I recently witnessed another trainer working with a client on a flat bench leg curl machine. While performing the exercise, the trainer stood on one side, placed a padded board on the client's back and pressed down to keep the low back from creeping up. Is this a valid technique? I would think that, unless you’re sitting on the client’s back and applying pressure evenly (although, I still don’t think I would do this), an imbalance would occur on the side not getting equal pressure. What are your thoughts?
First of all, let me suggest that if this is happening, the weight is likely excessive! The reason an individual's low back pops up during a lying hamstring curl has to due with length/tension and strength. Muscles are generally considered to be at their strongest - hence prepared for peak force production/strength - when they are at resting length or just slightly greater than resting length. As the knees flex during this exercise, the hamstring muscles (bicep femoris-long head, semitendinosis, semimembranosis) gradually get shorter and shorter. As more and more sarcomeres are involved, the muscle(s) begins to lose the ability to concentrically contract and gets "weaker" the closer it gets to its end range. The body then may try and cheat, as commonly seen in what you've noted, by flexing at the hip to try and re-establish or add length in order to regain an optimal length/tension relationship. In Diagram A below, the sacrum is flat and the hamstrings are notably shorter, contrasted with Diagram B where the sacrum has "popped" up flexing the hips, hence lengthening the hamstrings. Other contributing factors that ultimately go hand-in-hand with chronic exposure to isolated machine training are short/facilitated rectus femoris as well as other hip flexors and ultimately a weak and dysfunctional deep abdominal wall - or CORE (TVA, internal oblique, diaphragm, pelvic floor, TLF).
Trying to correct this problem via pushing down on the sacrum will not help the problem. It's similar to using a weight belt where the support is artificial and only serves to exacerbate existing neuromuscular imbalance and dysfunction. This could easily accelerate the rate at which this client is exposed to injury. Unfortunately, the injury will most likely appear to be some random unrelated event.
Now, although your question is valid, and it is important to understand the mechanics, the truth of the matter is that there are better alternative methods for training the hamstrings/legs. What needs to be understood first is the primary function(s) of these muscles is not only concentric contractions (which is what machine training focuses primarily on). The hamstrings must also function isometrically and eccentrically during gait patterns and the other dynamic movements of life and sport! Taking an integrated approach to training is a great way to ensure that the body is being taught to operate with multiple synergies of muscle groups, in multiple planes of motion and with functionally applicable speeds of motion. This absolutely can not be accomplished via isolation of muscle groups (such as in chronic machine training).
It should be immediately apparent that if we use a machine that supports our body in any way (particularly seated, prone, supine or leaning), we are not activating the body's static stabilizer or postural system. This is a critical concept to grasp when one considers the harsh reality that stability must always precede force generation. As the old saying goes, "You can't fire a cannon from a canoe!"
The nervous system is organized in such a way as to optimize the selection of muscle synergies and not the selection of the individual muscles. The nervous system thinks in terms of movement patterns and not isolated muscle function. Isolation and training individual muscles over prolonged periods of time creates artificial sensory feedback, faulty sensory-motor integration and abnormal forces throughout the kinetic chain. This ultimately acts to confuse the nervous system as muscles are being asked to perform a function that the nervous system does not understand. In essence, the muscles are re-programmed to perform:
- …A different task
The hamstring performing knee flexion on a hamstring curl machine rather than decelerating knee extension, hip flexion and internal rotation of the tibia and femur.
- …At a different speed
Consistently at slow controlled speeds rather than progressing to functionally applicable speeds (power training).
- …With a different muscle action
Emphasizing concentric rather than eccentric muscle actions for the hamstrings or concentric rather than isometric (dynamic stabilization) for the hip abductors (outer thigh machines).
- …In a different plane of motion
Working in the frontal plane (inner thigh machines) rather than sagittal and/or transverse planes for the adductors.
In summary, just because a muscle is getting "bigger," doesn't mean it's getting "better."
- Chek, P. (2000). Movement That Matters C.H.E.K Institute
- Clark, M. "The Essentials of Integrated Training - Part 7" Online. Personal Training on the Net. Internet 2002.
- Hittner, N. "Leg Extension/Curl Machines" Online. Personal Training on the Net. Internet 2003.