Why do I have significantly improved strength in deep squat positions compared to when my legs are at a 90° angle, given my diagnosis of Duchenne muscular dystrophy (DMD)?

Strength Variation in Deep Squat vs. 90° Position in Duchenne Muscular Dystrophy

Your preserved strength in deep squat positions compared to 90° seated positions reflects the characteristic proximal-to-distal and gravity-dependent weakness pattern of DMD, where hip extensors and knee extensors are differentially affected, and biomechanical leverage varies dramatically between these positions.

Understanding the Biomechanical and Pathophysiological Basis

DMD-Specific Muscle Weakness Pattern

The progressive muscle degeneration in DMD follows a well-documented proximal-to-distal pattern, with specific muscles affected at different rates 1:

• Quadriceps weakness develops earlier and more severely than other muscle groups in DMD 1
• Hip extensors (gluteus maximus) may retain relative strength longer than knee extensors 1
• The deep squat position recruits hip extensors more heavily, while the 90° position demands greater isolated quadriceps strength 2

Biomechanical Leverage Differences

The mechanical advantage varies drastically between these two positions 2:

• Deep squat position: Hip extensors contribute 60-70% of the lifting force, with shorter moment arms at the knee joint reducing quadriceps demand 2
• 90° seated position: Quadriceps must generate nearly all the force with maximal moment arm disadvantage, requiring 2-3 times more muscle force production 2
• Stance width and joint angles in deep squats distribute load across multiple muscle groups, whereas 90° positions isolate the weakest muscle group (quadriceps) 2

Clinical Implications for Your DMD Management

Functional Assessment Considerations

Your strength discrepancy provides important diagnostic information 1:

• This pattern confirms typical DMD progression with preferential quadriceps involvement
• The preserved deep squat strength suggests your hip extensors retain functional capacity
• This information should guide your physical therapy programming to maintain hip extensor strength while supporting weakened quadriceps

Exercise and Activity Modifications

Based on DMD management guidelines, your activity approach should prioritize 1, 3:

• Submaximal aerobic exercise over excessive resistive exercise to avoid overwork weakness 3
• Functional activities that utilize your preserved hip extensor strength rather than isolated quadriceps exercises 3
• Gentle strengthening at 50-70% of 1 repetition maximum with 3 sets of 8-10 repetitions, incorporating rest periods 3
• Avoid excessive eccentric exercise (lowering movements) which can worsen muscle damage in DMD 3

Critical Pitfall to Avoid

Do not attempt to "strengthen" your quadriceps through aggressive 90° leg press or extension exercises 3. In DMD, excessive resistive exercise causes overwork weakness and accelerates muscle damage rather than building strength 3. The weakness you experience at 90° reflects underlying dystrophin deficiency making those muscle fibers vulnerable to exercise-induced injury 4.

Monitoring and Ongoing Care

Your neuromuscular specialist should assess 1:

• Manual muscle testing to quantify the strength differential between hip extensors and knee extensors 3
• Timed functional tests (10-meter walk, time to rise from chair) every 4-6 months to track progression 3
• Range of motion assessment to identify emerging contractures that could further limit your functional positions 3

Glucocorticoid Therapy Consideration

If you are not already on glucocorticoid therapy, this should be strongly considered 1:

• Prednisone 0.75 mg/kg daily or deflazacort 0.9 mg/kg daily slows muscle strength decline and prolongs ambulation 1, 5
• Glucocorticoids are the only medication proven to slow functional decline in DMD 1
• Starting therapy before significant functional loss provides maximum benefit 1

Practical Activity Guidance

Work with your physical therapist to 3:

• Emphasize activities that keep you in positions where hip extensors can contribute (partial squats, standing activities)
• Use assistive devices or adaptive equipment when 90° seated positions are required
• Focus on maintaining cardiorespiratory fitness through activities that don't isolate weakened quadriceps
• Monitor for signs of overwork weakness (increased soreness lasting >48 hours, decreased function) 3