Could the employer have provided aids to stop the driver having to "tippy-toe"? Our drivers have various devices meaning their feet can remain firmly planted on the ground so I would contend your work methods are possibly wanting compared to "best practice" and have contributed to the injury.

As an observation I am noting a general theme beginning to appear in RIDDOR threads where every IP seems to have some "pre-existing condition" when the OP is being placed.

To answer the question. I would say this is work related. Pre-existing or not the IP was stretching for a strap as part of their job. They were injured whilst reaching for this strap.

The real question is whether this constitutes an accident under the strict definition of RIDDOR. 

"In relation to RIDDOR, an accident is a separate, identifiable, unintended incident, which causes physical injury. This specifically includes acts of non-consensual violence to people at work.

Injuries themselves, eg ‘feeling a sharp twinge’, are not accidents. There must be an identifiable external event that causes the injury, eg a falling object striking someone. Cumulative exposures to hazards, which eventually cause injury (eg repetitive lifting), are not classed as ‘accidents’ under RIDDOR."

I would say it wasn't an accident. It still warrants investigation. Also, if you knew they had an issue with their knee you should have taken steps (no pun intended) to reduce the risk of them aggravating that injury.

I'm with the Beaky one. ;)