Why There's Nothing Wrong with a Bouncy Stride

I've always had a pet peeve on this topic when I hear people claim that you should run so that your body does not "bounce" (i.e. you center of mass (CM) does not oscillate up and down much). It's a pet peeve because that assumption is generally wrong. I was motivated to write this seeing a new study on the topic discussed by Alex Hutchinson on his blog.

This is not a running-specific oversight, but a general issue when people attempt to make biomechanical insights without the proper training in biomechanical analysis.

You focus on the benefit of the output (moving forward), but you are forgetting the cost on the input (human body muscle demand).

1. In cycling, you do NOT necessarily want to push tangential to the crank arm.

2. In running, you do NOT necessarily want to push parallel to the ground (after accounting for body weight)

3. In wheelchair (WC) pushing (one area I studied in my phd thesis), you do NOT necessarily want to push tangential to the wheel.

In all 3 cases, yes you would want to push that way if all that mattered was optimizing mechanical output. But running / cycling / WC pushing economy is a balance between power output and the power input cost (muscle metabolic cost).

Gross Mechanical Efficiency ~ Power Output / Oxygen Input

You want to try to increase power output, but only if the load on the muscles (which relates to calories burned and oxygen consumed during aerobic exercise), does not.

An example of this is shown below, taken from one of my wheelchair studies. We are looking at how varying the angle you push on the wheel of a wheelchair affects the mechanical loading (moments, or torques) of the elbow and shoulder joints. At all angles, the tangential (propulsive) component is fixed, so the wheelchair movement is guaranteed, but the radial force component may vary.

A 90 degree force angle is purely tangential and what you would think is best. A lower degree requires a radial force component, and the force magnitude required overall increases. Wouldn't that increase mechanical demand of the upper extremity?

Not so fast. Notice the total moment demand is minimal at force angles less than 90 degrees in both subjects (S1 & S2). This is because while the overall force is greater, the upper arm and forearm have better leverage and effectively smaller moment arms and require less muscle activation to produce the force.

Net Joint Moments (NJM) as a function of force angle. Changes in force angle change how forces are applied to both forearm and upper arm. Force directions that "line up" parallel to the limb require less NJM (and less muscle demand). For instance, Subject S1 requires little shoulder NJM at an angle of 55 degrees as the force is close to parallel to the upper arm, and the moment arm about the shoulder is small. The key takeaway is that 90 degree forces may be smaller but have larger moment arms, and thus not necessarily the most efficient.


Same goes for cycling: You may think pushing tangential is best, but it really depends on the configuration of the femur and tibia relative to the pedal location. In some positions it probably makes sense to push tangential to the crank arm, but not always.

And in both cycling and wheelchair pushing, experimental evidence agrees: people don't push tangentially, for the reasons explained above.

Again, it's not just about force magnitude, but how that force passes through the segments of the body. What's easier, holding a bowling ball in your hand with your arm fully extended horizontally, or with your arm relaxed downward at your side? The latter is easier.

Back to Running

I have not personally studied running as much as wheelchair / cycling mechanics, but let's relate these concepts. "Ideally" you should push off your the ball of your back foot just enough to maintain your vertical position while moving horizontally. But what if your leg and body configuration really want you to push more vertically?

As shown above, the optimal force direction that gives the proper amount of horizontal acceleration but minimizes total muscular load may not be obvious. In running, your calves want to push you upwards, not just forwards, so it makes sense to me that vertical motion is just a side effect of the runner's force direction for optimal running economy.

Now, I imagine that if you have really great ankle dorsiflexion range of motion, then you can lean your body forward more before push off, and the force / stride angle can be reduced (and vertical motions reduced), and that may have the best running economy of all. But if you don't have that range of motion in your ankle, trying to run without oscillating will just make you less efficient.


Which of the following force angles do you think is most efficient? Both produce the same horizontal component (forward motion). The red is larger, but passes closer to the joints of the leg, and thus will have smaller moment arms. The green is smaller, but requires larger moment arms. Which requires more muscle activation?

Study: Barefoot Running (but Not Minimalist Shoes) reduces Patellafemoral Stress vs. Neutral Shoes

A study just came out that concludes that their model estimates of patellafemoral (PF) stress is reduced in barefoot running vs. regular running shoes.

Overview: This study is a side-kick to a previous manuscript (Free fulltext) which discussed the kinematic and kinetic outcomes of the experimental data from various shoe conditions. This study added the PF model estimates for two conditions (neutral shoes vs. barefoot). Runners who habitually run with shoes were recruited, and ran on a track in four shoe conditions (barefoot, minimalist shoe, flats, and neutral shoe). Kinematics and reaction forces were collected. For two conditions, PF stress were estimated using a model based on several equations estimating contact area, knee moment, quadriceps force, etc...

Findings: 1) From the first study: Peak knee flexion and knee extensor moments were LESS in the barefoot condition, but the SAME across all 3 shoes. 2) From the second study: Peak PF stress was less in barefoot condition vs. neutral shoes Here's a plot of moments from the first study:

Conclusion: The PF stress study only concludes that barefoot running reduces PF stress estimates vs a neutral shoe. However, given the knee flexion and extensor moment estimates from the initial study, it would seem that PF stress would be similar in all shoe conditions.

Why?: A big part of why barefoot running alters running mechanics is due to the fact that we don't like pounding the fat pads under our feet very hard. Even wearing a minimalist shoe aids in comforting our fat pads, which allows us to pound them harder to reach the same pain threshold than when barefoot.

Potential Limitation: 

The biggest - these runners were habitual shod (shoe) runners. What happens when they are fully adapted to barefoot running? Do fat pads stiffen up and allow runners to pound harder on their feet? Or do the differences increase more through increased comfort using the ankle joint as a shock absorber?

Another - The running velocities need to be kept constant across conditions or else the results don't mean much. The authors claim that the velocities ended up (on average) being EXACTLY the same across all 4 conditions, seemingly without feedback to the runners on their speed. This seems dubious to me. In addition, how different would the results be if moments and PF stresses were calculated for each trial and THEN averaged, vs. the other way around?

A Short Comment on Running Mechanics and Injury

The NYTimes has an article talking about a recent study indicating amount of ankle pronation was not associated with injury during running. I have a lot of say about this topic (with mechanical explanations), but I'll save that for later. Here's my comment to the article: The most important aspect of running isn't trying to achieve a specific form, or loading the body in a certain way. It's allowing the body time to adapt and recover to the stimulus. For instance, impact forces aren't inherently bad, the body actually likes them (somewhat). It's about how much, how quickly, and how often relative to the current conditioning of your joints, muscles, and tendons. Similar to altitude acclimation - go too high too quickly and you'll die, but go slowly and you'll adapt fully and be okay (up to a point). If people focused more on very slowly gradually increasing their mileage and pace, with many rest days built in, there would be a reduction of injuries - regardless of whether you run in shoes/heelstrike or barefoot, overpronate, etc...

Coming Soon: A Personalized Caloric Expenditure Calculator

In the next week, I will be implementing some algorithms that will allow you to enter in certain metrics (heartrate, bodyweight) and performance in several (optional) endurance movements, and the end result will be a personalized equation that allows you to calculate how many calories you are burning for a given run / walk pace or heartrate.

Stay tuned!

Basis for the Steroid Detection Calculator

The basis for the "Is He on Steroids?" calculator is pretty straightforward and based on the research done in this paper. Basically, there is a pretty clear limit in the amount of fat free body mass someone can gain naturally, and that number is generally in related to the Fat Free Mass Index (FFMI), which is the ratio of lean body mass over height squared. If your FFMI is above 25.0, there is a pretty good chance your getting some "help". Obviously, someone could be under 25.0 and still be using steroids. This calculator isn't going to detect that.

Muscle Force - Length Relationship

When you activate your muscle, it will not produce a constant force over time, for a variety of reasons.

One key reason is that the maximal force that a muscle can produce is dependent on the muscle length.

Read more

Your bicep, for instance, can produce a maximal amount of force when the elbow is flexed at around 90 degrees. At more extended and flexed positions, maximum force is less.

 
from the wiki article: A generic example of a muscle force / length relationship

The active force component of muscle force generation is related to actin-myosin cross-bridges pulling past one-another. When they have pulled too far past each other (contracted position), their ability to produce tension reduces. Similarly  when the components are too far away from each other, limited elements are in contact and the amount of tension generated is reduced.

When you get to really stretched positions, there is an increasing "passive" contribution which is really connective tissue increasing in resistance to further strain.

The dependency of muscle force on muscle length has implications in performance and training, and will be recalled in many discussions.

The Point of This Blog

Is to aid in the conceptual understanding of how humans move and interact with their environment, with specific interest in performance / injury in sport and training. We'll discuss issues as basic as muscle force generation & body center of mass coordination, to complex tasks such as running mechanics and impact / concussion biomechanics.