Monday, March 3, 2014

Why Upright and Recumbent Cycling should require the same Muscle Activations

A recent study (kudos to @CyclingScience1) concluded that there was no difference in EMG (muscle activation) patterns of the leg muscles between upright and recumbent cycling.

This post is not reviewing that paper, but offering a short, general conceptual explanation for why that conclusion would be true.

Main Effect - Why They are Similar

Coordinated contraction of lower extremity muscles are required to overcome inertial resistance in a cyclical motion (crank arm about the crank axis), regardless of the bicycle type.

Take a look at this figure. On the left is an upright position, on the right is a recumbent position.

As you may have guessed, I have simply rotated the upright image to create the recumbent one. Thus, the relative orientation of the lower extremities (foot, shank, and thigh) are in the same positions relative to the crank arm in both case.

In both cases, the cyclist is trying to push down on the pedal to produce a propulsive force (green arrow) that should rotate the crank arm. In turn, the pedal is pushing back with an equal but opposite force (red arrow) on the leg.

This red force creates moments - which are related to muscle demands - on the ankle, knee, and hip joints. The magnitude of the moments for each joint are dependent on the red force magnitude, as well as each joint's moment arm (the perpendicular distance from the joint and the force direction).

When you rotate the entire lower extremity, the forces and moment arms rotate accordingly. Thus, there is no difference in joint moment demands between the two cases.

The most important parameter in changing biomechanical loading is the distance between the hip and crank axis (also pedal position on the foot). As you change this distance, knee, hip, and ankle angles all change, changing the moment arm distances in the figure above, and thus changing the absolute and relative demands of the lower extremity muscles crossing those joints.

2nd Order Effects - Why They Could Be Different

It would be silly to dismiss any differences between the two cycling styles, but important to note that they have smaller effects than the fundamental mechanics as shown above.

1) The rotation of the body does change the body positioning relative to gravity. Gravity is always acting of course, but segmental positions when going with gravity and working against (lifting up) change and could slightly change overall effort.

2) Torso position. Even though the lower extremity may be in the same position, the torso could be more flexed forward when upright cycling than recumbent cycling. This is an important aspect. In recumbent style, it will be easier for the cyclist to keep their pelvis in a neutral position vs the posterior tilt seen in many upright cyclists.

The more flexed torso in upright cycling also keeps the hip in more flexed positions overall. A more flexed hip can change where hip muscles are working on the Force - Length and Force-Velocity relationships, altering how hard or easy it is for each muscle to generate the required force. And chronic cycling will change the # of sarcomeres in series between these two styles!

Also, I believe that the more flexed hip position has some consequences on reducing femoral arterial blood flow (like in speedskating) which can make the upright style more painful and demanding.


Tuesday, February 4, 2014

Muscle Force - Velocity Relationship

Previously, I discussed how muscle force is dependent on the length of the muscle.

Now I want to talk briefly about maximum muscle force is dependent on muscle contraction velocity. As seen in the figure below, maximum muscle force is inversely proportional to the contraction velocity. When a muscle is contracted very rapidly, much less force can be generated than if the muscle is contracted isometrically (no muscle shortening).

Conversely, peak force increases above isometric max if the muscle is stretched while contracted for small stretching velocities before topping out.

Why does the maximum possible force decrease? It has to do with the actin-myosin cross-bridges which form the foundation of a contraction. The cross-bridges are increasingly unable to reattach to continue to aid in muscle contraction. The less cross-bridges, the less tension the muscle can produce.

Why does this matter? Biomechanical engineering of devices and analysis of movement is dependent on understanding this concept. You don't want to place yourself in a context that has you trying to generate forces to inefficiently.


Wednesday, January 15, 2014

Walking Mechanics: What is Center of Mass and How Do We Control It?

Understanding mechanics provides the foundation for strong understanding of human movements.

Why would you care about movement mechanics? Well perhaps you have some questions about joint loading, energy expenditure, technique...and grasping basic principles may lead you to better understanding to answer such questions (and also realize that usually it is not as black and white as some may say).

For instance, why do we expend energy when walking on flat ground?

Short Take Home Message:

The need to keep angular momentum low (no rotating) constrains our choices in how we move.

Longer Explanation (Just go to "summary" if it's too long-winded)

Center of Mass
Now, biomechanics can have many levels of analysis, but we must start with the most basic, which would take us back to physics and representing the entire body as one point mass, called your Center of Mass (CM). We'll stick to 2D.

Thursday, December 19, 2013

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?

Tuesday, July 16, 2013

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?

Wednesday, June 26, 2013

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...

Wednesday, June 19, 2013

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!