Chapter 3 – Tendons fascia and muscle groups for motion reads

Chapter 3: Tendons, Fascia & Muscle Groups for Motion Reads

Created by Sarah Choi (prompt writer using ChatGPT)

Tendons, Fascia & Muscle Groups for Motion Reads

Why Motion‑Focused Anatomy Matters for Creature Artists

Most anatomy resources talk about individual muscles, but as a creature concept artist, you rarely need to name every fiber. What you do need is a practical understanding of how muscle groups, tendons, and fascia create motion—and how those motions show up as visible surface landmarks.

When a creature lunges, braces, lands, or recoils, the viewer reads that movement through:

How big muscle groups bulge, stretch, or compress.
How tendons create taut, straight lines and bony accents.
How fascia and skin slide and fold over the underlying structure.

Whether you’re on the concepting side (expressive sketches, cinematic keyframes) or the production side (clear turnarounds, pose sheets, callouts for animation), understanding these systems helps you:

Draw more believable and dynamic poses.
Place surface detail (veins, plates, wrinkles) in ways that support motion.
Communicate clearly with modelers and animators about where movement originates.

This article focuses on comparative anatomy & construction—not memorizing human anatomy charts, but learning how to see and use tendons, fascia, and muscle groups across creatures.

1. The Motion Engine: Bones, Joints, Muscles, Tendons, Fascia

1.1 The Basic Chain

Every visible movement in a creature can be thought of as a simple chain:

Bones form levers → Joints define rotation points → Muscles create force → Tendons transmit that force → Fascia/Skin show the result on the surface.

Bones: Hard structures that won’t deform much; they give you fixed lengths and angles.
Joints: Hinge or ball‑and‑socket areas where motion happens.
Muscles: Contractile masses that shorten to pull bones closer.
Tendons: Tough, fibrous cords or sheets that attach muscle to bone—often visible as taut, rope‑like or flat bands.
Fascia: Connective tissue layers that wrap muscles, fill gaps, and help transmit tension; visually, they show up as broad planes, subtle depressions, and sliding skin.

As a creature artist, your job isn’t to diagram every tendon—it’s to understand where the body is soft and where it is rigid, and how those areas shift during movement.

1.2 Muscle Groups vs. Individual Muscles

Instead of memorizing every muscle name, group them by function:

Shoulder/hip complexes: Move limbs at their roots.
Upper limb groups: Power pushes, pulls, and swings.
Lower limb groups: Handle stance, jumps, and landings.
Spinal/axial muscles: Bend and twist the torso, tail, and neck.

These muscle groups create predictable mass shapes and movement rhythms you can reuse across vertebrate and hybrid creatures.

2. Tendons: The Lines of Tension and Precision

2.1 What Tendons Look Like

Tendons are dense, fibrous connectors that link muscles to bones (or to other structures). On the surface, they often appear as:

Taut cords near joints (e.g., Achilles tendon at the back of the ankle).
Flat, strap‑like bands across the top of hands, feet, and certain joints.
Hard, smooth transitions from thicker muscle bellies to skinny anchor points.

They do not bulge like muscles; they tend to look straighter, shinier, and less fleshy.

2.2 Where Tendons Stand Out

In many animals (and creature designs), tendons are most visible:

Around distal limbs (wrists, ankles, backs of hands and feet).
At major leverage points: behind the heel, front of the knee, back of the knee.
On hands, claws, and talons, where extensor tendons create planar surfaces.

Because tendons are less elastic, they often define the limit of motion at a joint and create sharp contour changes when stretched.

2.3 Using Tendons in Motion Reads

When a limb is under tension:

Tendons align along the direction of pull, creating straight or slightly curved tension lines.
Muscles near that tendon may appear more compressed (shortened) or elongated depending on the pose.

You can use tendons to:

Show which limb is bearing weight (tendons taut, angles sharp).
Emphasize speed or force (stretched tendons on a sprinting leg).
Add visual contrast: soft vs. hard, bulging vs. taut.

In creature design, even when anatomy is stylized, hinting at tendon‑like lines near joints helps poses feel more mechanical and grounded.

3. Fascia & Skin: The Sliding Layer

3.1 What Fascia Does

Fascia is the connective tissue matrix that wraps muscles, groups them, and connects them to skin. You don’t see fascia directly, but it explains why:

Some areas look tight and planar (skin hugging the muscle sharply).
Others look soft and mobile (skin sliding over deeper forms).

In motion:

Fascia allows skin to glide over muscles in some regions.
In other areas, it anchors tightly, creating stable landmarks.

3.2 Tight vs. Loose Regions

On many vertebrate creatures (and their fantasy variants):

Tighter fascia/skin over:
- Bony landmarks (knuckles, elbows, knees, shins, spine).
- Tendon‑rich areas (back of hand, top of foot).
Looser fascia/skin over:
- Belly and flanks.
- Areas with fat pads or thick fur.

In motion reads, tight areas show sharp folds and tension lines, while loose areas show soft wrinkles and lag (e.g., belly lagging behind as a creature accelerates).

3.3 Using Fascia in Creature Design

You can stylize fascia behavior to sell creature personality:

Lean, predatory creatures: more tight fascia—skin hugs muscles and bones, lots of visible landmarks.
Bulky tanks or beasts of burden: more loose fascia and fat—forms squash and stretch, details are softer.

In both concepting and production, decide:

Where is the skin locked to structure (never slides much)?
Where does it stretch and ripple during motion?

Marking these zones in callouts can help animators know where secondary motion (jiggle, flutter, skin sliding) should occur.

4. Major Muscle Groups and How They Read in Motion

Instead of naming every muscle, we’ll treat them as functional groups across vertebrate‑based creatures. These patterns hold for many mammals, reptiles, and even stylized hybrids.

4.1 Shoulder & Forelimb Groups

Key functions: reaching, striking, pushing off, climbing.

Typical masses:

Scapular/shoulder mass: sits over or near the shoulder blade.
- Raises and rotates the limb.
- On quadrupeds, often bulges during striding.
Upper arm mass (biceps/triceps equivalent): front and back of the upper limb.
- Front group flexes the limb (bending elbow).
- Back group extends the limb (straightening elbow).

Motion reads:

When a creature braces on its front limbs, the shoulder mass and triceps region become more pronounced.
When reaching or climbing, the front upper limb mass and scapular area stretch and compress.

Landmarks:

Shoulder bumps (scapula and deltoid forms).
Elbow joint angles.
Tendon lines along the front/back of the elbow.

4.2 Hip & Hindlimb Groups

Key functions: propulsion, jumping, stabilizing landings.

Typical masses:

Hip and gluteal region: major power source in jumps and runs.
Thigh mass: large, rounded; handles extension and flexion of the hind limb.
Calf/ankle complex: smaller but critical for push‑off and fine control.

Motion reads:

In a sprint or leap, the hip and thigh muscle groups compress and then extend powerfully.
Landing poses show eccentric tension—muscles lengthened but tense; tendons taut at knees and ankles.

Landmarks:

Hip joint bump.
Knee cap or equivalent joint prominence.
Achilles‑like tendons attaching to heel or rear claws.

4.3 Spinal & Axial Muscle Groups

Key functions: bending and twisting the torso, stabilizing during limb motion.

Typical masses:

Paraspinal muscles along both sides of the spine.
Additional lateral muscles in the ribs and flank.

Motion reads:

In flexed poses (crouching, coiling), the spine curves and back muscles bunch on one side, elongate on the other.
In rearing or roaring poses, muscles around the neck and upper back become prominent.

Landmarks:

Spine ridge (visible or implied) along the back.
Flank compression lines where ribs and muscles fold.
Neck muscle masses connecting skull to shoulders.

4.4 Neck & Head Muscle Groups

Key functions: head orientation, biting, carrying objects.

Typical masses:

Neck muscles running from back of skull to shoulders and chest.
Jaw muscles (especially in predatory creatures) bulging at the cheeks or behind the jaw.

Motion reads:

Roaring or straining poses show expanded neck muscles and jaw bulges.
Subtle head tilts may show rope‑like neck tension lines.

Landmarks:

Angle of the jaw and cheek bulges.
Vertical or diagonal neck ridges when flexed.

5. Comparative Notes: Fish, Amphibians, Reptiles, Birds, Mammals

You can apply the same “muscle group” logic across different vertebrate bases:

Fish: Axial muscles dominate; surface shows rhythmic chevron patterns; limbs are fins with smaller muscle contributions.
Amphibians: Soft, rounded muscle groups; powerful hindlimbs in frogs; less bony definition.
Reptiles: Strong jaw muscles and axial muscles; more rigid, angular outlines; limbs often show strong tendon accents at joints.
Birds: Massive chest (flight muscles), lean limbs; wings show tendon‑like feather shafts at the leading edge.
Mammals: Balanced limb and spinal groups; clear scapula, hip, and thigh masses; tendon visibility in extremities.

When combining these into fantasy creatures, pick a dominant motion logic:

Fish‑dominant: undulating spine, softer limb reads.
Mammal‑dominant: stride‑based locomotion.
Bird‑dominant: strong chest, flapping or gliding motion.

Then layer other influences in a way that respects the main motion engine.

6. Motion Reads in Practice: Poses and What to Emphasize

6.1 Idle / Neutral Poses

In neutral poses (turnarounds, orthos):

Muscles are in a mid‑tone of tension—not fully flexed, not fully stretched.
Tendons appear but aren’t extreme; fascia is fairly smooth.

For production sheets:

Show enough muscle and tendon landmarks to indicate structure without over‑flexing.
Keep symmetry and balance clear.

6.2 Action Poses: Push, Pull, Coil, Strike

In dynamic poses, prioritize believability of force over anatomical perfection.

Push off / Jump:
- Hindquarters (or primary push limbs) show strong muscle contraction.
- Tendons at ankles and knees taut; toes/claws splayed.
Landing / Bracing:
- Limb muscles elongated but tense; fascia folds where compression happens.
- Spine and neck muscles engaged to stabilize head.
Coiling / Winding up:
- Axial muscles compress on one side of the body; opposite side stretches.
- Tendon lines appear across the inside of bends.
Striking / Swinging:
- Muscle group at the root of the motion (shoulder, hip, base of tentacle) visibly engaged.
- Distal segments show inertia—slightly lagging behind or snapping forward.

For concepting, push these cues to hit expressive beats. For production, moderate them so the design can be rigged consistently.

7. Landmarks Cheat‑Sheet for Creature Artists

Here’s a compact list of high‑value landmarks that signal motion and structure, useful in both rough sketches and final callouts:

Bony landmarks:
- Scapula (shoulder blade) plane changes.
- Spine line and spinous process ridges.
- Pelvis/hip protrusions.
- Elbows, knees, ankles, wrists.
Muscle bulges:
- Shoulder/hip masses.
- Upper limb (thigh/upper arm) bulges.
- Neck and jaw masses.
Tendon lines:
- Back of heel to calf (Achilles‑like area).
- Top/back of hand/foot in animals.
- Cable‑like lines behind knees or along the front of ankles.
Fascia/skin behavior:
- Tight, planar regions at joints and over thin bone.
- Soft, folding regions at belly, flanks, and under the neck.

If you place these landmarks thoughtfully, your creatures will feel structurally sound even without fully rendered anatomy.

8. Concepting vs. Production: Different Depths of Anatomy

8.1 On the Concepting Side

Your priority is energy and clarity, not labeling every muscle:

Use implied muscle groups (big masses) and a few key tendon lines to sell force.
Overlap forms and push curves to exaggerate motion.
Keep anatomy flexible enough to support multiple interpretations as the design evolves.

You might purposely simplify:

Smoothing some anatomical detail to keep silhouettes strong.
Using stylized tendon lines to emphasize action beats.

8.2 On the Production Side

You must support rigging and animation with clearer anatomical information:

Show where muscles insert relative to joints so deformations make sense.
Mark rigid vs. soft vs. highly mobile regions for secondary animation.
Maintain consistency: if a tendon shows in one view, it should logically appear in others when the limb is similarly posed.

Provide:

Neutral pose orthos where landmarks are clearly visible.
A few key poses with notes on which muscle groups drive each motion.

This helps downstream teams avoid guessing where skin should crease or stretch.

9. Practical Study Exercises for Motion‑Focused Anatomy

9.1 “Before and After” Motion Pairs

Take an animal reference (or video frames) and draw:

Pose A: The wind‑up.
Pose B: The follow‑through.

For each, mark:

Main muscle bulges.
Key tendon lines.
Areas where skin/fascia folds or stretches.

Compare how landmarks shift between poses.

9.2 Silhouette + Landmark Overlays

Draw a simple silhouette of a creature pose.

Then, on top:

Add spine, joints, and a few muscle masses.
Add tendon lines and key fascia folds.

This shows you how minimal anatomy cues can drastically improve motion believability.

9.3 Multi‑Species Motion Study

Choose three different animal bases (e.g., big cat, bird of prey, lizard):

Draw each performing a similar action (leap, strike, turn).
Note how the same functional groups appear differently (e.g., bird chest vs. cat shoulder).

This builds comparative intuition you can apply to fantastical creatures.

9.4 Creature Pose Sheets

For your own creature design:

Create a sheet with 4–6 poses: idle, walk/run, attack, hit reaction, special move.
For each, lightly indicate:
- Muscle group activation.
- Tendon tension.
- Skin/fascia folds.

This becomes a powerful tool for animators and a test of whether your anatomy supports the intended moves.

10. Bringing It All Together

Tendons, fascia, and muscle groups are the hidden machinery behind convincing motion. As a creature concept artist, you don’t need medical‑grade detail, but you do need a working model of how:

Bones and joints set limits and pivots.
Muscle groups create force and mass shapes.
Tendons and fascia turn those forces into visible tension lines and folds.

For both concepting and production:

Use major muscle groups to shape the creature’s power and personality.
Use tendon landmarks to clarify where limbs are working hardest.
Use fascia behavior (tight vs. loose regions) to add believability to motion and secondary effects.

Whenever a creature pose feels off or weightless, ask:

Which muscle groups are actually doing the work in this pose?
Where would tendons be stretched or slack, and can I show that with simple lines?
Are there regions where skin or fascia should fold, lag, or stay tight—and am I showing that?

If you can answer those questions in your drawings, you’ll create creatures that not only look cool in still images, but also feel ready to move convincingly in animation—bridging the gap between expressive concepting and reliable production anatomy.