The deadlift is one of the best moves for working your posterior chain. This is the group of muscles stretching from the back of your thighs up to your mid-back. Deadlifts will aid in building thigh muscle size, though they target specific parts of the thighs, and whether you build muscle also depends on a number of other factors.
Muscles Targeted
The posterior chain encompasses your hamstrings, glute muscles, the spinal erectors and quadratus lumborum of your lower back, as well as your mid-back and adductors. The hamstrings and to an extent the glutes can be classed as part of your thighs, so in that respect, conventional deadlifts will work your thighs, increasing strength and muscle size.
Increasing Thigh Activation
To take your lower- and mid-back out of the equation and focus more on the hamstrings, change how you perform deadlifts. Chad Wesley Smith of Juggernaut Training Systems recommends switching to Romanian deadlifts instead. These are performed with minimal knee bend. Keep the bar close to your body, your weight on your heels and push your butt back on every repetition, advises Smith. You can also use sumo deadlifts, performed using a wider stance, to increase hamstring activation.
Working the Quads
The hamstrings and glutes only make up the back part of your thighs -- to fully work your thighs you also need to target the quadriceps on the front. Exercises like squats, leg extensions and leg presses do this, but certain deadlift variations do too. Deadlifts using a trap or hex bar remove some of the tension from the posterior chain and place it onto the quads, according to strength coach Bret Contreras. This makes trap bar deadlifts superior for building the front of your thigh muscles.
Considerations
Getting bigger thighs isn't just about whether you deadlift or not. To build muscle you need to eat more calories than you consume, so if you're dieting or trying to lose weight then it's unlikely you'll get big thighs from deadlifting. You may experience some muscle growth if you're new to weight training, however. Second, you also need progressive overload to get bigger. This means using more weight, performing extra reps or adding sets each time you train. If you're not doing this, you're not giving your thighs the stimulus they need to grow.