The displacement of conventional electricity generation in favor of stochastic renewables will require extended regulation capacity from remaining dispatchable plants, including hydropower power plants (HPPs), which may be subject to accelerated aging of its mechanical parts due to more severe regulation duties. This paper focuses on transient pressures in the penstock of medium-head HPP due to load variations from primary frequency control. We propose and compare the performance of two algorithms to reduce pressure transients, ultimately leading to decreased penstock fatigue. The first is a linear filter applied to the input control signal of the HPP. The second, that we call fatigue-aware, uses a simplified model of the HPP to estimate the fatigue of the penstock and selectively remove those patterns from the input control signal that are conducive to accelerate fatigue damage. The performance of the two algorithms is validated by simulations considering a 230 MW medium-head HPP.