Muscle color, labeling, and material scheme

If you have a basic or full knee kit model, your kit comes with elastic muscle cords ("muscles"), color-coded by compartment and with removable labels. You can think of these elastic cords like approximate action lines for each muscle: as you simulate various knee motions, the cords shorten, lengthen, or stay the same length, enabling you to see whether—and if so, how—each muscle changes length during a particular motion.

If you have the basic knee model, you'll have four elastic muscle cords, each representing one of the quadriceps muscles.

Photo of basic model with all muscles

If you have a full knee model, you'll have cords for 23 muscles, representing all of the muscles between the superior and inferior cross-section plates, except articularis genu and fibularis tertius.

Photo of full model with all muscles

Color-coding by compartment

The human lower limb muscles are organized into fascial compartments: three in the thigh and four in the leg. The muscles in your knee kit are color-coded to match their corresponding compartment. The table below lists the color for each compartment.

Muscle compartment color coding
Thigh	Anterior:	Yellow
	Posterior:	Blue
	Medial:	Red
Leg	Lateral:	Black
	Medial:	White
	Superficial posterior:	Green
	Deep posterior:	Orange

You can also refer to your kit's Parts Plates: the background color for each muscle section corresponds to the compartment.

Photo of plate, caption explaining color backgrounds

There is one elastic muscle cord in your kit that bridges two compartments: the cord corresponding to the hamstring portion of the adductor magnus. This portion of the adductor magnus muscle is generally considered part of the posterior compartment of the thigh (with the hamstrings) whereas the rest of the adductor magnus is considered part of the medial compartment of the thigh (with the adductors). The cord has both blue and red components to reflect this dual designation.

Muscle name abbreviations

Your knee kit uses a unique 3-letter abbreviation (all capitalized) for each muscle where it is not possible to print the full name (e.g., on the clip-on labels, bone surface). You won't see these abbreviations elsewhere- these abbreviations were developed specifically for this kit as there is no standard set of abbreviations for the muscles of the human body (presumably to avoid ambiguity in the practice of medicine). The table below lists the 3-letter abbreviation for each muscle in your kit. Also included in this list is the iliotibial tract/band, a structure that is part tendon, part ligament, and part muscle.

Muscle abbreviations
ALO	Adductor longus
AMA	Adductor magnus
BFE	Biceps femoris
EDL	Extensor digitorum longus
EHL	Extensor hallucis longus
FDL	Flexor digitorum longus
FHL	Flexor hallucis longus
FIL	Fibularis longus
GAL	Gastrocnemius (lateral head)
GAM	Gastrocnemius (medial head)
GRA	Gracilis
ITT	Iliotibial tract
PLA	Plantaris
POP	Popliteus
REF	Rectus femoris
SAR	Sartorius
SEM	Semimembranosus
SET	Semitendinosus
SOL	Soleus
TIA	Tibialis anterior
TIP	Tibialis posterior
VAI	Vastus intermedius
VAL	Vastus lateralis
VAM	Vastus medialis

You can also refer to your kit's Parts Plates: the "docking site" for each muscle on the Part Plate includes the muscle's full name and abbreviation.

Photo of plate, perhaps zoomed in to show name of muscle and its abbreviation

Attachment site and muscle labeling

Every elastic muscle cord in your kit has two ends: one that attaches to the muscle's origin and one that attaches to the muscle's insertion. Since the designation of an "origin" vs. an "insertion" is somewhat arbitrary, this guide simply refers to them both as the muscle's "attachment sites." To help you know where to attach each muscle cord, every attachment site in the kit is labeled. The attachment sites are labeled according to the following scheme:

[3-letter muscle abbreviation]-[Unique letter, starting at "A"]

For example, the rectus femoris attachment sites are: REF-A and REF-B. So to attach the rectus femoris, you would attach one end at the site labeled "REF-A" and one end at the site labeled "REF-B."

Photo of rectus femoris with attachment site labels visible and circled/indicated with an arrow. No need to circle/indicate the clip-on label yet.

Every elastic muscle cord also has a rectangular label that clips onto (or off of) the cord. This clip-on muscle label has inscribed letters that tell you both the muscle name (abbreviated) plus the two sites where you attach that muscle, according to the following scheme:

[Attachment site #1]-[3-letter muscle abbreviation]-[Attachment site #2]

For example, the muscle label for the rectus femoris has the following inscription: A-REF-B. This tells you that one end attaches to "REF-A" and the other end attaches to "REF-B." These muscle labels should be oriented so that the inscribed letters point toward their corresponding attachment site.

Photo of rectus femoris with clip-on label circled/indicated. Add something to note the orientation of the label toward each corresponding end.

This same attachment labeling scheme is repeated at the "docking site" for each muscle on the Parts Plates to make it as easy as possible to detach and reattach the muscles: each end has the corresponding attachment site while the middle has the same inscription found on the muscle's label.

Photo of Parts Plate A with the attachment sites and middle all indicated.

Some muscles of the lower limb have broad origins and/or insertions (e.g., adductor magnus, the vasti muscles), and thus cover a large area of bone. Muscles like these have more than one muscle cord in your kit to represent the wide range of action lines contained within that single muscle. This is why the full knee kit model has 35 elastic cords representing 23 muscles. For muscles with more than two attachment sites (i.e., more than one elastic cord), the attachment site labels proceed in alphabetical sequence. For example, the labels and corresponding attachment sites for the adductor magnus muscle cords are:

A-AMA-B (attaches from AMA-A to AMA-B)
C-AMA-D (attaches from AMA-C to AMA-D)
E-AMA-F (attaches from AMA-E to AMA-F)

Elastic+Inelastic muscle cords

While both muscles and tendons can shorten and lengthen, muscles generally change length to a far greater extent than tendons. Thus, muscles are generally more "stretchy" than tendons. Most, if not all, of the muscles in the lower limb have a tendinous component. However, for most muscles, the tendon is contained within the muscle itself or forms part of the external surface of the muscle so that muscle fibers can attach to the tendon along its length, resulting in some degree of pennation. That is, if you were to cut the muscle in cross-section, you would see both muscle and tendon in the same cross-section slice. For this reason, most muscles in your kit are represented by a single elastic cord, without any stiffer tendinous component.

However, there are some muscles that have a tendinous component that continues beyond the muscular portion of the muscle (e.g., semimembranosus, semitendinosus, plantaris). That is, if you were to cut the muscle in cross-section, in some cross-section slices you would see only tendon. If you have a full knee model, the cords representing these muscles have an elastic cord (representing the muscle+tendon part) coupled to a thicker inelastic cord (representing the tendon only part).