There are many approaches to gaining muscle. First, determine what best fits your goals and schedule.
If you have time to exercise 5-6 times per week targeting individual muscles for certain routines is recommended. This allows proper time for your strained muscles to rest (1-2 days) and recover while ensuring you are exercising and building each desired muscle group.
If you plan to exercise 3-4 times per week focusing on more muscle groups during your routine is recommended. This allows proper time for rest and recovery while ensuring you are strengthening your desired muscles.
For stronger and more powerful muscles, use higher resistance and lower reps with 2-3 minutes rest between sets is recommended. (Example: 4 sets of 5 reps)
For toning muscles and building stamina, use lower resistance and higher reps with 1-2 minutes of rest between sets is recommended. (Example: 3 sets of 15 reps)
Muscle confusion: an important concept to achieve the most from your fitness plan by constantly mixing up your exercise routine. Your muscles, like many things, become accustom to your routine. This can result in a plateau or a sudden decrease in your muscle gains. Constant changes or varied exercises challenge your muscles resulting in more muscular gains. Therefore, using one day/week/month to focus on lower reps with higher resistance and the other day/week/month to focus on higher reps with lower resistance helps you continue to see results.