Hi DLM
i tend to agree with Fergus but for slightly different reasons (and for safety reasons too – having a swiss ball roll out from under you is no fun!)
our sprinters use to do a lot of swiss ball stability work – but to be honest except for specific prehab-rehab needs we found it basically a waste of good training time – running style and speed showed no improvement and gym related injuries went up! – even then for knees/ankles rehab we prefer running on foam or bosu board wobbles.
The unstable surface concept is ok in theory – but unstable surface for runners is “moving” on paddock, hills, sand etc not “standing” on a swiss ball!
The single most important factor in running speed is force applied during the period of foot-contact – you develop that power through stable surface hard training – unstable simulation can be better developed by the same on one leg for example – the rest is wastage of training time and energy.
That said – to be balanced and to move outside my probbaly biased opinions based on limited experience :>> – some athletes find the challenge of a swiss ball – fun – and fun is a lot of what it is about!
Train hard my friend and enjoy it all!