Professional longbowmen, as found in archaeological remains, were typically tall with strongly uneven arm and upper body muscle development. They would have been visibly muscular (unless an individual tended towards the wiry), unlike the whole armies of willowy elven archers shown in modern high fantasy.
English longbowmen of the High to Late Middle Ages trained from childhood - all boys were required to practice archery for military readiness, and the best were picked to become professional soldiers. Longbows were in military use among the English from the 13th to 16th century: http://www.archers.org/?page_id=421 They were a characteristically English weapon, and other European states had their own weapons of choice, for the example Genoese crossbowmen.
The replies here provide more detail about the training and development of longbowmen: https://www.reddit.com/r/AskHistorians/comments/xjuvs/was_the_welshenglish_longbow_itself_really_all/ , in particular answers from /u/BonzoTheBoss , /u/tobiov , and /u/Caedus_Vao
The thousands of hours of practice from a young age produced characteristic changes in the musculature and skeleton which are visible from archaeological remains of professional archers. These changes included asymmetric development such as one arm being considerably larger than the other. Among the most comprehensively studied are the skeletons of the archers from the sixteenth century English shipwreck the Mary Rose: these were elite professional archers , the majority 6ft+ tall. (The longbow itself was over 6ft, and to operate it archers had to pull a weight of up to 200lb up to 12 times per minute during battle - these arrows had enough force to puncture plate armour.)
A sports scientist studying the remains told the BBC: "on one of the skeletons we have looked at, the surface area of the joint between the lower arm and elbow is 48% larger than on the joint on the other arm." http://www.bbc.com/news/uk-wales-17309665
Here is a skeleton and a reconstruction of one of the archers from the Mary Rose museum: http://i2.cdn.turner.com/cnn/dam/assets/130530121104-mary-rose-skeleton-horizontal-gallery.jpg
The Mary Rose longbowmen also suffered from repetitive strain injury: http://www.telegraph.co.uk/news/science/science-news/9686177/Mary-Rose-scientists-identify-shipwrecks-elite-archers-by-RSI.html
If you can get hold of the BBC documentary The Mary Rose: a Timewatch Guide, you will be able to see more about the reconstructions, and use of longbows by contemporary archery experts. http://www.bbc.co.uk/programmes/b051h0gy
One skeleton from a mass grave of soldiers from the 1461 Battle of Towton (in the Wars of the Roses) shows injuries consistent with having tried to practice with an adult longbow before he was fully grown: http://erasgone.blogspot.co.uk/2013/03/towton-skeletons-confirm-brutality-of.html
Skeletal changes characteristic of archers have been found in other medieval archaeological remians, not only those specifically of longbowmen. This report about Hungarian finds from the 10th century, archers who probably also trained from childhood, features a diagram showing some of the muscles which are most strongly developed in archers: https://www.forbes.com/sites/kristinakillgrove/2015/09/30/brawny-bones-reveal-10th-century-hungarian-warriors-were-accomplished-archers/#6703eb05f710 Similar skeletal changes have been found in remains from as long ago as the Neolithic: http://dienekes.blogspot.co.uk/2014/03/middle-neolithic-european-archers-from.html although earlier bows were not as heavy as medieval English longbows, therefore the changes would not have been so very pronounced.
Here a modern re-enactor, /u/spionkop1900 , using a lighter longbow mentions his experience of uneven muscle development: https://www.reddit.com/r/AskHistorians/comments/w2n9n/how_did_ancientmedieval_people_deal_with_chronic/
You may also find points of interest here: https://www.reddit.com/r/todayilearned/comments/2sr2c1/til_that_medieval_english_longbows_could_fire_an/
https://www.reddit.com/r/todayilearned/comments/3r2vq2/til_archeologists_can_tell_who_were_archers/
About muscle groups used in archery https://www.reddit.com/r/Archery/comments/1xu9s9/what_muscle_groups_are_used_in_archery/
