Simulating various patterns of muscle co-contraction to move the knee

Now that the femur of your knee kit is fixed in place, you're ready to start simulating various patterns of muscle contraction to see these affect the knee joint and the motion of the tibia.

An extensor driving motion on its own

Attaching a muscle cord to a clip

You'll start by simulating the contraction of a single muscle on its own to see what happens when muscles don't co-contract (also called co-activate) with other muscles, using the rectus femoris muscle as an example. This muscle is one of the four quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius), which are knee extensor muscles (they extend the knee). To attach the rectus femoris manual muscle cord, follow the steps shown in the video or listed out below.

Video to add: Attaching a manual muscle cord from CS plate to REF clip

Thread the non-looped end of a manual muscle cord through the attachment site for rectus femoris in the cross section plate (labeled REF-A).
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Pull the cord through enough so that you can insert the non-looped end of the cord through the clip on the rectus femoris portion of the patellar tendon (labeled REF-B). Be sure to insert the cord through the labeled side first (i.e., you should see "REF-B" as you're inserting the cord).
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Pull the end knot through the clip and slide the cord into the slot on the top of the clip.
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Pull the cord back through the clip in the opposite direction, keeping the cord in the slot and allowing the knot to get become hooked inside the clip.
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Simulating muscle contraction

To simulate the contraction and shortening of the rectus femoris, pull on the looped end of the cord. Simulate shortening of the rectus femoris a few times. Get a feel for the range of flexion and extension you can achieve and how well you can control the motion.

Image to add: two images, one with cord tight and one with cord pulled and knee extended

Whenever you're pulling manual muscle cords through the cross section plate, pull directly backward (not upward). If you pull upward, the cord will pull up on the cross section plate, possibly lifting it out of the bracket.

Image to add: hand pulling on cord, arrows indicating to pull backward not upward

What's the problem with using a single muscle to move a bone? Write your answer on page 1 of the activity worksheet.

A flexor and extensor driving motion

In the previous simulation, your rectus femoris acted as an agonist (a muscle whose contraction moves a part of the body). However, you should have noticed a problem with controlling the motion using just one muscle. What you need to fix this problem is a second muscle that acts in opposition to the agonist: an antagonist; together, these form an agonist-antagonist pair. When an agonist and antagonist muscle co-contract, they either both stay the same length (isometric contraction) or one muscle shortens while the other one lengthens (concentric contraction and eccentric contraction, respectively).

The primary muscles acting in opposition to the quadriceps (extensors) are the hamstrings: the semimembranosus, semitendinosus, and the biceps femoris. The hamstring muscles are knee flexors (they flex the knee). Attach a second manual muscle cord to your knee kit representing the semimembranosus, using the instructions below.

Attaching a muscle cord to a bone

To attach a manual muscle cord to a bone, follow the steps shown in the video or listed out below.

Video to add: Attaching a manual muscle cord from the CS plate to a bone (e.g., semimembranosus)

Thread the non-looped end of a manual muscle cord through the attachment site for semimembranosus in the cross section plate (labeled SEM-A) and pull the cord through, just as you did for the rectus femoris.
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Use the forceps to push the knot (again at the non-looped end of the cord) into the semimembranosus attachment hole on the tibia (labeled SEM-B). The knot should be pushed fully below the surface of the bone, into the hole.
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Pull the cord into the slot of the hole until the knot is caught by the hook inside the hole and the cord is secured (you may feel the knot "click" into place). You might need to hold the knot inside the hole with the forceps while pulling on the cord to keep it in until it locks into place.
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Leave the "tail" after the knot sticking out of the hole to make it easier to remove the cord later.
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Whenever you're attaching a muscle to your knee kit, be sure that the two attachment sites have the same 3-letter muscle code, indicating they belong to the same muscle. For example, when you attached the rectus femoris from/through the cross section plate to the patellar tendon, both of these sites had the 3-letter code "REF," indicating that both are rectus femoris attachment sites. The sites will have different letters after the three letters (e.g., REF-A, REF-B, etc.); these letters indicate different sites for the same muscle. If you attach a muscle to sites with two different 3-letter codes, you're creating a muscle that doesn't exist. Is that fun? Yes, of course. But it's not within the scope of this activity.

Simulating muscle contraction

Now that you have both manual muscle cords attached, simulate co-contraction of the rectus femoris and semimembranosus to rotate the knee through flexion and extension as shown in the video below.

Video of co-contraction of REF and SEM

Based on your simulation, why do you think at least two muscles are needed to drive smooth, controlled motion? And why is muscle coordination (regulating the timing and force of muscles relative to other muscles) important when co-contracting multiple muscles (i.e., why can't you just contract all muscles with equal tension at the same time)? Write your answers on page 1 of the activity worksheet; check the hint below if you need some help.

HINT

In considering the importance of muscle coordination, pay attention to the relative timing and force you're using with your hands as you pull the two muscle cords to simulate flexion and extension.

Detaching a muscle cord

For the next simulation, you'll need to detach the rectus femoris and semimembranosus from your knee. To detach a muscle cord, follow the steps shown in the video or listed out below.

Video to add: Detaching the rectus femoris manual muscle cord from the tendon clip and detaching the semimembranosus muscle from the bone

To detach the rectus femoris from its attachment to the tendon clip, pull the cord up and out of the slot. The knot will then be free to pull through the clip and you can remove the muscle cord out through the cross section plate.
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To detach the semimembranosus from its bony attachment to the tibia, pull the cord so that it slides out of the slot portion of the hole. It can be helpful to use the forceps for this.
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Pull the "tail" of the cord sticking out from the hole until the knot and cord pull out of the hole and then remove the muscle cord out through the cross section plate.
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Two extensors driving motion

A co-contracting muscle pair doesn't have to consist of an extensor and flexor (i.e., muscles on opposite sides of a joint). Motion and stability at a joint can benefit from the co-contraction of multiple muscles on the same "side" of a joint. To figure out why, use your knee kit to simulate the following patterns of muscle contraction, using two additional quadriceps muscles as an example:

Simulate the vastus lateralis contracting on its own to extend the knee. Attach the vastus lateralis by threading it through any of its attachment sites on the cross section plate (labeled VAL-A, VAL-B, or VAL-C) and clipping it into the vastus lateralis portion of the patellar tendon (labeled VAL-D).
Simulate the vastus lateralis and vastus medialis co-contracting to extend the knee. Add the vastus medialis by threading it through its attachment site on the cross section plate (labeled VAM-A) and clipping it into the vastus medialis portion of the patellar tendon (labeled VAM-D).

Note, that in a real knee, the muscles and tendinous sheath surrounding the patella (not all of which is represented in your knee kit) all have some passive tension that holds the patella in place as long as there aren't any high forces acting on it; the patella wouldn't just flop forward or over to the side as it does in your knee kit. What you want to pay attention to is what happens to the knee (including the patella) when you are actively pulling the manual muscle cords and their resulting lines of action; that is properly represented in your model.

Based on your simulations, what's the problem with only contracting the vastus lateralis? Why is it beneficial (for knee function and stability) to co-contract the vastus lateralis and medialis and how could this relate to patellar tracking disorders (the deviation of the patella from its proper groove during knee motion)? Do you think this pattern of two muscles on the same "side" of a joint co-contracting is a common one for joints, beyond just the knee joint? If yes, why? If no, why not? Write your answers on page 2 of the activity worksheet; check the hint below if you need some help.

HINT

The ligaments that hold joints together can be damaged by excessive torques that would move the joint in abnormal ways. Additionally, some joints allow for a wide range of motions/rotations (e.g., the shoulder), which means that the muscles can't rely as much on ligaments to guide motion along a particular axis.

Two flexors driving motion

In the previous simulation you observed the effect of contracting one versus two knee extensors. Next, you'll simulate the effect of contracting one versus two knee flexors.

Use your knee kit to simulate the following patterns of muscle contraction:

Simulate the semimembranosus or semitendinosus contracting on its own (whichever you'd like). Attach the muscle by threading it through its attachment site on the cross section plate (labeled SEM-A or SET-A) and hooking it into its attachment on the tibia (labeled SEM-B or SET-A).
Simulate the semimembranosus/semitendinosus and biceps femoris co-contracting. Add the biceps femoris by threading it through its attachment site on the cross section plate (labeled BFE-A) and clipping it into its attachment on the fibula (labeled BFE-D).

Based on your simulations, why might it be beneficial (for knee function and stability) to co-contract the semimembranosus/semitendinosus and biceps femoris? Write your answer on page 3 of the activity worksheet; check the hint below if you need some help.

HINT

When simulating both muscles co-contracting, pull both muscle cords tight to fully flex the knee and then alternate pulling tight on one muscle while releasing the other slightly.

Based on your dual-extensor/flexor simulations, do you think this pattern of two muscles on the same "side" of a joint co-contracting is a common one for joints (beyond just the knee joint)? If yes, why? If no, why not? Write your answer on page 3 of the activity worksheet; check the hint below if you need some help.

HINT

Agonist-antagonist muscles stabilizing a joint

In your "Flexor and extensor driving motion" simulation, you saw how an agonist-antagonist pair of muscles can co-contract to generate opposing forces on a bone at a joint and drive smooth, controlled motion. But these opposing forces also have the effect of stiffening a joint. The stiffness of a joint is a measure of how much force it takes to produce motion at that joint; the greater the stiffness, the harder it is to produce motion at a joint. Imagine an old, rusted hinge joint: the rust and corrosion of the joint create additional friction in the joint, making it harder to open and close the hinge.

Older people may complain about "stiff joints"; this results from within the joint itself—not a good thing! However, stiffness at joints that results from active agonist-antagonist muscle co-contraction is actually a good thing and necessary for proper joint function. This dynamic stiffness (i.e., it can be turned on and off) helps joints resist external forces that could otherwise cause injury and results in the smooth, helps produce the controlled motion that you simulated previously, and—at a high enough level of stiffness—can immobilize a joint to prevent motion. In this way, agonist-antagonist muscle pairs stabilize joints. For this last set of simulations, you'll test how different combinations of co-contracting muscles can stiffen or even immobilize the knee joint.

Based on your previous simulations, which two muscles would you co-contract to stiffen or prevent flexion-extension rotation at the knee joint? Once you have an idea, test it out by simulating the co-contraction of these two muscles with your knee kit: while contracting (pulling) both muscles with one hand, push with your other hand on the tibia to see if it is prevented from flexing/extending. If not, try a different muscle pair. Once you find a pair that works, write the muscle names on page 4 of your worksheet.

Although you've managed to stiffen or prevent flexion-extension rotations, this isn't the only way in which knees can move. The tibia can also rotate about its long-axis.

Can the muscles you chose to stiffen/prevent flexion-extension rotation also stiffen/prevent tibial long-axis rotation? If not, add additional muscles to stiffen both of these rotations at the knee joint. Be sure to test it out with your kit. Once you find a combination that works, write the muscle names on page 4 of your worksheet.

In addition to rotating about its long axis, recall from the How many ways can you move your knee? activity that the tibia can also translate anteriorly and posteriorly. And recall from the What are the functions of the knee ligaments? activity that both excess anterior-posterior translations and excess long-axis rotations can damage the knee ligaments. Use this information to answer the following questions relating muscle strength and coordination to knee ligament injuries.

Can co-contraction of muscles help to prevent ACL and MCL injuries? If yes, why? If no, why not? If you need some help, check the hint below

HINT

Recall the excess/abnormal motions of the knee that can damage the ACL and MCL. Use your knee kit to test whether co-contracting the muscles you've attached to stabilize the joint, can reduce or prevent those motions.

What stabilizes the knee joint: ligaments, muscles, or both? If both, what do you think the relative contribution of each should be in a healthy knee?

HINT

Surprising as it may seem, there are individuals who rupture their ACL and return not just to normal life but performing sports without reconstruction surgery (Thoma et al., 2020). These individuals are referred to as "copers" because they are able to cope without an ACL. This should tell you something about the potential roles of ligaments versus muscles in stabilizing the knee.