Why is it the knee or elbow take long to heal when hurt?

Question: Why is it the knee or elbow take long to heal when hurt?
- Keywords:
  elbow,
  knee
Asked by 783bmay33 to Peter, Jonathan, Grace, Doris, David, Ann on 5 Jun 2019.
- Grace Kago answered on 5 Jun 2019:
  
  hmm… it could be because we use those particular body parts quite a bit so even when they’re hurt, they don’t have a chance to just hide and rest… they’re still gonna be exposed so they’re likely to keep getting hurt/still be under stress.

Question: Why is it the knee or elbow take long to heal when hurt?