Canadian scientists found that the more emotional or mental challenges a person faces in a day, the less resolute they become to work out physically.

However, the findings should not deter us from keeping fit. Experts believe that willpower is like a muscle, which needs to be exercised to avoid the temptation of a night on the couch instead.

Kathleen Martin Ginis, an associate professor of kinesiology – the science of movement – who led the study, said we all have a limited amount of willpower which is whittled away by stress.

Miss Martin Ginis, from Canada's McMaster University, explained: “Cognitive tasks, as well as emotional tasks, such as regulating your emotions, can deplete your self-regulatory capacity to exercise."
Er, correct me if I'm wrong but isn't the "willpower gene" more of an "I'm tired and don't want to exert more physically gene"?

Ironically, I turned down an after-work workout last night for much the same reason. Ha! It's my willpower gene! Free excuse!