Categorical or Continuous ?

Hello everyone,

I have done an experiment in Ergonomics and now I'm dealing with the analysis. It's about a manual lifting task and heart rate recovery time. I have one main response which is heart rate recovery in minutes (obviously continuous), and three Indep variables: weight of the object(10 kg, 10 KG), Frequency of lift (3 lifts/minute, 9 lifts/minute), and duration of lift (5 min, 10 min)

As you can see each of my independent variables (fixed values) has a high level and low level and a total of 8 combinations will be obtained as a result of their interaction.

My question is if I want to perform a multiple linear regression, what type of data are my independent variables?

My guess is they are not continuous, but categorical. And within categorical, they are not nominal but are ordinal.
In case they are ordinal, can I treat them as continuous in my analysis?

Thank you in advance for your kind replies!