All bikes are subject to twisting forces when they're being pedalled or when they encounter irregularities in the road. All decent ones cope with that twisting force pretty well. ThatBritBloke mentions the prospect of some flexing at high speeds (eg, on downhills). I'd suggest that a 'step-thru' frame is a little more subject to flexing when being pedalled than one with a higher crossbar - so some of your effort on the pedals is wasted in frame flex. In both cases, this effect will be quite minor and you are probably not going to be riding at speeds that will even make this very noticeable. All bikes are a compromise. Buy the bike that you think suits you best. ENJOY!