I have a client who insists on doing leg extension/curl exercises with his legs parallel, then toes turned out, then toes turned in! I feel this is going to put a tremendous amount of stress on his knees (especially toes turned in). Any thoughts on this?
I would suggest explaining it to him from an anatomical point of view:
The knee joint is a modified hinge joint, which basically means it has one degree of freedom (i.e., sagittal plane extension and flexion). REMEMBER: The machine will not dictate which muscles are used! Rather, muscle fiber alignment and the direction of resistance will. The quadriceps and hamstrings have a fiber alignment set up to concentrically perform extension and flexion. Externally and/or internally rotating the knee (note: ultimately, this rotation is coming from the hip) will do nothing by way of altering the recruitment of the muscles to emphasize either the lateral or medial heads. What this will do is place the knee joint in an extremely compromised situation. This is due to the fact that when the knee is rotated, either internally or externally, the direction of resistance remains the same. So what one winds up with is the knee joint wanting to extend/flex at an angle approaching the frontal plane, and the resistance remains constant in the sagittal plane. This results in abnormal forces ripping throughout the knee joint, increasing the likelihood of injury and pain.
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 quads/hamstrings/legs. What needs to be understood first is that these muscles primary function(s) is not concentric contractions. The quads and hams must also function isometrically, and most important of all, eccentrically during gait patterns and the movements of life.
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 sensorimotor 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.
Taking an integrated approach to training is a great way to ensure the body is being taught to operate with multiple synergies of muscle groups, in multiple planes of motion, with functionally applicable speeds of motion. This absolutely can not be accomplished via isolation of muscle groups (such as in chronic machine training).