Weakness in MS does not mean that the muscles themselves are weak. (Although that can certainly happen as well, as MS can often lead to being less active and the muscles will become less well-conditioned as a result.) Rather, it's the muscles aren't getting the message to do their jobs because of damage to the nerve fibers.
Muscles in both the arms and legs have had some weakness issues for me. In my most recent relapse, the entire left side of my body was weak. It was endlessly fascinating to me to not even be able to hold my arm up in the air without it falling back down as captured in this video that I shared a while ago:
After time and steroids, the left side weakness is better. I can now lift a 5-lb weight again. And I can stand in the security line scanner airport pose without my arm dropping!
In the past, weakness has just meant that large muscle groups (especially my quads) would reach total exhaustion WAY sooner than they should have. Pre-MS Kayla could have comfortably biked for miles; post-MS Kayla's legs would completely give after only a handful of blocks. Activities like biking and climbing stairs were impacted. Walking was as well, but not nearly to the same degree of intensity. There have been some not-too-long-ago examples of weakness impacting walking ability more significantly, though. A 1-mile walk near my home that I take pretty regularly saw my speed going from maybe 3 mph at the beginning of the walk to maybe 0.25 mph by the halfway point, as every step felt like moving lead.
So there has been some pretty significant acute weakness for me in the past as well as some chronic weakness issues that continue.