From your question you seem assuming that atmospheric density has an effect on punching/kicking.
The only reasons why this would be possible are:
- changed air resistance because of the different air density
- changed metabolism due to the different air amount
We can safely exclude the first effect. Air drag becomes significant above 30 km/h, or about 8 m/s. Punch velocities are estimated to be lower than that, around 1 m/s to 7 m/s. Also, the time it takes for a punch to hit is so short that any practical influence of the above effect would be negligible, as opposed for example with what would be on riding a bike trying to break the hour record.
About the second point, it is known that different air density affects muscular performance, due to the different intake of oxygen with breathing.
However the organism slowly adapt to the different conditions, with the process called acclimation
is the process in which an individual organism adjusts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH), allowing it to maintain performance across a range of environmental conditions. Acclimatization occurs in a short period of time (hours to weeks), and within the organism's lifetime.
The text above seems suggesting that the performances are kept unchanged after acclimation, therefore I would expect no particular changes in the hitting yield.
Again, a single punch is not so demanding that any significant effect can be perceived even during acclimation (if I was to be hit by Mohamed Ali immediately after his landing in Kinshasa I would have been knocked out anyway). Different is the story if you extend the effort.