There isn't any. Train the trainer is just one way of showing competence. I would also say someone with say a NEBOSH diploma and teaching qualification (e.g. PTLLS) could teach at most levels. Also it depends on what level you're talking about - if it's a manager carrying a toolbox talk on manual handling then a good undertsanding of the work and manual handling might be enough. also train the trainer courses often offer additional stuff like course material, certification facilities and fresh ideas, oh and provide a 'safe' environment for the newb to training to have a go!