The velocity slowdown is the decrease in velocity of the striker from the beginning of the test to a given point, expressed as a percentage of impact velocity. International standards prescribe a limited slowdown during instrumented impact tests, therefore this parameter shall be monitored and reported to verify that the test is valid.

ASTM D3763 prescribes that the velocity slowdown be no more than 20% from the beginning of the test to the point of peak load. ISO 6603-2 prescribes that the decrease of velocity during the whole penetration test be not greater than 20%. ISO 7765-2 says that a decrease in the velocity of the striker of (up to) 20% is acceptable. ISO 179-2 prescribes that the decrease of velocity during the whole penetration test be not greater than 10%. The main goal of these requirements is to ensure comparable results when testing materials showing a viscoelastic behavior, since the application of load at different velocities produces different effects.