Prompt Title: Muscle Gain Workout Plan.

Great, let's create a 90-day workout plan to help you gain muscle mass, working out four days a week with 45-minute workout sessions.

Before we dive into the workout plan, it's essential to mention that to gain muscle mass, you need to focus on progressive overload, which means gradually increasing the weight and/or reps of each exercise you perform over time. Also, make sure you're eating enough protein and calories to support muscle growth and recovery.

Day 1: Upper Body

1. Warm-up: 5 minutes of light cardio (jogging, jumping jacks, etc.)
2. Bench Press - 3 sets of 8 reps
3. Incline Dumbbell Press - 3 sets of 10 reps
4. Bent Over Rows - 3 sets of 8 reps
5. Lat Pulldowns - 3 sets of 10 reps
6. Shoulder Press - 3 sets of 8 reps
7. Bicep Curls - 3 sets of 10 reps
8. Tricep Pushdowns - 3 sets of 10 reps

Day 2: Lower Body

1. Warm-up: 5 minutes of light cardio (jogging, jumping jacks, etc.)
2. Squats - 3 sets of 8 reps
3. Lunges - 3 sets of 10 reps (per leg)
4. Deadlifts - 3 sets of 8 reps
5. Leg Press - 3 sets of 10 reps
6. Calf Raises - 3 sets of 15 reps

Day 3: Rest Day

Rest and recover to allow your muscles to repair and grow.

Day 4: Full Body

1. Warm-up: 5 minutes of light cardio (jogging, jumping jacks, etc.)
2. Pull-ups - 3 sets of 8 reps
3. Push-ups - 3 sets of 10 reps
4. Dumbbell Squat and Press - 3 sets of 8 reps
5. Bent Over Rows - 3 sets of 10 reps
6. Bicep Curls - 3 sets of 8 reps
7. Tricep Pushdowns - 3 sets of 10 reps
8. Leg Press - 3 sets of 10 reps

Day 5: Rest Day

Rest and recover to allow your muscles to repair and grow.

Day 6: Upper Body

1. Warm-up: 5 minutes of light cardio (jogging, jumping jacks, etc.)
2. Incline Bench Press - 3 sets of 8 reps
3. Dumbbell Flys - 3 sets of 10 reps
4. One-Arm Dumbbell Rows - 3 sets of 8 reps (per arm)
5. Seated Cable Rows - 3 sets of 10 reps
6. Arnold Press - 3 sets of 8 reps
7. Hammer Curls - 3 sets of 10 reps
8. Cable Tricep Extensions - 3 sets of 10 reps

Day 7: Rest Day

Rest and recover to allow your muscles to repair and grow.

Day 8: Lower Body

1. Warm-up: 5 minutes of light cardio (jogging, jumping jacks, etc.)
2. Deadlifts - 3 sets of 8 reps
3. Leg Extensions - 3 sets of 10 reps
4. Leg Curls - 3 sets of 10 reps
5. Leg Press - 3 sets of 8 reps
6. Calf Raises - 3 sets of 15 reps

Day 9: Rest Day

Rest and recover to allow your muscles to