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Differential Back Muscle Flexion-Relaxation Phenomenon in Constrained versus Unconstrained Leg Postures.

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Summary

Leg posture significantly impacts the flexion-relaxation phenomenon (FRP) in back muscles. Constrained leg posture and low flexibility amplify FRP, while flexible individuals show less FRP with unconstrained legs.

Keywords:
flexibilityflexion–relaxation phenomenonleg posturemuscle activitytrunk flexion

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Area of Science:

  • Biomechanics
  • Human Movement Science
  • Musculoskeletal Physiology

Background:

  • The flexion-relaxation phenomenon (FRP) is crucial for understanding back muscle function during trunk flexion.
  • Previous FRP studies show inconsistent results, often attributed to uncontrolled leg postures.
  • Individual flexibility is a key factor influencing muscle responses during movement.

Purpose of the Study:

  • To investigate how leg posture control and individual flexibility affect FRP in back and lower limb muscles.
  • To analyze the interplay between erector spinae, biceps femoris, and gastrocnemius muscle activity during trunk flexion.
  • To determine the correlation between lumbosacral angle (LSA) and FRP under varying conditions.

Main Methods:

  • Recruited 32 male participants, divided into high- and low-flexibility groups.
  • Measured erector spinae, biceps femoris, and gastrocnemius muscle activity during 0°-90° trunk flexion.
  • Recorded lumbosacral angle (LSA) under constrained and unconstrained leg posture conditions.

Main Results:

  • Leg posture and flexibility significantly influenced erector spinae activity, especially at 60° flexion.
  • Low flexibility with constrained legs showed the highest FRP; high flexibility with unconstrained legs showed the least.
  • Constrained leg posture increased gastrocnemius activity by 33-50% (30°-90° flexion) while biceps femoris remained stable.

Conclusions:

  • Inappropriate leg posture during FRP assessments can lead to overestimations of the phenomenon.
  • Flexibility and leg posture are critical variables that must be controlled in future FRP research.
  • Understanding these factors enhances the accuracy of biomechanical analyses of the spine and associated musculature.