Cause muscle several times a week.

Cause muscle several times a week. How many sessions per week pecs? The item is not on the chest in particular, but it is that comes up most often in the questions I get muscle.
There are several ways to train the full body, split, etc ...
Double or even triple the same muscle every week is possible under certain conditions.
You have a low point and it requires special attention, I am among those who think one training per week is useless to try to catch up muscle behind. The rest will continue to be strong point and weak point remains low.

Cause muscle a few times a week: aches

"Should we train on a stiff muscle? "
Yes and no ... Muscle pain causes a decrease in strength so it is not ideal to be the best. Train heavy on aches is not good because if the muscle is stiff is that the recovery cycle is not finished. So I do not recommend heavy work on aches.
By cons, light in long series on a weak point stiff interesting. The fact feel pain due to soreness during long and light series (more than 30 reps) will increase your sensitivity between the nervous weakness and brain. At the time you will not gain muscle but then when your next heavy sessions you will gain muscle recruitment and you target better and better your weak point. More long runs help congest in many irrigate the muscle and said good irrigation always said a good amino acid transport in the future.
For those who follow me on Facebook you could see at present I add a series of 100 repetitions of various exercises. I believe in this method the results it gave me in the past my shoulders. Muscle burn during very long series is unbearable but it is the price to pay to make up for a muscle. I got the first results after 2 months of testing at 3 sets of 100 reps per week (for the same muscle) from different angles but still work against an isolation exercise.
For those who have my ebook you can integrate a series of 100 repetitions without difficulty.