How to Perform the 45 Degree Leg Press Machine
Most of us begin our leg workouts with a heavy compound barbell movement like the back squat, deadlift, or clean. As the workout progresses volume increases and the complexity of movements decreases.
The 45-degree leg press machine is an outstanding compound push exercise to target the quadriceps and glutes. This plate-loaded machine can be found in even the most hardcore gyms, using a lever or sled apparatus to hold the weight.
In most bodybuilding and powerlifting routines, it's commonly placed after the main free weight barbell exercise(s) of the day. It allows the lower body to move in a motion similar to the squat while supporting the glutes and upper body. Some lifters find they're able to move their legs through an increased range of motion and use more weight compared to performing conventional back squats.
The 45degree leg press machine targets the quadriceps, a muscle group comprised of four heads (Rectus Femoris, Vastus Lateralis, Vastus Intermedius, & Vastus Medialis (Internus).  This exercise engages the gluteus maximus, adductor magnus (inner thigh), and soleus (calf) to assist in completing the movement. The hamstrings and gastrocnemius (calf) act as stabilizers during the exercise. 
Even though the 45-degree leg press machine is performed seated, it's no joke. It can leave even the most experienced trainees with epic delayed onset muscle soreness (DOMS).
MTS Nutrition CEO Marc Lobliner shows you how to perform the 45 degree leg press.
How to Perform the 45 Degree Leg Press MachineApproach the leg press machine and select the appropriate working weight. Distribute the weight evenly on both sides of the lever or sled apparatus.
don't place 55lbs on one side and 35lbs on the other side of the machine; doing so won't improve your gains and will likely lead to an injury. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions.
Before sitting down on the machine ensure the angle of the padded back support is roughly perpendicular to the glute support. For most this will be the most comfortable position but feel free to adjust as desired.
Sit on the machine so that your back and glutes are resting on the padded support areas. Raise your legs and place your feet flat on the lever apparatus or sled platform. Take a shoulder width or slightly wider stance with the toes pointed forward or slightly outwards. At this point your knees may still be slightly bent; this is ok as the weight is resting on the safety pins.
After setting your stance, take a deep breath, brace your abdominals, and push through your heels to fully extend the hips and knees. Release the safety pin apparatus and then hold the handles on either side of the machine. Holding these handles will minimize upper body involvement and ensure your back and glutes remain on the padded supports throughout the entire motion.
This isn't just another pre-workout. It's da RUCKUS! Order one of 5 amazing flavors now.
Begin lowering the lever apparatus or sled by flexing the hips and bending the knees. Allow the platform to descend until you've hit the desired depth, which for most will be until the hips are flexed. Those who are more flexible can descend until the thighs hit the calves. You should still be holding a big breath and bracing your abdominals.
To initiate the raising portion of the exercise begin pushing through your heels to fully extend your knees and hips. At the top of the movement, often referred to as the lockout, legs are straight but not hyperextended.
Some choose to breath between each rep while others prefer to breathe out during the ascent; experiment and see which feels most comfortable and natural to you. The knees should be pointed in the same direction as your toes throughout the entire rep.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, tri-sets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc) but strive to improve every time you walk into the gym.
45 Degree Leg Press Machine Form TipsAvoid Half Reps - The 45-degree leg press machine provides maximum benefit if performed using full range-of-motion reps. Half or quarter reps place additional stress on the knees, reinforces poor movement patterns, and doesn't provide as much stimulus to the target and supporting muscle groups.
Switch Up Your Stance - To emphasize glute involvement take a wider stance and/or place your feet slightly higher on the sled or platform. To emphasize quadriceps involvement take a narrower stance and/or place your feet slightly lower on the sled or platform. Regardless of what stance tweaks you make, aim to perform full range-of-motion reps with knees pointing in the same direction as your toes.
Stay on the Padded Support - Throughout the entire movement doesn't let your glutes and back come off the padded support. Coming off the padded support changes the movement pattern, shortens the range of motion of the rep, and doesn't provide as much stimulus to the target and supporting muscle groups.
Hands Off The Knees - don't put your hands on your knees during any portion of this exercise. Pushing your knees with your hands to help lock out the lever or sled will take the emphasis off the target and supporting muscle groups.
Use One Leg - If you're having trouble getting a good quad contraction or if one quad is lagging in size or strength compared to the other, perform the leg press with one leg at a time. This will help improve the mind-muscle connection during the exercise as well as even out any imbalance that may have developed between quadriceps. To perform a one-legged variation simply flex one knee and keep the other leg off the platform; perform the desired number of reps and then switch legs.
Pause at the Bottom - Pausing at the bottom of the rep increases the intensity by extending the duration of the set and time under tension as well as stretches the adductors (inner thigh muscles) and hips.
References1. Griffing, James, et al. "Quadriceps." ExRx.net. N.p., 2015.
2. Griffing, James, et al. "Sled 45° Leg Press." ExRx.net. N.p., 2015.