Collocations for slacken - verb

a bit, a little, slightly, somewhat (esp. AmE)
His grip slackened a little and she pulled away.

never
Willis never slackened his pace.

off
Slacken off the ropes.
Growth in the economy was beginning to slacken off.

Slacken is used with these nouns as the subject:
grip, muscle

Slacken is used with these nouns as the object:
grip, pace