“… physical injury, either to their head, torso, or pelvic area...”
Muscle- powered warfare… spears, arrows, swords, axes… horrible wounds.
Brutal.
In a battle, the legions began with a long wait for their adversaries to charge in, then they launched their javelins. Thousands of javelins came whistling into the ranks of the attackers, wounding and killing hundreds or thousands, then the generally untrained horde met the disciplined, trained, and generally experienced legions. Ow.
The front rank would fight for 2? 3? minutes, the Centurion would blow a whistle, and the second rank would move into action. The former first rank would fall in at the back of the formation. This kept them able to fight for as many hours as needed to defeat and/or annihilate opponents. One of the spot-on tidbits in the beautiful but otherwise silly "Cleopatra" movie (Taylor-Burton-Harrison) was when the tutor character said "and with so few men". That always struck Roman opponents as odd. For the better part of a year, Caesar held Alexandria with a half legion, got some anticipated reinforcements, then went out and wiped out Ptolemy's (largely ethnically Greek?) army.
After the defeat of Tony and Cleo, Augustus had cut the size of the army in half, to 28 legions, and added 28 auxiliary legions from various conquered tribes (generally specialized skills not part of the Roman methods). All legions were brought up to strength, but the century was reduced from 100 legionaries (which made sense) to 80, and increased the number of Centurions to improve command and probably to slightly improve the career path. The size of each legion was increased from about 4200 to about 5200, with 300 cavalry throughout.
Even the size of a legion wasn't fixed, centuries would be reassigned from quiet spots to places with more activity. The plum place to serve (and to get land upon retirement) was probably Britain. It was rare for more than two legions to be stationed there. The other pretty quiet spots (usually) were Greece, Egypt, and North Africa. I'd imagine that Greece was another popular retirement or vacation destination for higher-end Romans.
A legion didn't typically serve all in one spot (smaller units were spread along the frontier forts and/or walls/limes), and a cohort (varied in size) of a particular legion would be stationed in one or more forts in geographic proximity. One of the best preserved is in Jordan, I think the link is up above. :^)