The wide grip that many people use slackens the back muscles, which provide crucial support for the weight and transfers the load to the spine.
Does “which” in the above sentence point to “back muscles” or “the wide grip”?
Without the commas, does the meaning change?
‘which’ in this sentence refers to the back muscles (notice this is a plural); and since this is a plural, ‘provide’ is used in the sentence later.
(back muscles provide; back muscle provides)
Going by this same logic, ‘which’ here cannot refer to ‘wide grip’ (you seem to be confused between ‘wide grip’ and ‘back muscles’).