Acidosis and reduction of pH are one and the same thing (pH is the negative logarithm of proton concentration). My understanding of the literature is:
a. that metabolic acidosis is not the result of protons dissociating from so-called 'lactic acid',
b. the lactate dehydrogenase reaction consumes a proton thus retarding acidosis,
c. that acidosis may not actually be responsible for 'muscular contraction slow down' as you call it (i would call it fatigue
) and
d. that the exact mechanisms of fatigue are still unclear, although phosphate ion accumulation is one of the candidates.
L.