nucleus; backbone; mainstay; key figure
medium-level; mid-level; middle-ranking; midsize
main body (of troops); crack troops; select troops
《baseball》center field; centre field; center fielder; centre fielder
athlete competing in the middle-number match in a team competition, i.e. second in 3-on-3, third in 5-on-5 (kendo, judo, etc.)
彼は中堅として後輩を引っ張る。
He leads the juniors as a key mid-level figure.
会社は中堅社員を育成している。
The company is training its key mid-level employees.
業界では中堅人材の不足が深刻な課題です。
A serious shortage of key mid-level personnel is an issue in the industry.