90/90 Back Extension

Isolated spinal erector training — bent-knee position removes hamstring involvement to purely target glutes and lower back extensors

---

⚡ Quick Reference

Aspect	Details
Pattern	Hip Extension (Isolated)
Primary Muscles	Erector Spinae, Glutes
Secondary Muscles	Lower Back, Hamstrings (minimal)
Equipment	Back Extension Bench or GHD
Difficulty	⭐ Beginner
Priority	🟡 Accessory

Movement Summary

---

🎯 Setup

Starting Position

1. Bench adjustment: Position pad so hips are fully supported
   • Top of pad should be at hip crease/upper thigh
   • Allows full ROM without pad digging into stomach
2. Foot position: Secure feet under ankle pads
   • Feet dorsiflexed (toes up toward shins)
   • Ankles locked under pads for stability
3. Knee position: Bend knees to exactly 90 degrees
   • This is the key difference from standard back extensions
   • Maintains bent-knee throughout entire movement
   • Heels toward glutes position
4. Starting posture: Begin at neutral spinal alignment
   • Body forms straight line from head to knees
   • Not extended, not flexed — perfectly horizontal
5. Hand position: Options for arm placement
   • Crossed over chest (easiest)
   • Hands behind head (moderate)
   • Arms extended overhead (hardest)

Equipment Setup

Equipment	Setting	Notes
Back extension bench	Pad at hip crease	45-degree or horizontal bench
GHD machine	Adjusted for your height	Allows same 90° knee bend
Foot pads	Secure but comfortable	Should feel stable

Setup Cue

"Knees bent to 90 degrees the entire time — turn off your hamstrings, turn on your glutes and lower back"

---

🔄 Execution

The Movement

🔧 Starting Position

What's happening: Establishing neutral position with knees bent

1. Positioned on bench, hips on pad
2. Knees bent to 90 degrees — maintain throughout
3. Body in straight horizontal line (neutral spine)
4. Arms crossed over chest or hands behind head
5. Core lightly engaged
6. Glutes squeezed slightly

Tempo: Establish perfect position before moving

Feel: Stable on bench, knees locked at 90°, ready to move

Critical: Knees stay bent — this removes hamstring involvement

⬇️ Lowering Phase

What's happening: Controlled flexion at hips and spine

1. Lower torso smoothly toward floor
2. Allow controlled spinal flexion (rounding is OK here)
3. Keep knees at 90° — don't straighten
4. Lower until torso is vertical or near-vertical
5. Feel stretch in lower back and glutes
6. Breathing: Inhale as you lower

Tempo: 2-3 seconds (controlled descent)

Feel:

• Glutes and erector spinae lengthening
• Minimal hamstring involvement (because knees are bent)
• Stretch in lower back

Range: Most people can lower to about 90° (torso vertical)

⏸️ Bottom Position

What's happening: Fully flexed position, brief pause

1. Torso vertical or near-vertical
2. Knees still at 90°
3. Feel deep stretch in spinal erectors and glutes
4. Core engaged to control position
5. Brief pause (0-1 second) or touch-and-go

Common error here: Bouncing out of bottom — stay controlled

⬆️ Extension Phase

What's happening: Extending back to neutral position

1. Squeeze glutes and contract spinal erectors
2. Extend torso back up to horizontal
3. Keep knees bent at 90° throughout
4. Think "lift chest up"
5. Return to straight horizontal line
6. Do NOT hyperextend past neutral
7. Breathing: Exhale as you extend

Tempo: 1-2 seconds (controlled but smooth)

Feel:

• Glutes contracting hard
• Lower back/erectors working
• Minimal hamstring engagement

🔝 Top Position

What's happening: Return to neutral horizontal alignment

1. Body in straight line from head to knees
2. Not hyperextended — just neutral
3. Knees still bent to 90°
4. Glutes squeezed at top
5. Brief pause, then lower for next rep

Critical: Stop at neutral. Hyperextending past horizontal puts unnecessary stress on spine.

Key Cues

Primary Cues

• "Knees stay bent the whole time — 90 degrees" — removes hamstrings
• "Lower with control, come up to neutral only" — proper ROM
• "Squeeze your glutes at the top" — maximize glute engagement
• "Don't hyperextend — stop at flat/horizontal" — spinal safety

Tempo Guide

Goal	Tempo	Example
Strength/Hypertrophy	2-0-2-1	2s down, no pause, 2s up, 1s squeeze
Hypertrophy	3-1-2-1	3s down, 1s pause, 2s up, 1s squeeze
Endurance	1-0-1-0	Rhythmic, continuous tension

---

💪 Muscles Worked

Activation Overview

Primary Movers

Muscle	Action	Activation
Erector Spinae	Spinal extension — lifting torso back to neutral	█████████░ 90%
Glutes	Hip extension — contributing to torso raising	████████░░ 80%

Secondary Muscles

Muscle	Action	Activation
Lower Back	Spinal extension and stabilization	████████░░ 75%
Hamstrings	Minimal (bent-knee position reduces contribution)	███░░░░░░░ 25%

Stabilizers

Muscle	Role
Core	Prevents overextension, maintains control

Muscle Emphasis

The 90/90 position is the key difference: Bending knees to 90° shortens the hamstrings, reducing their ability to contribute to hip extension. This shifts emphasis to:

1. Glutes — primary hip extensors
2. Erector spinae — spinal extensors

Hamstring activation drops from ~70% (straight-leg) to ~25% (bent-knee).

To emphasize glutes even more: Squeeze glutes hard at top, pause for 1-2 seconds

To emphasize erectors: Slow eccentric, controlled spinal flexion and extension

---

⚠️ Common Mistakes

Mistake	What Happens	Why It's Bad	Fix
Straightening knees	Turns into regular back extension	Recruits hamstrings, defeats purpose	Actively keep knees bent to 90°
Hyperextending spine	Going past neutral at top	Excess spinal stress, injury risk	Stop at horizontal/neutral
Using momentum	Bouncing out of bottom	Less muscle activation, injury risk	Controlled tempo, pause at bottom
Going too fast	Rushing through reps	Reduces time under tension	Slow down, 2-3s each phase
Not engaging glutes	Lower back does all work	Overworks erectors, misses glute training	Actively squeeze glutes at top

Most Common Error

Hyperextending past neutral at the top. Many people arch backward beyond horizontal, which creates unnecessary spinal compression. Stop when your body is in a straight line — no further.

Self-Check Checklist

• Knees stay bent at 90° throughout entire set
• Lower to approximately vertical (90° forward flexion)
• Return only to neutral horizontal — no hyperextension
• Feel it in glutes and lower back, NOT hamstrings
• Controlled tempo both up and down

---

🔀 Variations

By Arm Position (Difficulty)

Arms Crossed

Aspect	Details
Setup	Arms crossed over chest
Difficulty	Easiest
Best For	Beginners, high-rep work, learning pattern

Hands Behind Head

Aspect	Details
Setup	Hands clasped behind head or neck
Difficulty	Moderate
Best For	Intermediate, more challenge without weight

Arms Overhead

Aspect	Details
Setup	Arms extended straight overhead
Difficulty	Hardest (bodyweight)
Best For	Advanced, maximum bodyweight difficulty

By Loading

Bodyweight

Standard 90/90 back extension with no external load

Weighted (Plate)

Hold weight plate to chest or behind head — adds resistance

When to use: Can do 3x20+ bodyweight easily

Banded

Resistance band around neck anchored to floor — variable resistance

When to use: Home training or unique resistance curve

By Training Focus

Variation	Change	When to Use
Paused	2-3s pause at top	Maximum glute contraction
Tempo Eccentric	5s lowering	Build eccentric strength
Partial ROM	Reduced range	Rehab or building strength in specific range
Pulse Reps	Small pulses at top	Glute burnout finisher

---

📊 Programming

Rep Ranges by Goal

Goal	Sets	Reps	Rest	Load	RIR
Strength	3-4	10-12	90s	Bodyweight or weighted	2-3
Hypertrophy	3-4	12-20	60-90s	Bodyweight to moderate weight	1-2
Endurance/Rehab	2-3	20-30+	60s	Bodyweight	3-4

Workout Placement

Program Type	Placement	Rationale
Lower body day	Late in workout	After main compounds
Pull day	End	Posterior chain finisher
Rehab/Warm-up	First or last	Low-fatigue spinal erector activation
Core day	Middle	Posterior core work

Programming Note

90/90 back extensions are excellent as:

• Warm-up for deadlifts or squats (1-2 sets of 15, bodyweight)
• Accessory after main lifts (3-4 sets of 12-20)
• Finisher with high reps or pauses (2-3 sets to failure)

Frequency

Training Level	Frequency	Volume Per Session
Beginner	2-3x/week	2-3 sets, bodyweight
Intermediate	2-3x/week	3-4 sets, can add weight
Advanced	2-4x/week	3-4 sets, often as warm-up or accessory

Progression Scheme

Progressive Overload

Most people progress from arms crossed → hands behind head → arms overhead → weighted.

When you can do 3 sets of 20 with arms overhead, add weight (start with 5-10 lb plate).

Sample Progression

Week	Variation	Sets x Reps	Notes
1-2	Arms crossed	3x15	Establish baseline
3-4	Arms crossed	3x20	Build reps
5-6	Hands behind head	3x15	Progress difficulty
7-8	Hands behind head	3x20	Build reps
9+	Weighted or arms overhead	3x12-15	Continue progression

---

🔄 Alternatives & Progressions

Exercise Progression Path

Regressions (Easier)

Exercise	When to Use	Link
Bird Dog	Cannot do back extensions yet, learning spinal control
Superman	Floor-based spinal extension, very low load
Prone Cobra	Minimal range spinal extension, rehab focus

Progressions (Harder)

Exercise	When Ready	Link
Standard Back Extension	Want to involve hamstrings more
Weighted 90/90 Back Extension	Can do 3x20+ bodyweight easily
Reverse Hyperextension	Want to load hip extension without spinal flexion

Alternatives (Same Goal, Different Movement)

Spinal Erector Focus

Alternative	Benefit
45-Degree Back Extension	Standard variation, more hamstring
Romanian Deadlift	Loaded hinge, more functional
Good Morning	Barbell-loaded erector work

Glute Focus (Hamstring-Minimized)

Alternative	Benefit
Glute-Focused Back Extension	Similar concept, different setup
Hip Thrust	Pure glute, no spinal extension
Reverse Hyperextension	Glute emphasis, no spinal flexion

Rehab/Low-Load

Alternative	Benefit
Bird Dog	Floor-based, anti-rotation
Prone Cobra	Minimal load, thoracic extension
Dead Bug	Supine alternative, core stability

---

🛡️ Safety & Contraindications

Who Should Be Careful

Condition	Risk	Modification
Low back pain	Spinal flexion/extension may aggravate	Start with very small ROM, progress slowly
Disc issues	Repeated flexion/extension cycles	Get medical clearance, consider bird dog instead
Pregnancy	Prone position uncomfortable	Avoid after first trimester
Balance issues	Minimal concern — very stable exercise	Should be safe

Stop Immediately If

• Sharp pain in lower back
• Shooting pain down legs
• Muscle cramping in lower back
• Dizziness or lightheadedness

Injury Prevention

Strategy	Implementation
Don't hyperextend	Stop at neutral horizontal, no further
Controlled tempo	No bouncing or momentum
Proper knee angle	Maintain 90° bend throughout
Progress gradually	Add reps before adding weight
Listen to your body	Stop if lower back feels overworked

Spinal Safety

Key points:

• This exercise involves spinal flexion and extension — this is NORMAL and safe for healthy spines
• Do not hyperextend past neutral — unnecessary stress
• Controlled ROM prevents injury
• If you have disc issues, get clearance from medical professional

Spinal Flexion Note

Unlike loaded exercises (deadlifts, squats) where spinal flexion under load is dangerous, unloaded or lightly-loaded spinal flexion in back extensions is generally safe for healthy individuals. The load is minimal and the movement trains spinal erectors through full ROM.

However, if you have existing back issues, consult a professional.

---

🦴 Joints Involved

Joint	Action	ROM Required	Stress Level
Hip	Extension	0-30° extension	🟡 Moderate
Spine	Flexion/Extension	90° flexion to neutral	🟡 Moderate
Knee	Static flexed position	90° constant	🟢 Minimal

Mobility Requirements

Joint	Minimum ROM	Test	If Limited
Hip	Neutral to slight extension	Can extend hips to neutral when prone	Hip flexor stretches
Spine	Flexion and extension	Can round and flatten back	Gentle mobility work
Knee	Can hold bent position	Can maintain 90° knee bend	Should not be an issue

Joint Health Note

The 90/90 back extension is very joint-friendly:

• Low stress on knees (static position)
• Controlled spinal movement
• Minimal hip ROM requirements
• Excellent for building resilience in spinal erectors

Great for both beginners and advanced lifters.

---

❓ Common Questions

What's the difference between this and a regular back extension?

Knee position and muscle emphasis:

90/90 Back Extension (knees bent):

• Knees bent to 90°
• Hamstrings shortened and less active (~25%)
• More glute and erector emphasis (~80-90%)

Regular Back Extension (legs straight):

• Legs straight or nearly straight
• Hamstrings highly active (~70%)
• More balanced posterior chain work

Choose 90/90 when you want to isolate glutes and spinal erectors without hamstring fatigue.

Why would I want to minimize hamstring involvement?

Several reasons:

1. Glute isolation — You want to target glutes without hamstrings fatiguing first
2. Lower back focus — Building spinal erector strength/endurance
3. Hamstring fatigue — You already did heavy hamstring work and want posterior chain without more hamstring load
4. Rehab — Recovering from hamstring injury but can train glutes/erectors
5. Variety — Different stimulus for posterior chain

How do I know if my knees are at 90 degrees?

Visual check:

• Your lower leg (shin to ankle) should be perpendicular to the bench
• Heels pointing toward ceiling or slightly back toward glutes
• If someone looked from the side, your knee would form a right angle

Feel check:

• Your hamstrings should feel short/slack, not stretched
• You should NOT feel hamstrings working much during the movement

Practice holding the position before starting reps.

Should I feel this in my hamstrings at all?

Minimal hamstring activation is normal (~25%).

You should feel:

• Primary: Lower back/spinal erectors working hard
• Primary: Glutes squeezing at top
• Minimal: Light hamstring engagement

If you feel hamstrings working a lot, check that:

1. Knees are actually bent to 90° (not straighter)
2. You're squeezing glutes at the top
3. You're not using momentum

How high should I come up at the top?

Stop at neutral/horizontal — no hyperextension.

Your body should form a straight line from head to knees. Going beyond this (arching backward) puts unnecessary stress on your spine with no additional benefit.

Think: "Flat, not arched."

When should I add weight?

When you can do 3 sets of 20+ reps with perfect form and it feels easy.

Progression options:

1. Change arm position first (crossed → behind head → overhead)
2. Once overhead arms is easy for 3x15-20, add 5-10 lb plate
3. Progress weight slowly

Most people don't need much weight — bodyweight with arms overhead is plenty challenging.

---

📚 Sources

Biomechanics & Muscle Activation:

• ExRx.net Back Extension Analysis — Tier C
• Strength Training Anatomy — Frederic Delavier — Tier C
• EMG studies on bent-knee vs straight-leg back extensions — Various journals — Tier B

Programming:

• NSCA Essentials of Strength Training — Tier A
• Practical Programming — Rippetoe & Baker — Tier B

Rehabilitation:

• McGill, S. (2015). Back Mechanic — Tier A
• Low Back Disorders — Stuart McGill — Tier A
• Physical therapy protocols for spinal erector training — Tier B

Technique:

• Squat University — Dr. Aaron Horschig — Tier C
• Stronger by Science — Greg Nuckols — Tier B

---

For Mo

When to recommend this exercise:

• User wants to build glute and lower back strength without taxing hamstrings
• User has hamstring fatigue/soreness from other training
• User wants to isolate spinal erectors for hypertrophy or endurance
• User is rehabbing from hamstring injury (with clearance) but can train glutes
• User wants posterior chain accessory work with minimal equipment

Who should NOT do this exercise:

• Acute low back injury without clearance → Suggest bird dog or dead bug
• Severe disc issues without medical approval → Need clearance
• Pregnancy (after first trimester) → Prone position uncomfortable
• Cannot maintain 90° knee bend comfortably → Try standard back extension

Key coaching cues to emphasize:

1. "Keep your knees bent to 90 degrees the entire time — don't straighten them"
2. "Come up to flat/horizontal only — don't arch backward"
3. "Squeeze your glutes hard at the top"
4. "You should feel this in your lower back and glutes, NOT your hamstrings"

Common issues to watch for in user feedback:

• "I feel this all in my hamstrings" → Knees not bent enough or too much momentum
• "My back hurts" → Likely hyperextending; cue to stop at neutral
• "I don't feel my glutes" → Cue to actively squeeze glutes at top, pause
• "How many reps?" → Start with 3x12-15, build to 3x20+ before progressing

Programming guidance:

• Pair with: Hip thrusts, deadlifts (as finisher), leg curls
• Can use as warm-up for deadlifts: 2x15 bodyweight
• Typical frequency: 2-3x per week
• Place late in workout after main compounds

Progression signals:

• Ready to progress when: 3x20+ with perfect form feels easy
• Regress if: Cannot maintain knee bend or experiencing back pain
• Next step: Change arm position or add light weight

Red flags:

• Hyperextending past neutral → spinal stress, cue to stop at horizontal
• Knees straightening → becomes regular back extension, defeats purpose
• Sharp pain in lower back → stop immediately, assess form or rest

---

Last updated: December 2024