Core muscle strength and coordination are important components to resistance training and sports performance. Unstable training (UT), further divided into unstable surface training (UST) and unstable load training (ULT), is commonly chosen to improve core muscle strength. ULT has been accomplished by suspending resistance from a barbell with bands, allowing it to move vertically, but research has not been done on a horizontal type of ULT. The purpose of this study was to measure the impact of horizontal dynamic instability (HDI) on prime mover and core stabilizer muscle activity, during the seated overhead press, through use of electromyography (EMG). HDI was compared to a traditional type of ULT as well as stable load training (SLT). Thirteen resistance trained males (age = 24.1+/-2.4 years, height = 178+/-7.2cm, mass = 84.1+/-9.4 kg) participated in all three condition assignments. Subjects tested their 5-RM strength on the seated overhead press and performed five repetitions at 50% 5-RM strength for condition assignments. Electrodes were attached to 8 muscles (anterior deltoid, triceps brachii, upper trapezius, right & left external oblique, right & left rectus abdominis, erector spinae). Concentric and eccentric muscle actions were analyzed separately across all repetitions. Mean voltage of activation was acquired and processed (band pass filter 10--500 Hz, rectified, RMS 50ms window and normalized) and significance was set at p <0.05. A two-way RM ANOVA was used to compare condition x muscle action. Anterior deltoid and upper trapezius activity was higher during ULT concentric compared to SLT and HDI concentric. Erector spinae activity was higher during SLT and HDI concentric compared to ULT concentric. Left external oblique activity differed significantly between HDI and ULT eccentric, but no differences occurred at the right external oblique. These results suggest that HDI may require external oblique activity to a similar magnitude as traditional ULT. Continued research on HDI should address other exercises or loading intensities.