The GCN crew, inspired by the spindly Spanish climber Alberto Contador's having mentioned in an interview that he regularly made of point of training in the mountains by riding out of the saddle for up to 20 minutes at a time, once did a video shoot where a bunch of the GCN presenters tried to replicate that training method.

As shown in that video, Emma Poole, a featherweight former women's world champion on the road, had no trouble pedaling out of the saddle for the entire ride. Sy Richardson, a tall former pro who can still ride time trials at a power level of around 500 watts, struggled to stay out of the saddle and soon gave up the attempt.

Edit: found the video. (At about 2:45 in the video, Sy mentions that the FDJ pro team conducted a study on the topic and found that their lightest riders were equally efficient in or out of the saddle but the heavier riders were less efficient out of the saddle, with the difference becoming significant after around 30 seconds of standing and climbing.)