What causes decreased flexibility and hamstring tightness in an adult patient?

Causes of Decreased Hamstring Flexibility

Decreased hamstring flexibility results from multiple mechanisms including muscle-tendon unit stiffness (viscoelastic properties), reduced stretch tolerance (pain threshold during stretching), and neuromuscular factors affecting muscle compliance, rather than actual muscle shortening in most cases.

Primary Mechanisms

Muscle-Tendon Unit Stiffness

• Passive stiffness of the hamstring muscle-tendon complex is the primary mechanical factor limiting flexibility 1
• This represents the viscoelastic properties of the muscle and tendon tissue itself, independent of neural factors 1
• Stiffness changes occur more rapidly than flexibility improvements—passive stiffness returns to baseline within 30 minutes after static stretching, while range of motion improvements persist longer 1

Stretch Tolerance (Pain Threshold)

• Stretch tolerance—the ability to withstand discomfort during stretching—is often the limiting factor rather than actual tissue length 2, 3
• Many individuals classified with "tight hamstrings" based on traditional testing actually have normal tissue extensibility but reduced pain tolerance during stretching 4
• Improvements in flexibility after stretching interventions frequently reflect increased stretch tolerance rather than changes in muscle stiffness 3

Neuromuscular Control Factors

• Muscle compliance (the inverse of stiffness) is influenced by neuromuscular control mechanisms 2
• "Apparent hamstring tightness" occurs when patients perceive tightness and test positive on clinical exams, but demonstrate immediate range of motion gains following neuromuscular interventions that don't address tissue length 4
• The sciatic nerve and its branches cross both hip and knee joints, making the hamstring vulnerable to neural tension that can limit flexibility 5, 6

Contributing Factors

Postural and Biomechanical Issues

• Thoracic kyphosis, increased anterior-posterior chest diameter, shoulder protraction, and trunk flexion alter body mechanics and can affect hamstring flexibility 6
• Postural deviations are associated with altered breathing mechanics and increased work of breathing, which may indirectly affect lower extremity flexibility 6

Musculoskeletal Conditions

• Progressive musculoskeletal deformities, scoliosis, and exaggerated lumbosacral lordosis can develop over time and affect hamstring flexibility 6
• Muscle weakness and gait disturbances may lead to compensatory tightness 6
• Orthopedic deformities of the feet and spine can alter biomechanics affecting hamstring length-tension relationships 6

Measurement Considerations

• Static flexibility measures (end range of motion) differ from dynamic-passive measures (stiffness/compliance) and dynamic-active measures (muscle contracted stiffness) 3
• Dynamic measures are less dependent on patient discomfort and more objective than static measures 3
• Traditional clinical tests (active knee extension, straight leg raise) may misclassify patients with apparent versus true hamstring tightness 4

Clinical Implications

Assessment Pitfalls

• Distinguishing between true tissue length restrictions versus stretch tolerance limitations is critical for appropriate treatment 3, 4
• The American Society of Anesthesiologists notes that while hamstring flexibility assessment is discussed for perioperative positioning, no consensus exists on specific assessment protocols 6
• Positions that stretch the hamstring beyond comfortable preoperative range may increase sciatic nerve injury risk during procedures 6, 5

Treatment Response Patterns

• Passive stiffness improvements from stretching are not maintained as long as changes in range of motion and stretch tolerance 1
• Muscle force decreases for 30 minutes after static stretching, which should be considered before activities requiring maximal strength 1
• Multiple bouts of static stretching provide superior acute and sub-acute improvements in range of motion compared to dynamic stretching 7
• Long-term static stretching effects are better than dynamic stretching for improving hamstring range of motion 7

Neuromuscular Factors

• Reactive neuromuscular training can produce immediate and sustained flexibility improvements in patients with apparent hamstring tightness 4
• This suggests central nervous system factors and motor control patterns significantly contribute to perceived tightness 4