Simple reasons why muscles stop growing even though you train hard and tips on how get them to grow again.

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Training to hard
Muscles need time to recover from the intense focus so train each muscle group only once or twice a week to start and build up from that point.
 
Training too long
It should be intense but not than an hour.  It is likely that after 45 minutes Cortisol levels will increase and this hormone breaks down muscle cells.  See Stretching the Limits.
 
Too little sleep
Muscles grow when you sleep therefore it makes sense that less sleep will effect muscle growth.  8 hours is a good start.
 
Alcohol
Alcohol breaks down muscle mass as well as the better known numerous other effects.
 
Routine
Muscles adapt to routine so change it every 6-8 weeks to prevent the effects of muscle memory decreasing growth.
 
Stretching the limits
If the limits are not pushed, there is no reason for muscles to grow.  Increase reps or load each time to encourage growth. Choose a weight that allows you to complete 8 to 15 reps; over 15 is about muscle endurance not definition.. If  8 is to much then the weight is too heavy; if 15 reps is easy to complete then its too light, aiming to increase by about 5 percent.  Always consider how challenging the weight is for the different sessions.
 
What you eat
Protein is the building block for your muscles and a 1gram of protein per pound of body weight is the recommended norm.  If the diet is not providing this then supplements are a good alternative.  Personally I prefer shakes.

Conclusion
Let face it, exercise is good for how we look and how we feel.  Weither it is for health or vanity, it makes no real difference, as it all contributes to both our phyiscal and mental wellbeing.

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