Add the hinge pattern: Deadlift, RDL, Glute Bridge, Hip Thrust, Back Extension

The biggest hole in the coverage matrix - the hinge had zero entries in
any modality. Deadlift and Romanian Deadlift are the library's first
standing (camera.zoom) and first barbell motions.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Back Extension/frames/frame-1.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
+  <title>Back Extension</title>
+  <path d="M 60.0 160.4 L 264.9 160.4 L 264.9 150.0 L 60.0 150.0 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
+  <path d="M 102.0 124.0 L 154.0 106.0 L 154.0 106.0 L 102.0 124.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="9" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 130.0 122.0 L 130.0 150.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 58.0 142.0 L 88.0 142.0 L 88.0 142.0 L 58.0 142.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_l" d="M 230.6 141.1 L 200.7 139.0 L 222.3 159.5" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_l" d="M 155.9 102.8 L 113.8 121.1 L 76.8 135.9 L 83.0 144.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="girdle" d="M 230.6 141.1 L 224.7 141.0 L 224.7 142.9" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="pelvisBar" d="M 155.9 102.8 L 150.0 102.0 L 150.0 103.2" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="spine" d="M 150.0 102.0 Q 192.4 114.2 224.7 141.0" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 224.7 142.9 L 194.8 140.8 L 216.3 161.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_r" d="M 150.0 103.2 L 110.7 126.8 L 72.0 136.7 L 76.9 146.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <circle cx="80.1" cy="142.6" r="4.5" fill="#6b7180"/>
+  <circle id="head" cx="239.1" cy="154.7" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
+  <line id="nose" x1="236.6" y1="164.4" x2="234.9" y2="171.1" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+  <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">
+    <line x1="224" y1="16" x2="238" y2="16" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+    <text x="243" y="20">near</text>
+    <line x1="273" y1="16" x2="287" y2="16" stroke="#a9afba" stroke-width="4" stroke-linecap="round"/>
+    <text x="292" y="20">far</text>
+  </g>
+</svg>

Exercise Library/Back Extension/frames/frame-2.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
+  <title>Back Extension</title>
+  <path d="M 60.0 160.4 L 264.9 160.4 L 264.9 150.0 L 60.0 150.0 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
+  <path d="M 102.0 124.0 L 154.0 106.0 L 154.0 106.0 L 102.0 124.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="9" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 130.0 122.0 L 130.0 150.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 58.0 142.0 L 88.0 142.0 L 88.0 142.0 L 58.0 142.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_l" d="M 155.9 102.8 L 113.8 121.1 L 76.8 135.9 L 83.0 144.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_l" d="M 230.9 62.9 L 216.3 88.7 L 244.3 78.1" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="girdle" d="M 230.9 62.9 L 225.0 62.8 L 225.0 64.7" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="pelvisBar" d="M 155.9 102.8 L 150.0 102.0 L 150.0 103.2" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="spine" d="M 150.0 102.0 Q 190.5 85.8 225.0 62.8" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_r" d="M 150.0 103.2 L 110.7 126.8 L 72.0 136.7 L 76.9 146.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 225.0 64.7 L 210.4 90.6 L 238.4 80.0" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <circle cx="80.1" cy="142.6" r="4.5" fill="#6b7180"/>
+  <circle id="head" cx="242.9" cy="54.2" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
+  <line id="nose" x1="252.8" y1="55.7" x2="259.7" y2="56.8" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+  <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">
+    <line x1="224" y1="16" x2="238" y2="16" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+    <text x="243" y="20">near</text>
+    <line x1="273" y1="16" x2="287" y2="16" stroke="#a9afba" stroke-width="4" stroke-linecap="round"/>
+    <text x="292" y="20">far</text>
+  </g>
+</svg>

Exercise Library/Back Extension/info.md

@@ -0,0 +1,39 @@
+# Back Extension
+
+The roman-chair hinge — hips anchored on the pad, fold forward, then raise
+the trunk back in line with the legs.
+
+- **Category:** Main circuit
+- **Type:** Machine-based back extension (hip hinge)
+- **Targets:** Spinal erectors, glutes, hamstrings
+- **Prescription:** 3 × 10–15
+
+## Setup
+
+Set the pad so its top edge sits at the crease of your hips — you must be
+able to fold freely over it. Hook your heels under the ankle rollers,
+cross your arms over your chest.
+
+## Execution
+
+1. Fold forward over the pad with a controlled, mostly-flat back.
+2. Lower until the hamstrings stop you, without going slack.
+3. Squeeze glutes and back to raise the trunk until it lines up with the
+   legs — no higher.
+
+## Cues
+
+- Movement comes from the hips with the back held long.
+- Rise to a straight body line, not into an arch.
+- Slow down; momentum does nothing here.
+
+## Common Mistakes
+
+- Pad set too high, blocking the hinge.
+- Hyperextending past the body line at the top.
+- Swinging up with momentum.
+
+## Progression
+
+Slow the lowering phase, add a pause at the top, then hold a plate to the
+chest for load.

Exercise Library/Back Extension/motion.json

@@ -0,0 +1,37 @@
+{
+  "name": "Back Extension",
+  "primary": 2,
+  "working": ["spine"],
+  "props": [
+    {"type": "scene", "shapes": [
+      {"kind": "line", "pts": [[102, 124], [154, 106]], "w": 9, "depth": 8},
+      {"kind": "line", "pts": [[130, 122], [130, 150]], "w": 5},
+      {"kind": "line", "pts": [[58, 142], [88, 142]], "w": 5, "depth": 6}
+    ]},
+    {"type": "roller", "at": ["foot_r", "foot_l"], "r": 4.5, "side": 1, "back": 4}
+  ],
+  "frames": [
+    {
+      "hold": 0.4, "tween": 1.0,
+      "root": {"pos": [150, 102], "pitch": 112},
+      "spine": [0, 12],
+      "neck": 10, "head": -30,
+      "shoulder_r": 30, "elbow_r": 140,
+      "shoulder_l": 30, "elbow_l": 140,
+      "hip_r": 48, "knee_r": 12, "ankle_r": 12,
+      "hip_l": 48, "knee_l": 12, "ankle_l": 12,
+      "pins": {"foot_r": [72, 136], "foot_l": [76, 138]}
+    },
+    {
+      "hold": 0.6, "tween": 1.1,
+      "root": {"pos": [150, 102], "pitch": 65},
+      "spine": [0, -6],
+      "neck": 5, "head": -55,
+      "shoulder_r": 30, "elbow_r": 140,
+      "shoulder_l": 30, "elbow_l": 140,
+      "hip_r": 2, "knee_r": 12, "ankle_r": 12,
+      "hip_l": 2, "knee_l": 12, "ankle_l": 12,
+      "pins": {"foot_r": [72, 136], "foot_l": [76, 138]}
+    }
+  ]
+}

Exercise Library/Back Extension/visual.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
+  <title>Back Extension</title>
+  <path d="M 60.0 160.4 L 264.9 160.4 L 264.9 150.0 L 60.0 150.0 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
+  <path d="M 102.0 124.0 L 154.0 106.0 L 154.0 106.0 L 102.0 124.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="9" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 130.0 122.0 L 130.0 150.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 58.0 142.0 L 88.0 142.0 L 88.0 142.0 L 58.0 142.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_l" d="M 155.9 102.8 L 113.8 121.1 L 76.8 135.9 L 83.0 144.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_l" d="M 230.9 62.9 L 216.3 88.7 L 244.3 78.1" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="girdle" d="M 230.9 62.9 L 225.0 62.8 L 225.0 64.7" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="pelvisBar" d="M 155.9 102.8 L 150.0 102.0 L 150.0 103.2" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="spine" d="M 150.0 102.0 Q 190.5 85.8 225.0 62.8" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="leg_r" d="M 150.0 103.2 L 110.7 126.8 L 72.0 136.7 L 76.9 146.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 225.0 64.7 L 210.4 90.6 L 238.4 80.0" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <circle cx="80.1" cy="142.6" r="4.5" fill="#6b7180"/>
+  <circle id="head" cx="242.9" cy="54.2" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
+  <line id="nose" x1="252.8" y1="55.7" x2="259.7" y2="56.8" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+  <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">
+    <line x1="224" y1="16" x2="238" y2="16" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+    <text x="243" y="20">near</text>
+    <line x1="273" y1="16" x2="287" y2="16" stroke="#a9afba" stroke-width="4" stroke-linecap="round"/>
+    <text x="292" y="20">far</text>
+  </g>
+</svg>