Many clubs have used Challenge Cup success as a launchpad for a more sustained challenge at the upper end of the Super League table.
Invisible as the Falun Gong protests are to most Chinese, they represent the most coordinated and sustained challenge to Communist Party rule since the pro-democracy movement of 1989.
The first sustained challenge to the authenticity of "Falling Soldier" came in the mid-1970s, in Philip Knightley's book "The First Casualty".
A Democrat — Elizabeth Warren, Sherrod Brown, Deval Patrick — should give Hillary Clinton a serious, sustained challenge, for her and the party's sake if nothing else.
And while small steps are being made towards a sustained challenge for Champions League qualification next season Rodgers is under no illusions about the severity of the task.
The runners-up Aberdeen, who pushed Celtic close last season before fading, will hope they can mount a more sustained challenge when they start their season with a trip to Dundee United.
A more sustained challenge to Ringling Brothers has come instead from the many advocacy groups that push for an end to circus herds, arguing that forcing rare animals to traverse the country in tight cages by train constitutes cruelty by itself.
Richardson, who is against the removal of the Rhodes statue, speaks at a time when the role of universities is under sustained challenge from a generation of students who have different ideas and expectations of what an education should and should not entail.
There was always the danger that the Russian would be too exhausted to offer Federer a sustained challenge, having battled his way through three five-setters in his previous four matches and in total spending eight hours longer on court than Federer over the fortnight.
Safin, though, is notoriously volatile, and - until his performances are governed by something more predictable than whether he has got out of bed on the right side - it is difficult to see him mounting a sustained challenge to Federer's dominance.
