Leg Lock Lab
BMEN 2320 Statics · Fall 2026 · Term project

The Double
Leg Lock

ΣF = 0 · ΣM = 0 · tap before it breaks

Animated double leg lock Side view of the attacker in a blue gi trapping the defender's crossed legs. Step 1 she squeezes her legs, step 2 she leans back and pulls his feet into her chest, step 3 his ankle reaches its limit and he taps the mat. T_A squeeze P pull HIM · defender · 175 cm · 56.7 kg HER · attacker · 157 cm TAP! ankle limit 47 N·m
She squeezes her legs together. Her adductors (T_A) push his crossed shins into each other.
–days until the problem description is due (Nov 20)
8rigid members in our frame
19 ≤ 21unknowns ≤ independent equations
Anklegives out first, every time
Open the group doc ↗Upload folder ↗My guideTeam Hub / request a change

Read this first. Everything here is reference material. We can't hand in AI-written work, so write every section in your own words, check the course AI policy, and Leya redraws every diagram by hand.

The belt path to submission

Plan

Problem description is 20% of the grade and the final report is 60%. Here's the road to Nov 20.

Sat Nov 14Everyone reads their guide and the example doc
Mon Nov 16Leya shares measured angles + hand-drawn diagrams
Tue Nov 17Drafts in the Google Doc by Tuesday night
Wed–Thu Nov 18–19Wed: ask the professor. Then fix, combine, review
Fri Nov 20Submit the problem description

What we decided

PositionDouble leg lock, both athletes on their sides, head to feet
PlaneThe mat plane (top view). Weight goes into the mat, so it drops out.
MembersHis pelvis, 2 thighs, 2 shanks; her trunk, 2 legs = 8
Questions1) Which joint gives out first, and how much effort? 2) All joint reactions. 3) Internal loads (N, V, M) in his left shin.
LimitsKnee 101 N·m [1], ankle eversion 47 N·m [2]
ScaleWallet, 13.3 × 8.9 cm [4]

Who does what

PersonWrites
Anusha§1 Introduction and §3 Biomechanical system
Adithi§2 Problem statement and §6 Solvability check
Veera§4 Schematic + cover page
Leya§5 FBDs, §7 Assumptions, all diagrams, formatting
EveryoneAdd references as you go
Pick your name

Step-by-step guides

Tap your card to start your mission. Tap each task as you finish it. Upload your work after every step. When a step is done you get a stripe and jump to the next one.

Everything in one place

Documents

Examples to copy the structure from, not the words. Each button asks you to confirm the download.

Nothing goes missing

Deliverables tracker

Every single thing we need, with its label, the exact file name, where it goes and when it's due. Name your files exactly like this so we can find them.

Tap a status to change it: Missing → In progress → Done. Everyone sees the change.

Talk to the team

Team Hub

Post a message, ask a question, or request a change to the site or the plan. Everyone sees it here, live.

Type

Latest

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Hand in your part

Team uploads

Everyone puts their drafts, photos and drawings in one shared folder so the whole group can see them.

Group Google Doc

The one doc we all write the problem description in. Type only under your own heading.

Shared upload folder

Drop in your draft, sources (PDFs), hand drawings and measurement tables.

  • Name files like Veera_Schematic_v1 or Leya_FBDs_v2.
  • New version? Upload a new file with v2, v3. Don't delete old ones.
  • Photos of hand drawings: take them straight on, in daylight, no shadow.
Measure from the photos

Tools

Wallet long side = 13.3 cm, short side = 8.9 cm
Last result
–
No scale set yet
#TypeValue
Numbers we use

Data

Our athletes

Her (attacker)Him (defender)
Height5′2″ · 157 cm5′9″ · 175 cm
Weight125 lb · 56.7 kg · 556 N125 lb · 56.7 kg · 556 N
Thigh 0.245H0.385 m0.429 m
Shank 0.246H0.386 m0.431 m

CDC U.S. adult averages (20+)

WomenMen
Height63.5 in · 161 cm68.9 in · 175 cm
Weight171.8 lb · 77.9 kg199.0 lb · 90.3 kg
Waist38.5 in40.6 in

CDC/NCHS FastStats, Body Measurements, NHANES Aug 2021–Aug 2023 [5]. Use it to say our athletes are close to average height but much lighter than average.

Results at 1000 N squeeze + 1000 N pull

UnknownValueMeaning
H_Ly111.5 NHis hip pin
K_Lx, K_Ly117.8, −158.1 NHis knee pin
M_KL19.3 N·mKnee ligament moment
J_Lx, J_Ly−701.4, 303.8 NHer hip pin
N_X231.1 NShins pressing where they cross
W_L294.2 NHer shin on his thigh
N_CL1115.1 NHis foot on her chest
Ankle11.0 N·mAnkle moment
Max |M| shin23.2 N·mAt legs-cross point, 34.2 cm from knee

Right side mirrors the left. All contact forces came out positive, so the model behaves physically.

Stuck? Start here

Learn & where to find data

FreeBody

Leya's statics study site. Use it for FBDs, frames, equilibrium and shear/moment diagrams. Need access? Post a request in the Team Hub and Leya will add you.

Open FreeBody ↗

Watch & read

GuideKinovea: measuring anglesFree app for measuring joint angles on photos and video. GuideKinovea: tracking distances and anglesIf anyone films the move instead of taking photos. VideoFrames & machinesTaking a frame apart into member FBDs. VideoTwo-force membersWhy muscles act along one line. VideoShear & moment diagramsFor the extra-credit shin diagram. VideoThe move itselfSee the double leg lock from other angles.

Where to look for data

Body sizeCDC FastStats: Body MeasurementsU.S. average height, weight, waist. SegmentsWinter, Biomechanics and Motor ControlSegment length and mass fractions (Table 4.1). Knee limitKajzer et al. 1993Knee lateral bending tolerance, about 101 N·m. Ankle limitFunk et al. 2002Ankle eversion 47 N·m, inversion 21 N·m. Still neededHip adductor strengthFind a published squeeze force for women, in N. BackgroundBJJ injury studiesFor Anusha's introduction. ScaleWallet product listingProof of the 13.3 × 8.9 cm scale object.
Read before you write

Help

Common mistakes (don't do these)

  • Copying sentences from the example or the sample report. Rewrite everything.
  • Forgetting units. Every number needs N, m, N·m, kg or degrees.
  • Calling his weight an in-plane force. Weight goes into the mat, not along it.
  • Saying "ball and socket" for the ankle. It's a hinge (talocrural) plus the subtalar joint.
  • Mixing up who is who: she attacks, his leg gets locked.
  • Using a source but not citing it with [#] in the sentence.
  • Wrong file names. Use the exact names in the tracker.
  • Changing someone else's section in the group doc. Leave a comment instead.

Grade at a glance

Problem description20%Due Nov 20
Final report60%Later
Extra credit+10 ptsMost complex problem; shear/moment diagram

Copy-paste group texts

Words you'll see · flash cards, tap to flip

FrameA structure with at least one member that has 3 or more forces on it. Ours has 8 members.
MemberOne rigid piece, like his thigh or her trunk.
Free body diagram (FBD)A drawing of one piece by itself with every force and moment on it.
Pin jointA connection that stops sliding but lets it rotate. 2 unknowns (x and y).
Smooth contactTwo surfaces pushing with no friction. 1 unknown, straight out from the surface.
Two-force memberA piece with forces at only 2 points, so the force acts along the line between them. Our muscles.
Moment (N·m)A twisting effect: force × perpendicular distance.
BeamA long member with 3+ forces. The rubric needs at least 2. We have 7.
EversionThe ankle rolling so the sole turns outward. Her lean-back does this to his ankle.
AdductorsInner-thigh muscles that squeeze the legs together. Her squeeze force, T_A.
Shear (V) and moment (M) diagramA graph of the inside forces along a beam. Ours is his shin, for extra credit.
SolvableUnknowns ≤ independent equations. Ours: 19 ≤ 21.
IEEE format · copy and paste

References

After Nov 20 · 60% of the grade

Final report roadmap

Graded out of 100 + 10 extra credit, using the professor's final report rubric. Here's the split; change it if the group wants. The full sample is in Documents → Sample final report.

Things the final rubric needs that are easy to miss
  • Write the equations out. Solver numbers alone lose points. Every member needs ΣFx = 0, ΣFy = 0 and ΣM = 0 written out, then solved.
  • Units and directions on every answer, like "K_Lx = 117.8 N →".
  • A section-cut FBD of his shin for the internal loads, separate from the member FBDs.
  • Assumptions from multiple sources, with realistic weights and measurements cited.
  • Conclusions compare to literature values, for example our ankle moment vs. the 47 N·m limit [2].
  • Show the professor's feedback was applied to the problem description.

What carries over

  • §1–7 of the problem description, with fixes from the professor's feedback.
  • Updated angles, coordinates and FBDs if anything changed.
  • References, renumbered as new ones get added.

Our key results (to check against)

Gives out firstAnkle, in every case
Squeeze = pull4.28 kN each
Max shin moment23.2 N·m at 34.2 cm
SolverDocuments → Solver code
What the professor grades

Rubric

The professor's exact rubric for the problem description: 15 points, 5 per section. Each section needs all 5 of its criteria to get full points.

The assignment

Choose a static biomechanical system (human or animal) that can be modeled as a frame or machine, with at least 3 members, 2 of them beams (3+ forces). Give a short description of the system and what we plan to solve, and a schematic of the frame or machine.

Ours goes beyond this: 8 members, 7 beams, full FBDs and a solvability check.

Problem description20% · 15 pts · due Nov 20
Final report60% · later
Extra credit+10 pts · most complex problem in the section (final report)