"Now you wouldn't believe me if I told you, but I could run like the wind blows. From that day on, if I was going somewhere, I was running!"
--Forrest Gump
It's a beautiful day. The sun is shining and glistening off the ocean's waves, causing a beautiful reflection of colors in the cloudless yet slightly misty morning sky. In other words, it's a perfect day for an outdoor run. But it's hot, too. And you have pale skin and burn easily. It's also early in the morning, meaning that there aren't too many sunbathers to gawk at, or at least check out discretely under your Ray-Bans. OK, so indoor treadmill running it is!
My good friend Forrest Gump would never contemplate the nuances of such a choice. Rather, he would just run. But we're not Forrest Gump. So here's the dilemma: Should you run on a treadmill or overground? To answer this, let's disregard the rhyme and reason behind the choice and focus solely on the science.
Running Pattern
A recent study out of the University of Delaware found that individuals demonstrate similar hip, knee and ankle kinematics (movement patterns) during the stance phase of overground (OG) and treadmill (TR) running. However, during OG running, individuals demonstrate 4.5 degrees more of rear foot dorsiflexion. This may be attributed to a shorter stride length (increased cadence for controlled speeds) in TR running and suggests more of a heel-strike contact pattern. OG running also results in increased peak knee varus (bowing out) and, according to an analogous study conducted at the University of Virginia, an increase in peak knee extension and flexion during the swing phase. The aggregate results suggest that TR running may be better suited for individuals with a mid-foot strike, or who wish to develop such a running pattern. Conversely, it is plausible that OG running may help to limit knocking of the knees.
Forces and Mechanical Demand
Whether attributed to the differences in mechanical stiffness properties between the ground and a TR, or the stiffness of the human-TR vs. human-ground interaction, TR running results in reduced peak anterior propulsive forces and medial ground reaction forces (GRFs). As a result of the deviancy in magnitude of the GRFs as well as the altered kinematics at the ankle and knee, OG running results in greater average varus torques about the knee. Furthermore, there appears to be greater mechanical demand (increase in energy generation) to the knee in OG running and an increase in energy absorption to the ankle in TR running. These findings translate clinically. For example, individuals who may have knee pain may prefer to run on a TR in order to reduce the demand to this joint. In contrast, individuals with calf or Achilles strains may benefit from running OG, but with shorter steps in order to minimize demand to these structures.
Muscle Activation
In addition to the altered movement patterns, reaction forces and joint mechanical demand, Chung et al. (2009) reported significant differences in muscle dynamics. Specifically, these researchers found that TR running results in significantly reduced muscle activation of the quadriceps and hamstrings throughout the stance phase, as well as during toe-off. This appears at least partly attributed to the fact that when on a TR, the posterior movement of the belt results in passive extension of the hip, thereby limiting the required activation of the muscles surrounding this joint and potentially hindering training effects to these muscles.
Energy and Speed
All the aforementioned differences between OG and TR running were predicated upon studies that were performed on level ground and that controlled for running speed between conditions. However, energetic costs (as measured by oxygen consumption) on an indoor TR are theoretically less than outdoors OG running, because of the lack of air resistance. In light of this, researchers from the University of Brighton found that in order to equalize energetic costs, a TR grade of 1 percent is required. However, this is still under controlled running velocity conditions. But here's the caveat: People don't run as fast on TRs as they do OG. In fact, in a recent investigation, Kong and colleagues (2012) found that healthy runners demonstrate nearly a 32-percent reduction in perceived running speed on a TR than when OG. In other words, when running on a TR, people think they are running faster (and harder) than they really are. This could theoretically limit the selected speed and/or distance one chooses to run.
Forrest once told me that he used to "run to get to where [he] was going, but [he] never thought it would take [him] anywhere." Lo and behold, he used those powerful legs of his to see the beauty this country has to offer, all by running OG for 3 years, 2 months, 14 days and 16 hours. But whether you're going to run like Forrest or stay indoors on that fancy treadmill, recognize that just like most things in running (and life, for that matter), the choice depends on intrinsic characteristics and goals. So whatever choice you make, make sure you follow Forrest's lead in one thing: Enjoy yourself and "run like the wind blows."