That one. The statement against it isn't irrational because the idea itself was irrational.
The fact that the foot is overdesigned and that you can eliminate one part of it does not negate the overall design of the foot.
It is not overdesigned. Most of us (and not all of us) simply retain the vestiges a feature that our ancestors needed. It was perfectly "designed" for them, irrelevant for us, which is why it is on the way out in our line. It is at a point where it has much more potential disadvantage (injuring something you don't need at all) than any advantage.
The remarkably short and slender plantaris muscle with its long slender tendon serves
Interesting you have to go to a creation site to find such "research." I like to read from those neck-deep in the subject and not on a site dedicated to one side of the debate. In fact, here's a short discussion on a podiatry board with one great paragraph in response to a creationist with an agenda:
I have noted no change in biomechanical function or proprioceptive ability of the numerous patients that I have seen over the past two decades who have had either plantaris rupture or plantaris tendon excision for tendon repair. Your suggestion that the plantaris is the "brain of the triceps surae" is not supported by any clinical evidence or research that I know of, your suggestion does not make good physiological sense and your suggestion will probably be met with resistance from any intelligent biomechanist, physiologist, orthopedic surgeon or podiatrist.The guy is a podiatrist and adjunct associate professor of applied biomechanics
All complex systems have a minimum number of vital components, without which they will fail to provide functionality. What is so troubling about that fact to an evolutionist besides the fact that it forces them to provide a naturalistic account for the development of such systems in a detailed manner?
While I believe vestigial organs do exist, I find it surprising the evolutionists continue to exaggerate them despite a century of embarassments. The continual pattern has been to assert that a given organ is "vestigial" in a state of ignorance. Then as more information is obtained from the progress of science, it becomes clear the organ does have a function after all and the evolutionary claim is refuted.
There are a few cases of degeneration that seem safe enough (such as blind troglodytes and degenerate wings on non-flying island insect and bird populations), but the overall track record has been miserable. Evolutionists often falsely claim that creationists do not make predictions, so let me make one here: the sensory function of the plantaris will turn out to be vindicated in the end.
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Wow, you do get into some esoteric discussions, don't you! I haven't thought about the plantaris muscle since mom tore her Achilles tendon before you were born. If you've never heard the story, she was playing in the finals of the Detroit Public Parks tennis tourney when she went down when her tendon snapped. It was on 4th of July weekend and only residents were available in the emergency room at Providence Hospital when we arrived. The young kid diagnosed it as a torn plantaris and sent us home claiming it was an expendable muscle and wouldn't cause any long term problems. We got home and she had to walk upstairs to our apartment, and of course couldn't do it very effectively. I put my finger where her Achilles tendon was supposed to be and new we had a problem. We got back in the following Monday to see an orthopedic surgeon and he sent her in the next day for surgery. Of course she was in a cast for a lengthy period thereafter - and that's when you came into the picture!! You were born the following summer.
Ok, back to plantaris. First of all it's not even a given. Not everyone even has one, and it's small size doesn't allow it to accomplish much mechanically - which seems to be part of what you were discussing with someone (maybe when we next talk you can fill me in about your interest in this whole discussion). If you looked it up in an anatomy reference you might even have trouble finding much written about it. In fact many of my texts skip right over it.
The name, which includes "plantar" refers to its possible mechanical role in plantar flexion, i.e., depressing the foot as in flexing the ankle joint when you jump or push down on an accelerator peddle. It's considered an extrinsic muscle in that the belly of the muscle is distant from the structure it supposedly moves, assuming that plantar flexion of the foot is what its purpose is - which may be where your discussion is headed. Can it play a role at the knee joint since it's a biarticular muscle, crossing both the knee and ankle joints. Biarticular muscles logically can influence both of the joints crossed but are usually leveraged to affect one more than the other in normal function. A good example would be the gastrocnemius muscle which is also biarticular and is situated very close to plantaris, but with much greater force capability because of its much larger cross section of contractile tissue. It shares a distal attachment on the calcaneous (heel bone) and thus mechanically has the same leverage characteristics as plantaris - but again, the gastroc is much more powerful!
So, what are their roles at the knee joint? First of all, you have to recognize that when a muscle shortens and applies its tension force it applies the same force to BOTH its proximal and distal attachments. Has to, according to the laws of physics (think Newton's 3rd law of action and reaction). Which attachment provides the greater movement depends on the inertial resistance at the respective attachments. This subject was actually the focus of my doctoral dissertation where I studied what are considered "reversible joint actions". When the hands are fixed to a chinning bar the elbow flexor torque produces movement of the body up to the bar, but if you strap someone's feet to the floor and pull on a bar cabled to an equivalent to body weight load in a weight stack (something like a so-called lat machine in weight training facilities, for example), the elbow flexors (such as biceps brachii and brachialis) produce the same muscular activity (recorded on an electromyogram) even though the so called "origin" and "insertion" attachments are reversed. That's why I abhor those terms as used in anatomy texts and always had my students use the more descriptive anatomical terms of "proximal" and "distal" attachments which bear no prejudice about the impending movement caused.
Muscles simply contract and apply their same tension force to both their attachments and the resulting movement depends on the resistance to that tension force at each respective attachment. Theoretically one attachment may move a lot while the other moves little (such as when an unresisting hand moves toward your mouth when you feed yourself - which anatomically would be considered a regular or normal movement), or, as described above, you can have a reversal of the normal joint actions for a given torque about a joint (such as in the chin up example).
Actually, most of the locomotor functions (such as walking, running, etc.) are reversals of movement from what is normally described in anatomy texts as movements from the anatomic position - which is how students studying musculoskeletal function usually are taught.
Back to the knee joint. When one stands relaxed, the knee joint is mostly extended but is normally just slightly flexed. That is, you don't force the knee into a hyperextended, locked position. To maintain normal relaxed standing posture there has to be a fine interplay between the inertial loads on the joints, range of tightening in joint capsule and ligaments, shapes of opposing bone structures, and closely controlled fine bursts of muscle activity under the feedback circuits of the neural sensors embedded in the joints and muscles - so called proprioception.
A simple experiment demonstrates how reflexes are involved in normal standing posture. I used to call an unsuspecting student to the front of the room and have them turn around and face the class. Then while talking to them and assuring them to relax while I lined up directly behind them, I would suddenly take both of my knees and simultaneously bump the back of their knee cubit area so that a sharp, quick stretch was applied to the upper gastrocnemius region just below the knee joint. Invariably the unsuspecting student would experience (and demonstrate to the class) two classic reflex responses: first the sharp, quick stretch induced relaxation in the postural control muscles posterior to the knee joint and the person starts to collapse and flex the knee joint; immediately following is a slower, stretch reflex induced in the quadricep muscles that posturally maintain the knee joint in an extended position. The person "catches" himself and the quads fire to again extend the knee joint.
This little experiment demonstrates that the gastrocnemius, and perhaps the attendant plantaris, are involved in a feedback loop with slight controlling postural muscle activity of knee flexion torque - even though in normal standing the knee joint is in a primary position of extension. In other words, when this knee flexion torque is interrupted, the knee joint begins to collapse under gravitational influence in flexion until the resulting stretch reflex is activated in the knee joint extensors of the quadriceps femorus.
This so called stretch or myotatic reflex is similar to what you experience when a doctor tests your patellar tendon reflex when he taps the patellar tendon just below your knee and your leg involuntarily extends in a kicking motion. The patellar tendon is the distal attachment of the quadriceps femorus group of muscles.
I doubt many studies of plantaris have been conducted because of its apparent lack of importance. I did see one mention that in comparative anatomy studies, the plantaris is often quite a bit larger and more important in plantar flexion of some animals as its distal tendon attaches below the foot with more leverage, rather than on the calcaneous as in the human. Perhaps this type comparison is what you are looking at if some evolutionist is arguing some comparison between animals and humans.
I'll look forward to talking to you about it tomorrow.