I used to smoke everyday before lifting for about a year or so. It put me in the zone and I felt a greater mind muscle connection. I also had more endurance, stamina, and intensity. Most of the time I would feel some bronchodilation and easier breathing (thus, aiding with the endurance/stamina), but other times I would feel a bit tight at the chest. Strength wise, no effect really; if your blown, the weight might feel heavier. Gains wise, no effects; I saw normal results. The key is moderation; don't get too lit, but just enough to get in the zone. In the end, it's every personal. How you react to it and the mindset you have while high are the main factors. Nowadays, I don't smoke as much or as often. Still train high on occasion, but it's most times I'm not. I think the main thing for me was the intensity, I would really zone out while I was in there.
To answer your question specifically, I think anyone who has ever smoked will tell you they felt less tense and relaxed, so there is definitely a muscle relaxing effect. However, I don't think it is so great to effect your strength and actually I believe it maybe enhance performance via facilitating compliance at the joints through greater ROM, allowing better stretching/contraction. Again, I believe the psychological aspects of marijuana to be a greater determinant of strength. Just like being not in the mood vs being hyped up while not high make a difference in strength, the same goes while high.