Get Fit in 90 Days: Week 4 (The Plan)

Your goal this week: To strengthen your core. Each workout will end with a plank, which targets your core muscles. (See how to do this exercise here.)

Monday: Cardio Session

We're stepping it up again this week! Choose the treadmill, elliptical, stationary bike, or head outside for a run or a bike ride.

• 5-minute warm up

• 5 minutes at a comfortable pace (5-6 on a scale of 10)

• 1 minute at an uncomfortable pace (working at a higher intensity, 8-9 on a scale of 10)

• Repeat 3 times

• Cool down for 5 minutes

After you cool down, find a place to perform a plank, and work to hold it for 30 seconds.

Tuesday: Upper-Body and Lower-Body Circuits

Good news: You only have to do each one once!

Upper-Body Strength Circuit: Find a weight that you can lift for a full set, but still feel fatigued on the last rep. In between each set, do 30 seconds of jumping jacks. (Click here for the how-to instructions for each exercise below.)

• 10 push-ups on toes, if possible (targets chest and shoulders); 30 seconds of jumping jacks

• 15 single-arm rows on each side (targets back and shoulders); 30 seconds of jumping jacks

• 20 tricep dips off of an elevated surface like a chair (targets triceps); 30 seconds of jumping jacks

• 16 alternating bicep curls and hammer curls using no less than 5-pound weight in each hand (targets biceps); 30 seconds of jumping jacks

Lower-Body Strength Circuit: You will need a jump rope. If it's not something you are willing to buy, simulate turning a jumping rope. If you aren't comfortable jumping, try marching in place. In between each exercise, jump rope for 60 seconds. Do the entire circuit 2-3 times through. (Click here for the how-to instructions for each exercise below.)

• 20 squats with feet shoulder width apart (targets glutes, quads, and hamstrings); jump rope for 60 seconds

• 20 alternating front lunges with a two-second hold (targets glutes, quads, and hamstrings); jump rope for 60 seconds

• 12 single-leg extensions by either balancing or holding onto a chair (targets quads and hips); jump rope for 60 seconds

• 20 plié squats (targets inner thighs and glutes); jump rope for 60 seconds

After you cool down, find a place to perform a plank, and work to hold it for 30 seconds.

Wednesday: Repeat Monday's Cardio Session

If you feel yourself getting bored, mix it up by either choosing a different exercise completely, or switch halfway through the routine. After you cool down, find a place to perform a plank, and work to hold it for 30 seconds.

Thursday: Quick Full-Body Circuit Parts I and II

Quick Full-Body Circuit Part I: Start with a 5-minute warm up like jogging, jumping jacks, or jumping rope, then start the circuit below. (Click here for the how-to instructions for each exercise below.)

• 20 squats with feet shoulder width apart (targets glutes, quads, and hamstrings)

• 10 push-ups on toes, if possible (targets chest and shoulders)

• 10 lunges on each leg (targets glutes, quads, and hamstrings

• 15 tricep dips (targets triceps)

Quick Full-Body Circuit Part II: Start with an easy 5-minute warm up like jumping rope or jogging/marching in place. (Click here for the how-to instructions for each exercise below.)

• 20 squats to alternating knee lifts (targets glutes, quads, hams, hips, and core)

• 10 push-ups to T-rotation on knees or toes, whichever gives you the best form (targets chest, shoulders, and core)

• 10 traveling lunges on each leg (targets glutes, hamstrings, quads, and core)

• 10 tricep dips with right leg up; 10 dips with left leg up (targets triceps and glutes)

After you cool down, find a place to perform a plank, and work to hold it for 30 seconds.

Check out the short clip below to see this workout come together.

Friday: Repeat Monday's Cardio Session

If you feel yourself getting bored, mix it up by either choosing a different exercise completely, or switch halfway through the routine. After you cool down, find a place to perform a plank, and work to hold it for 30 seconds.

Sara Haley (sarafitness.com) is a Fitness DVD Personality, Master Trainer and Creative Consultant for Reebok.