What’s Wrong with Impact?

Have you ever heard certain exercises referred to as being “high impact”?

There often seems to be a negative connotation to something being a high impact activity. Running and jumping are considered high impact activities, while cycling and water aerobics are low impact.

It seems that every time I jump down from a log while out on a volunteer trail maintenance crew there is somebody on hand to tell me that I’m damaging my knees with all that impact and will regret it when I’m older.

Which is what moves me to clarify what impact is, and if there’s really anything wrong with it.

Impact is what happens when our body contacts another object.

If you swing a tennis racquet there is impact that travels from the ball into your hand and arm. If you take a step there is impact that travels from the planet into your foot and leg. The impact doesn’t stop at your arm or leg. It spreads through the whole body.

Taking a step has a lower impact, and running has a higher impact. This is because there is less momentum or acceleration in the step than in the running stride.

In the tennis stroke the ball has momentum and the arm is being accelerated by coordinated transmission of force from the ground, through the muscles, and into the hand.

When you run, you land on one leg with the entire weight of your body, but the load isn’t just the weight of your body, because your body has momentum behind it. The weight multiplied by the momentum makes the force something like 8 times what your body weighs. (I don’t remember precisely, I’m going from memory.)

The first thing your muscles have to do when your foot hits the ground is to decelerate, which means that your muscles are contracting while lengthening. This is a really efficient way to do resistance training because it’s so tiring so quickly, but when you’re doing 100 reps (100 strides in a run doesn’t seem like a whole lot…I wonder how many strides there are per leg in a mile?) it will probably overload the muscle pretty quickly and require a longer recovery time.

Since we’re talking about running it’s also interesting to note that cardio aerobic capacity tends to stay with us for a longer period of time than muscle and tendon strength does. We decondition quicker in our muscles than our cardio parts.

So, the very efficient fatigue of trying to decelerate our bodies and the relative tenacity of aerobic conditioning, possibly coupled with a bit of starting and stopping in our attempts to take up running are not a recipe for long term success.

Now…getting back to impact.

If we can hurt ourselves by running and jumping then maybe we just shouldn’t do these things, right?

Nope. Definitely not right. And the reason isn’t because we need cardio. (I’m not saying we do or don’t, I’m just saying that’s not the reason.)

The reason is because we need impact.

You will have heard of osteoporosis. It’s a bone wasting disease caused both by dietary imbalance and a lack of stress on bones. Bones are continually remodeling along lines of stress. When a load or force travels through a bone it stimulates the bone building cells called osteocytes to make bone. When there is either a lack of stress or a lack of building material the bones don’t get built.

Types of stress that build bone; load bearing exercise, and impact. A load bearing exercise could be walking, but just as walking is a low impact exercise, it’s also a relatively light load. Squatting with a lot of weight is more load and thus better for your bone mineral density, and jumping (landing specifically) is really great for your bones.

So, now we have a conundrum, don’t we?

Jumping is high impact (frowny face). Jumping makes strong bones (smiley face).

It’s not really a conundrum, it just looks like one until we understand the path forward.

If you picture a body landing after it’s jumped down off a log, you’ll see a lot of bending joints…and with those flexing joints there will be a lot of muscles being forcibly lengthened. The calf muscles will lengthen, the glutes (butt muscles) will lengthen, the quads will lengthen. But can they handle it? That’s the thrilling question on everyone’s mind. Will they have to stop lengthening in order to save themselves from rupture, thus stiffening and taking away their magnificent shock absorbing capacity?

What’s under the skin, the strength or your muscles, ligaments and tendons (and of course bones) is changeable. It’s not hereditary, it’s activity based, and it’s specific.

Because of the decelerating, lengthening contractions and the extra momentum behind the mass, the accumulated fatigue of running and jumping can lead easily to overload injuries. It’s easy to do more than we think we’ve done.

It’s also easy to ignore the protective stiffening of muscles (or maybe they start out stiff) that transmits force into joints.

The path forward then, is to gradually work on your body’s ability to lengthen muscles under load. For instance, you could jump as high as you can and practice soft landing 10 times and then stop. Or if that’s not where you’re starting at you can work on slowly lowering yourself into a chair and then standing up again. There are a lot of ways to work on your body’s ability to control force. (I can help you with that, by the way.)

In summation:

• Impact can land in your joints and often does when the soft tissues aren’t strong enough.
• Impact is essential for healthy bones. (We spent millions of years evolving in the same gravitational field…gravity is not the problem.)
• Decelerating a mass as heavy as a human body requires a lot of strength.
• When you have a lot of strength in your soft tissues, you can control where the force lands and impact is not a problem.
• You can develop all the strength you need. Do it gradually and persistently. (And let me help, it’s fun!)

I hope that was interesting and informative. Feel free to reply to this email with your thoughts.

Cheers,
Roselie