Bike Fitting Glossary: Key Terms & Concepts
This glossary gathers the technical terms and concepts BikeFit AI uses. Each term is explained. Target values, however, aren't universal: they depend on your discipline and your body—which is why BikeFit AI measures them on your video and places them relative to your profile's target, rather than handing you a one-size-fits-all number.
1. Anatomy and Biomechanics
Body Landmarks
Joints measured by BikeFit AI:
| Joint | Description | Role in Cycling |
|---|---|---|
| Knee | Femur-tibia junction | Leg extension/flexion, power transfer |
| Hip | Pelvis-femur junction | Trunk-thigh opening/closing, aero position |
| Ankle | Tibia-foot junction | Pedaling technique, "ankling" style |
| Shoulder | Trunk-arm junction | Arm position on the bars |
| Elbow | Upper-arm-to-forearm junction | Shock absorption, arm tension |
Key Muscles
Vastus medialis (VMO): Inner quadriceps, just above the knee on the medial side. Stabilizes the kneecap during pedaling. Weakness can lead to poor tracking and pain.
Quadriceps: Four-muscle group at the front of the thigh. Powers knee extension—the pushing phase (0–90°).
Hamstrings: Muscles at the back of the thigh. Contribute to knee flexion and the pulling phase (180–270°) for riders clipped in.
Gluteus maximus: The largest glute. Essential for power, especially on climbs and during accelerations.
Gastrocnemius (calf): Stabilizes the ankle and transmits force—especially important for toe-down pedalers.
Foot Anatomy
Metatarsals: The five long bones of the foot, between the toes and the arch. The "bump" at the base of the big toe is the first metatarsophalangeal (MTP) joint.
Cleat positioning: The pedal spindle should sit at or slightly behind the ball of the foot. Too far forward overloads the calves.
2. Pedal-Stroke Positions
The Four Cardinal Points
The stroke is divided like a clock face:
12 o'clock (0°)
TDC
│
▼
┌────────┐
9 o'clock ────│ ● │──── 3 o'clock
(270°) │ │ (90°)
└────────┘
│
▼
6 o'clock (180°)
BDC
TDC – Top Dead Center
Definition: Pedal at the highest point (12 o'clock, 0°).
What's measured here:
- Maximum knee flexion (most closed).
- Maximum hip closure (trunk-to-thigh).
Why it matters: A hip that's too closed compresses the diaphragm and limits breathing. A knee that's too flexed overloads the joint.
BDC – Bottom Dead Center
Definition: Pedal at the lowest point (6 o'clock, 180°).
What's measured here:
- Maximum knee extension (leg most straight).
- Ankle angle (foot position).
Why it matters: This is THE reference for setting saddle height. A knee that's too extended strains the posterior structures; too flexed, it overloads the kneecap. The ideal is a slight bend—whose exact value depends on your profile and is measured while pedaling.
Pedaling Phases
| Phase | Position | Action | Primary Muscles |
|---|---|---|---|
| Push | 0–90° | Leg extension | Quads, Glutes |
| Bottom transition | 90–180° | End of extension | Calves, Hamstrings |
| Pull | 180–270° | Upstroke (if clipped in) | Psoas, Hamstrings |
| Top transition | 270–360° | Preparation | Hip flexors |
3. Frame Geometry
Stack
Definition: Vertical distance from the center of the bottom bracket to the top of the head tube.
Impact: High stack = more upright, more comfortable. Low stack = more aggressive, more aero.
Typical values:
- Endurance road: 560–600 mm
- Performance road: 520–560 mm
- TT/triathlon: 480–520 mm
Reach
Definition: Horizontal distance from the center of the bottom bracket to the top of the head tube.
Impact: Long reach = stretched position. Short reach = compact position.
Typical values:
- Size M road: 380–395 mm
- Adjustable via stem length and saddle setback.
Drop
Definition: Height difference between saddle top and bar top (or hoods).
Impact: Greater drop = more aggressive position.
Typical values:
- Comfort/recreational: 0 to –2 cm
- Endurance: –2 to –5 cm
- Performance: –5 to –10 cm
- TT/Pro: –10 cm or more
Setback
Definition: Horizontal distance from the seatpost axis to the saddle's lowest point (or nose).
Impact: Positions the knee relative to the pedal spindle. Affects quad/glute load distribution.
Verification: At 3 o'clock (90°), the front of the knee should be over or slightly behind the spindle.
Q-Factor
Definition: Distance between the outer faces of the two crank arms. Determines foot spacing.
Typical values:
- Road: ~150 mm
- MTB: 170–180 mm
- Theoretical optimum: < 130 mm
Impact: Q-factor that's too wide decreases efficiency (–2–3%) and comfort regardless of pelvis width.
4. The Measured Angles
Measurement Conventions
Flexion angle: Measured from the "leg straight" position. The larger it is, the more the joint is bent. (An internal angle, conversely, is measured between the two segments: the two conventions are supplementary (they add up to 180°).)
Static: Bike at rest, foot manually positioned.
Dynamic: While pedaling at normal cadence (80–90 rpm).
Static-to-dynamic gap: Measurements while pedaling systematically differ from those at rest. BikeFit AI measures dynamically, because that's the reality of pedaling.
Knee Angle
Position: At BDC (6 o'clock), maximum extension.
At bottom dead center, you want a slightly bent knee: neither too extended (strain on the posterior structures, pelvis rocking) nor too flexed (patellofemoral overload). The right range depends on your discipline and your body—BikeFit AI measures your actual flexion dynamically and places it relative to your profile's target.
Hip Angle
Position: At TDC (12 o'clock), maximum trunk-thigh closure.
Hip closure varies by discipline: more open for comfort riding, touring, and gravel; more closed for TT and triathlon, where the search for aerodynamics dominates. Too closed, it compresses the diaphragm and hampers breathing.
Torso Angle
Position: Back angle relative to horizontal.
The torso is lower the more the position targets performance: very upright for comfort/urban use, intermediate on the road, very low for TT/triathlon. It's the primary aerodynamic lever.
Ankle Angle
Convention: 90° = neutral (foot perpendicular to shin).
Variations:
- < 90°: Dorsiflexion (toes toward shin, "heel-down").
- > 90°: Plantarflexion (toes down, "toe-down").
How much and how the ankle moves over the cycle defines your pedaling style (see section 5).
5. Pedaling Styles
BikeFit AI automatically detects your style via ankle-angle analysis.
Ankling (Optimal)
Characteristics:
- Moderate, steady ankle movement over the full cycle.
- Fluid, natural movement.
- Efficient calf use without overload.
Advantage: Best efficiency/endurance trade-off.
Toe-Dipper
Characteristics:
- Toes pointed down (marked plantarflexion).
- Heel constantly raised.
- High calf activation.
Risks: Premature calf fatigue, cramps, potential Achilles issues.
Recommendation: Ankle-mobility drills; consciously drop heels.
Heel-Dropper
Characteristics:
- Heel low at BDC (strong dorsiflexion).
- Minimal calf engagement.
Impact: May require a slightly lower saddle. Less power transmission in sprints.
Note: Not necessarily a flaw—can suit endurance riding.
Piston Pedaler (Locked Ankle)
Characteristics:
- Very little ankle movement over the cycle.
- Rigid ankle, "piston" movement.
- Limited joint mobility.
Risks: Shock transmission to the knee, inefficiency at end of push.
Recommendation: Ankle-mobility exercises (circles, calf stretches).
6. Calculation Formulas
Saddle-Height Formulas
Several formulas exist; each suits different contexts.
Genzling Formula (1980)
Saddle height (cm) = Inseam (cm) × 0.883
- Origin: Developed with pro cyclists of the '80s.
- Best for: Adults with inseam > 88 cm (tall builds).
Hamley Formula (1967)
Saddle height (cm) = Inseam (cm) × 1.09 − Crank length (cm)
- Origin: Academic efficiency study.
- Best for: Children and shorter builds (inseam < 80 cm).
LeMond Formula
Saddle height (cm) = Inseam (cm) × 0.883
- Origin: Popularized by Greg LeMond.
- Note: Identical to Genzling, often cited separately.
Actual Cyclist Preference
Saddle height (cm) = Inseam (cm) × 0.875
- Origin: Observational studies on cyclists' spontaneous choices.
- Finding: Cyclists typically choose slightly lower than theory.
Which Formula to Use?
| Profile | Recommended | Multiplier |
|---|---|---|
| Child / inseam < 80 cm | Hamley | × 1.09 − crank |
| Average adult (80–88 cm) | Genzling or test both | × 0.883 |
| Tall adult (> 88 cm) | Genzling | × 0.883 |
| Comfort priority | Cyclist preference | × 0.875 |
| Pain with 0.883 | Try 0.875 | × 0.875 |
Example:
- Inseam = 84 cm
- Genzling: 84 × 0.883 = 74.2 cm
- Preference: 84 × 0.875 = 73.5 cm
- Difference: 7 mm (significant for comfort!)
Formulas Aren't Enough
They don't account for:
- Flexibility (tight hamstrings = lower saddle).
- Pedaling style (heel-dropper vs. toe-dipper).
- Crank length (except Hamley).
- Individual proportions (femur/tibia ratio).
True validation:
STEP 1: Formula → Starting point
↓
STEP 2: Heel method → Quick check
↓
STEP 3: Knee angle measured dynamically → FINAL VALIDATION ✓
That's exactly what BikeFit AI does: we don't stop at a formula—we MEASURE the actual angle while you pedal.
Heel Method
Procedure:
- Sit on the saddle; pedal at 6 o'clock.
- Place HEEL on pedal (not ball of foot).
- Leg should be straight (knee locked).
- If knee stays bent → saddle too low.
- If pelvis tilts to reach → saddle too high.
Accuracy: Approximate (±1–2 cm). Useful for initial setup but doesn't replace dynamic measurement.
7. Scientific Sources
Academic Studies
| Reference | Year | Contribution |
|---|---|---|
| Holmes et al. | 1994 | Framework for knee-injury prevention |
| Ferrer-Roca et al. | 2012 | Static/dynamic difference demonstrated |
| Ferrer-Roca et al. | 2020 | Dynamic knee angle at BDC |
| Foss & Hallén | 2004–05 | Optimal cadence ~80 rpm for TT |
| Heil et al. | 1994–97 | Saddle tilt's effect on hip angle |
Professional Sources
- BikeDynamics (UK) — UK bike-fitting reference.
- Bike Fit Adviser — In-depth joint-angle analyses.
- Slowtwitch / F.I.S.T. — Triathlon specialists.
- Retül / Trek Precision Fit — Professional fitting systems.
Static vs. Dynamic
The most important distinction in practice: angles measured at rest systematically differ from those measured while pedaling.
- The best-known historic ranges were based on static measurements.
- While pedaling, the body naturally "protects" its joints, and cadence influences the angles.
- For a dynamic measurement (like BikeFit AI), the targets shift accordingly—hence the importance of measuring in motion rather than applying a static number.
8. Additional Terms
Cadence
Definition: Pedal revolutions per minute (rpm).
Reference values:
- Endurance: 80–95 rpm
- Optimal TT: ~80 rpm (Foss & Hallén)
- Sprint: 100–120 rpm
- Climbing: 70–85 rpm
ROM (Range of Motion)
Definition: A joint's movement arc. In cycling, the difference between max and min angle over a pedal cycle.
Example: The more your knee moves from deep flexion (at TDC) to a nearly straight leg (at BDC), the greater its ROM.
Gross Efficiency
Definition: Ratio of mechanical power output to metabolic energy expenditure. Typically 20–25% in trained cyclists.
MAP (Maximum Aerobic Power)
Definition: Power output at VO2max. Used as a benchmark in testing and training.
Wrap-Up
This glossary forms the BikeFit AI knowledge base. Our philosophy: no "magic numbers" pulled from thin air. The right values aren't universal—they depend on your discipline and your body. Rather than handing you a one-size-fits-all range, BikeFit AI measures your actual angles while you pedal and compares them to your profile's target.
Questions about a definition or recommendation? Our AI Coach can explain the science behind any piece of advice.
Related Articles
- Bike Fitting: What It Is, Why It Matters, and How to Get Started
- Knee Angle in Cycling: Understanding Optimal Flexion
- Saddle Adjustment: How to Dial In Height, Setback and Tilt
- Home Bike Fitting: DIY Guide + Free AI Video Analysis
Glossary maintained by the BikeFit AI team. Sources: international biomechanics literature, PubMed studies, professional bike-fitting references.