There are many opinions on this. One consistent observation, hiking / trekking / walking poles do help distribute impact to the arms, reducing stress on knees. Upper body and arm muscles tend to become more toned, rather than bulk mass buildup. This makes for more efficient and stronger muscles with the same mass. Some users have observed their muscles lengthen in the arms, below the elbow.
Body builders and people who focus on developing muscle notice actual wear down and reducing of muscles from hiking poles, but more toned.
Helpful blogs on this issue include: