The flexion-relaxation phenomenon consisting of an erector spinae silent period occurring with trunk flexion can place considerable stress upon tissues. Since individuals often flex their trunks while unstable, the purpose of the study was to examine the effect of an unstable base on the flexion-relaxation response. Fourteen participants flexed at the hips and back while standing on a stable floor or an unstable dyna-disc. Hip and trunk flexion were repeated four times each with one-minute rest. Electromyographic (EMG) electrodes were placed over the right lumbo-sacral erector spinae (LSES), upper lumbar erector spinae (ULES), lower abdominals (LA), biceps femoris and soleus. In addition to the flexion-relaxation phenomenon of the ES, a quiescence of biceps femoris and a burst of LA EMG activity was observed with the majority of stable trials. There was no effect of instability on the flexion-relaxation phenomenon of the ULES or LSES. The incidence of a biceps femoris silent period while stable was diminished with an unstable base. Similarly, the incidence of a LA EMG burst was curtailed with instability. Soleus EMG activity increased 29.5% with an unstable platform. An unstable base did not significantly affect LSES and ULES EMG flexion-relaxation, but did result in more persistent lower limb and LA activity.

• An unstable base did not affect the flexion relaxation response of the erector spinae.

• An unstable base decreased the incidence of biceps femoris quiescent period.

• An unstable base diminished the incidence of the lower abdominals EMG burst.