Yes, STRAINED is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, A 1, I 1, N 1, E 1, D 2.

Is it a valid word?

STRAINED is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

to exert much effort or energy
test the limits of
use to the utmost; exert vigorously or to full capacity
separate by passing through a sieve or other straining device to separate out coarser elements
cause to be tense and uneasy or nervous or anxious
become stretched or tense or taut
remove by passing through a filter
rub through a strainer or process in an electric blender
alter the shape of (something) by stress

Also: tense up, separate out, filter out.