Muscle stiffness and spinal stretch reflex sensitivity in the triceps surae.
BACKGROUND
Greater musculotendinous stiffness may enhance spinal stretch reflex sensitivity by improving mechanical coupling of the muscle spindle and the stretch stimulus. This heightened sensitivity would correspond with a shorter latency and higher-amplitude reflex response, potentially enhancing joint stability.


OBJECTIVE
To compare spinal stretch reflex latency and amplitude across groups that differed in musculotendinous stiffness.


METHODS
Static group comparisons.


METHODS
Research laboratory.


METHODS
Forty physically active individuals (20 men, 20 women).


METHODS
We verified a sex difference in musculotendinous stiffness and compared spinal stretch reflex latency and amplitude in high-stiffness (men) and low-stiffness (women) groups. We also evaluated relationships between musculotendinous stiffness and spinal stretch reflex latency and amplitude, respectively.


METHODS
Triceps surae musculotendinous stiffness and soleus spinal stretch reflex latency and amplitude were assessed at 30% of a maximal voluntary isometric plantar-flexion contraction.


RESULTS
The high-stiffness group demonstrated significantly greater stiffness (137.41 +/- 26.99 N/cm) than the low-stiffness group did (91.06 +/- 20.10 N/cm). However, reflex latency (high stiffness = 50.11 +/- 2.07 milliseconds, low stiffness = 48.26 +/- 2.40 milliseconds) and amplitude (high stiffness = 0.28% +/- 0.12% maximum motor response, low stiffness = 0.31% +/- 0.16% maximum motor response) did not differ significantly across stiffness groups. Neither reflex latency (r = .053, P = .746) nor amplitude (r = .073, P = .653) was related significantly to musculotendinous stiffness.


CONCLUSIONS
A moderate level of pretension (eg, 30%) likely eliminates series elastic slack; thus, a greater change in force per unit-of-length change (ie, heightened stiffness) would have minimal effects on coupling of the muscle spindle and the stretch stimulus and, therefore, on spinal stretch reflex sensitivity. It appears unlikely that differences in musculotendinous stiffness influenced spinal stretch reflex sensitivity when initiated from a moderate level of pretension. Consequently, differences in musculotendinous stiffness did not appear to influence dynamic joint stability with respect to reflexive neuromuscular control.
Blackburn
J Troy
JT
University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-8605, USA. troyb@email.unc.edu
Padua
Darin A
DA
Guskiewicz
Kevin M
KM
D000328:Adult; D001132:Arm / Q000502:physiology*; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D009127:Muscle Rigidity / Q000503:physiopathology*; D009470:Muscle Spindles / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology*; D013131:Spine / Q000502:physiology*