Postural after-contractions in man attributed to muscle spindle thixotropy

Area of Science:

• Neuroscience
• Human Physiology
• Biophysics

Background:

• Non-volitional arm abduction movements, termed postural after-contractions, are an established phenomenon following forceful voluntary contractions.
• These movements are often accompanied by a sensation of arm lightness.

Purpose of the Study:

• To test the hypothesis that postural after-contractions result from tonic reflex responses.
• To investigate if enhanced resting discharge in primary muscle spindle afferents, due to thixotropy-dependent stiffness of intrafusal muscle fibers, underlies these after-contractions.

Main Methods:

• Experiments conducted on ten human volunteers.
• Utilized a conditioning procedure designed to mimic animal experiments effective in producing thixotropy-dependent excitation of primary spindle endings.
• Investigated the effect of intramuscular temperature changes on the strength of after-contractions.

Main Results:

• The arm abductor after-contraction phenomenon was most readily evoked by the specific conditioning procedure.
• Intramuscular temperature variations influenced the strength of after-contractions in a manner consistent with the thixotropy hypothesis.
• Similarities were noted between the after-contraction phenomenon and tonic reflex responses induced by muscle vibration.

Conclusions:

• The results support the hypothesis that postural after-contractions are induced by primary muscle spindle afferent activity.
• Thixotropic properties of intrafusal muscle fibers are identified as the cause of this enhanced afferent activity.
• Central excitability changes post-voluntary effort may also contribute to the phenomenon.