If you've done any research at all you've probably heard by now that to build muscle bulk, especially in the upper body, you have to work your legs.
(If you haven't heard that...
well, now you have.
) The problem upon discovering this rule is deciphering what lower body exercises are best to use to create the perfect proportions for your body.
In addition, what if you have any back issues? Which of those exercises are less painful or cause less aggravation to your back? Here is a list of the primary total lower body exercises you can do to help build muscle bulk.
All of these moves use the glutes, quads, and hamstrings as the primary muscles.
Here is a little description of each and the impact associated with your back.
Squat:

• Free weight barbell squat (FWS) - This is a free weight exercise where you are squatting with a weighted barbell on your shoulders.
  Your range of motion is not limited in any manner which has its benefits.
  However since there is no back support, this exercise has some of the highest potential for back injury.
  
• Smith Machine barbell squat - This move is very similar to the free weight barbell squat.
  The main difference is that your barbell rides in a track on a Smith Machine that guides the barbell as you squat and limits you regarding any forward or backward motion that a FWS would allow.
  Although the Smith squat doesn't prove as great a risk as the FWS does, this move still poses a moderate threat in causing injury to your back.
  
• Hack squat - The hack squat is a move using a machine whereby you rest your back against an upright carriage and your shoulders sit just under heavily padded shoulder pads.
  The machine limits the amount of free movement your upper body has but provides sufficient motion for pressing the weight up and lowering it.
  Regarding squats, this exercise gives the least potential for back injury.
  

Leg Press

• Lying leg press - The is a machine based exercise that either has you on your back and slightly elevated with your legs at approximately a 90 degree angle in relation to your body, or lying flat on your back with your legs in a horizontal position with your upper body.
  With both variations on this machine, your feet press against a plate and your legs push the weight until your legs are extended.
  The risk of back injury is low to moderate for either variation.
  
• Sitting leg press - This is also a machine exercise very similar to the lying leg press, the only difference between these two options is the sitting position verses the lying position.
  Because of the overall support the seat and back provide, this move is the least impacting to your back.
  

Lunge:

• Walking lunge - This exercise will have you stepping into a lunge position with one leg and then alternating to do the same with the other leg, typically covering a short distance moving forward with each step.
  Adding weight using dumbbells or barbells is an option.
  Due to the lack of back support, this exercise has a moderate to high risk of back injury associated with it.
  
• Stationary lunge - Just as in the walking lunge the method in this exercise is similar with the exception of you being stationary.
  You perform a lunge one leg at a time, returning the stepping leg to its original position after the lunge is complete.
  The move carries the same or slightly lower risk of back injury than the walking lunge.
  

It should be noted that there are obviously risks involved with any weight bearing exercise you would perform.
However using proper technique and form while doing the exercise, is essential in limited the level of risk potential.
In addition, utilizing the appropriate set and rep technique for these exercises will be essential in letting these moves become major players in your attempt to build muscle bulk.