The top Army and Marine Corps generals told senators Tuesday that it will take up to three years to fully integrate women into all combat jobs, which are now all open to them, and that women should also have to register for the draft.

The military service leaders say they will not lower standards to bring women into the more grueling jobs. But they warned that inherent physical differences and different injury rates between men and women will have an impact on how the integration moves ahead.

The sometimes contentious hearing before the Senate Armed Services Committee laid bare the deep reluctance of Marine Corps leaders to bring women into certain demanding infantry, armor and special operations jobs.

After the Pentagon ordered all branches of the military to integrate their ground combat units by the end of last year, or explain why women can’t do the job, the Marine Corps conducted an experiment to help determine whether women should fight on the front lines.

The results showed that in almost every area, the all-male units performed significantly better than those with both men and women Marines.

Still, three years after then-Defense Secretary Leon Panetta directed that all positions become open to women by January 1, 2016, Defense Secretary Ash Carter announced the opening of all combat roles to women. But he also vowed that no standards would be lowered.

Carter has said that he made the decision to fully integrate the military just before the deadline, after receiving recommendations from the secretaries of the Army, Navy, Air Force and Special Operations Command.

The Marine Corps initially sought to keep certain infantry and combat jobs closed to women, citing studies, and arguing that allowing women to compete for ground combat jobs would make the Marine Corps a less-efficient fighting machine.

But both Navy Secretary Ray Mabus and Carter rejected that argument. In response, Marine leaders, including Marine Gen. Robert Neller, immediately began to lay out implementation plans, but they continue to make clear their reservations.