"Is the only solution to put more light??".
This is explained here and many other places: https://photo.stackexchange.com/questions/6598/what-is-the-exposure-triangle .
There are three ways, the three factors are:
- Shutter — how long you let light in
- Aperture — how large of an opening you let light through (how much at once)
- ISO — how sensitive your film or sensor is to light
Half the Shutter Speed will double the brightness, a lens with a larger Aperture will let in more light but also reduces the Depth of Field and is more expensive, finally a camera with a more sensitive sensor will also result in a brighter image at a greater cost.
Reducing the Shutter Speed is likely the better method if you want the strings to have a bit of blur and depict motion. IF you actually wanted tack sharp frames of the strings moving you would not want to film at 25FPS but instead at a very high Frame Rate.
Do you need to do this only in the evening?, if most of the Film is set in the evening then do a few closeups during the day and use multiple lights to remove all shadows and increase the light so it's extremely bright; film that and rely upon the darkening of the fast Shutter to make it appear dark.