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.