Thursday, May 13, 2010

Some Thoughts on Knee Loading Mechanics

When hiking and doing other physical activities, our musculoskeletal system undergoes increased loading - in the muscles, tendons, cartilage, etc... Understanding how different postures / mechanics change loading may be beneficial in terms of hiking - how do you walk downhill to redistribute load in an ideal way for you?

For instance, many people end up with some knee problems from hiking downhill. There are several specific problems that could arise in the knee that are affected by different loadings, but an important one is the moment / torque --> muscular demand of the muscles crossing the knee joint. To illustrate how the magnitudes of these loads could change, we'll use a simple example of a weighted squat.

Here are three different positions where one could hold a barbell while performing a squat. A fundamental requirement of whole body movement is the balance constraint. To not lose your balance, you must keep your center of mass (CM) in a horizontal position in which you can maintain. Your CM accounts for the weighted average position of all your body segments (and the weight and position of the barbell, in this case). To remain stable, the force acting on your feet must be vertical and go directly through the CM (dashed lines in figure above).

If the force acts vertically but is in front of your CM, you will start to tip backwards. This is because you are generating a moment about your CM. Same effect if the force is behind the CM. And the CM must be aligned horizontally to be withing the horizontal range of your feet, or else you won't be able to keep the force aligned!

Given these requirements, if one changes where the barbell is positioned, this will change the overall CM location. In order to ensure the CM sits at the middle of the feet, one must change the kinematics of their segment / joint orientations, as seen in the figure above.

The consequence of this can also be seen in the figure - look at how the distance between the dashed line and middle of the knee decreases from left to right in the figures. The moment arm about the knee joint is decreasing. Given a force "F" that includes the barbell weight + bodyweight, and a moment arm "x", the moment / torque that is needed to be generated about the knee is T = F * x. As x decreases, the moment needed will decrease.

What does this imply? Well the smaller the moment demand, the less force the quadriceps muscles will have to generate. Less force will mean less contact force between the femur (thigh), patella (knee cap), and tibia (shank). This may be good, depending on your current state and injury history. On the other hand, the decrease in moment about the knee means an increase in moment about the hip.

Now there is a lot more complexity than this, but the take home message is that you can redistribute muscle and joint loading by orientation of your body segments, whether in squatting or hiking or anything else. Changes may be beneficial if you are suffering from musculoskeletal problems.

Tuesday, May 4, 2010

Salt Stains and Tips on Electrolyte Replenishment

Here's a decent article on electrolyte loss from sweating and replacement. Of course, this can be extremely important on strenuous hikes on hot days. Especially because you'll be a little more stranded in the wilderness than in a race!