In recent years, the size and style of play from running backs has changed, with the introduction of running back by committee approaches and complementary (or "scat") backs.
Because of this, there's been a reduction in the size requirements at the running back position that NFL personnel men were once notorious for desiring.
However, weightier backs are still important in today's game because they can do damage to defenses by wearing them down, as seen by the likes of the Michael Turners and Brandon Jacobs of the world as well as bruising them with broken tackles and picking up the tough yards.
There are many in the NFL that do this along with the aforementioned Turner and Jacobs, and there are four main traits that define these kinds of ball-carriers: low center of gravity, balance, mentality and strength.
Low Center of Gravity
Growing up playing pee-wee football, ball-carriers were (correctly) taught that they need to play lower to the ground in order to have an advantage when engaging in contact with the opposition. This lesson was described with the simple saying "low man wins."
Playing "low" to the ground can come in two different ways: pad level and natural advantage. The former comes in way of the ball carrier dropping his pad level down when nearing contact, as can be seen in the example of Peyton Hillis running over Dawan Landry while Hillis was with the Cleveland Browns.
Hillis is a bigger back in both stature and weight than most players at his position, and because of this, he is a bigger target to defenders than most. In order to avoid taking a greater beating than he already is, Hillis has to get his pad level lower than Landry's to give himself a chance at gaining yards after contact—which he did, as can be seen at the 28-second mark in the video.
While Hillis has to work harder to be the low man in contact, Jacksonville Jaguars running back Maurice Jones-Drew does not. MoJo has a natural leverage advantage against defenders because he stands only slightly over 5'6".
Jones-Drew is one of the league's best in breaking tackles year in, year out and his height (or lack thereof) is a significant reason why. He's able to already play lower to the ground than the defenders, and when he makes his body even more compact, he is very tough to bring down as seen by some of his long runs throughout his career.
While a leverage advantage is necessary, regardless of the form it comes in, it is not the sole reason why ball-carriers are successful at running with power and breaking tackles. There's also balance, which is arguably the most important trait of any in running back.
The reason it is so crucial is because, when it comes to running the ball in between the tackles, ball-carriers can often be bounced around like a pinball, so if they have good balance, they can hold up and potentially escape through despite receiving contact.
Perhaps the best example of this is Seahawks running back Marshawn Lynch's run against the New Orleans Saints defense in the 2011 Wildcard playoff game.
After taking the hand-off, Lynch bounced of several defenders between the tackles, and at first it appeared that the play was going nowhere. However, Lynch stayed on his feet despite the contact, displaying very good balance and lower body strength (more on that later) that enabled him to continue running downhill and eventually, into the end zone for a touchdown.
Attempting to do specific things while playing is a tough task that not every person is suited for. An example is playing the nose tackle position; it requires a person to be very tough, gritty and able to handle all the dirty things that go on in the trenches.
In some ways, running the ball with power is similar because the ball-carrier has to be very tough mentally and determined to overcome any obstacles that come in his way.
Unfortunately for defenders, they are the obstacles, as was seen in the example of Marshawn Lynch's run, it doesn't always end well for them. This is also often the case when new San Francisco 49ers running back Brandon Jacobs gets the ball.
Jacobs, who is known for having a rather short and violent temper, squares his shoulders and bowls over defenders constantly. There are multiple examples which illustrate Jacobs' power and violence as a ball-carrier, one of which came against the New Orleans Saints last season.
After receiving the hand-off on the counter play, Jacobs ran right, squared his shoulders and set safety Roman Harper in his sights. Harper was coming down in run support, arguably what he does best, to stuff Jacobs.
Despite Harper getting his pads low (even though he did drop his head), it was not enough to stop the bulldozing Jacobs into the end zone. Jacobs, determined to get in, ran right over Harper at the goal line and walked in for the touchdown.
Strength is (obviously) another trait that is needed in a ball-carrier looking to run with power, and this one is rather simple. Ball carriers not only need to have a strong upper body, as the legs are where he'll be generating the most of his power.
When it comes to power, few are built like the stocky Michael Turner of the Atlanta Falcons. Turner, who is known as a bruiser, is strong in his upper body and is especially strong in his lower half, where he has massive thighs that enable him to really run with power when nearing contact.
Turner's strength in the upper and lower body was on display this past season when the Falcons took on the Tennessee Titans. The former Charger did damage when he shrugged off safety Michael Griffin, who was looking to make a tackle, as well as his teammate with a strong right arm and then hurdled over them to continue his thunderous path.
The running back position is not one for everybody. Athletes playing it have a short career because of the beating they take when running the ball, which is why teams are reluctant to spend big money on it. Despite this, it continues to be a crucial aspect of offenses and players that can do more damage to defenses are more valuable.
Because of this, when a team gets a ball-carrier that plays with a low center of gravity, balance, a tough mentality and great strength, they look to keep them due to their ability to pick up yards after contact. They bring an edge to the offense with their rough and tough playing style.
There are several ball-carriers in the NFL that have this type of style, most notably Turner, Jones-Drew, Lynch and Jacobs. All four of these players possess these traits and are consistently one of the league's best at breaking tackles.