Focus on form, not weight.
Good form means aligning your body correctly and moving smoothly through an exercise. Poor form can cause injuries and hinder strength gains because you aren’t isolating muscles properly. Start with very light weights because I want them to get their alignment and form right. “It’s good to start off using light to moderate weight when learning an exercise routine.” Concentrate on performing slow, smooth lifts and equally controlled descents while isolating a muscle group. You isolate a muscle group by holding your body in the position specified for each exercise while consciously contracting and releasing certain muscles.
Tempo, tempo. Control is important.
Tempo helps you stay in control and avoid undercutting gains through relying on momentum. And sometimes switching speed — for example, lowering for three counts and lifting for one count instead of taking two counts for each — can enhance power.
Blood pressure rises if you hold your breath while performing strength exercises. Exhale as you work against gravity (when you’re lifting, pushing, or pulling); inhale as you relax.
Challenge your muscles.
Choose a weight that tires the targeted muscle or muscles by the last two reps while still allowing you to maintain good form. If you can’t do the last two reps, choose a lighter weight. When it feels too easy to complete all the reps, challenge your muscles again by adding weight (roughly 1 to 2 pounds at a time for arms, 2 to 5 pounds for legs); adding a set to your workout (up to three sets per exercise); or working out additional days per week (as long as you rest each muscle group for 48 hours between strength workouts). If you add weight, remember that you should still be able to do all the reps with good form and the targeted muscles should feel tired by the last two reps.
A complete upper- and lower-body strength workout two or three times a week is ideal.
Give muscles time off.
Strength training causes tiny tears in muscle tissue. Muscles grow stronger as the tears knit up. Always allow at least 48 hours between sessions for muscles to recover. You can always do “split sessions” — for example, you might do upper body on Monday, lower body on Tuesday, upper body on Wednesday, lower body on Thursday, etc.
From Harvard medical school newsletter.