closed eyed standing time repeatedly. As a result, maximal flexion and extension muscular strength in both cervical and lumbar joint were increased in MSG through muscle activation whereas maximal flexion and extension muscular strength in both cervical and lumbar joint were decreased in MRG through muscle inactivation. ROM during flexion and extension in cervial and lumbar joint were increased in both MSG and MRG. But, the rate of increase in MSG was higher than in MRG. Balance was increased in MSG whereas balance in MRG was decreased. In conclusion, hydro-exercise program in MSG was shown to be effective on improvement in truck muscular strength and ROM. Muscular strengthening in MSG was more effective on trunk ROM improvement than muscular relaxation in MRG. And it is suggested that hydro-exercise program in this study should be applied to elderly exercise program and exercise rehabilitation program.