Trunk muscle activity increases with unstable squat movements.
Anderson K, Behm DG.
School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John's, Newfoundland, A1C 5S7.
The objective of this study was to determine differences in electromyographic activity of the soleus -SOL-, vastus lateralis -VL-, biceps femoris –BF-, abdominal stabilizers -AS-, upper lumbar erector spinae -ULES-, and lumbo-sacral erector spinae -LSES- muscles while performing squats of varied stability and resistance. Stability was altered by doing the squat movement on a Smith machine, a free squat, and while standing on two balance discs. Fourteen male subjects performed the movements. Activities of the SOL, AS, ULES, and LSES were highest during the unstable squat and lowest with the Smith machine protocol p < 0.05. Increased EMG activity of these muscles may be attributed to their postural and stabilization role. Furthermore, EMG activity was higher during concentric contractions compared to eccentric contractions. Performing squats on unstable surfaces may permit a training adaptation of the trunk muscles responsible for supporting the spinal column as well as the muscles most responsible for maintaining posture
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So based on these findings the muscles that stabilize the body are less active. That means you'll have greater prime mover, in this case quad, force with lower stability. If the biceps femoris activity decreases while the quad activity increases you have increased shear force on the knee joint and greater compression of the patella on the femur.
Less activity of the trunk musculature means less spinal stabilization, yet the load lifted will be as much or greater than that which can be lifted in a traditional squat. That means greater load on spinal structures which over time means spinal destruction.
In other words, you'll gain nothing because the motor pattern learned in the Smith Machine is entirely different from the one learned in the free standing squat. There is zippo carry over and just more wear and tear on the body.
Before you read this and go thinking that you should squat on unstable surfaces, only do so if you plan to live your life in shoes with soles made from dyna-discs. Train in the environment in which you live. Feet stable, body stabilizing the load above the feet.
Bill
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