Hi, sirwhen we holding 5kg mass steady both flexor and extensor contracted but how about when lifting the mass upwards?will the extensor also contract?

Contract differently: When holding a weight up, against gravity, both flexors and extensors should contract, but with those muscles doing most of the work doing most of the contracting, and the other muscles on the other side of the arm just contracting a little bit to keep the person in balance, and to control the steadiness of the arm.

Tey will not: They will not contract, extensors work when you extend, flexors work when flexing motion occurs, this muscles groups work independant of eatch other.