PRACTICAL SIGNIFICANCE
This study revealed that differences exist between sexes in spinal control of movement as well as the activation of the neuromuscular system. This information can be used to develop interventions and training programs.

STUDY BACKGROUND
Females’ display different lower extremity movement patterns, particularly at the hips and knees, when compared to males.  These movement patterns potentially put females at increased risk for lower extremity injury.  However, the underlying neural mechanisms controlling these movement patterns are unknown.  Specifically, differences in motor neuron excitability, intrinsic and extrinsic pre-synaptic inhibition, postsynaptic inhibition, and supraspinal drive between the sexes are unidentified.  Additionally, sex differences in rate of torque development and electro-mechanical delay are not fully elucidated.

OBJECTIVE
To determine if sex differences exist between spinal control mechanisms and functional neuromuscular variables.

DESIGN AND SETTING
A between-subjects design was used to compare differences in rate of H-reflex excitability, rate of intrinsic pre-synaptic inhibition (IPI), rate of extrinsic pre-synaptic inhibition (EPI), level of postsynaptic recurrent inhibition (RI), level of supraspinal neural drive (V-wave), maximum rate of torque development (RTD), and electromechanical delay (EMD) between healthy active males and females.  The study took place in a controlled laboratory setting.

SUBJECTS
Nineteen males (age = 23.0 ± 4.3yrs, height = 177.45 ± 5.44cm, mass = 77.52 ± 13.18 kg) and 18 females (age = 24.7 ± 2.9yrs, height = 165.31 ± 5.85cm, mass = 62.44 ± 8.76 kg) healthy, physically active individuals participated in the study,

MEASUREMENTS
Participants were seated on the chair of the Biodex System 3 dynamometer in a semi-recumbent position with the knee flexed to 60 degrees and ankle in anatomical position.  All subsequent testing was performed in this position. Hreflex recruitment curves of the soleus were elicited at the tibial nerve.  IPI recruitment curves were obtained by utilizing paired pulses.  Two stimuli of the same intensity and an interstimulus interval of 100 ms were given to the tibial nerve.  To obtain a EPI recruitment curves, the H-reflex was conditioned by stimulating the tibialis anterior (50% TA Mmax) 100 ms prior to stimulating the soleus muscle.  The first derivative of all three recruitment curves was calculated.  RI was determined by the difference between S1 alone, which was 25% of soleus Mmax, and S1 conditioned by S2, which was set to Mmax.  Ten trials of each were completed.  V-waves were elicited by a supramaximal stimulation to the tibial nerve once 90% of maximum isometric plantarflexion torque was reached.  Five trials with 60 seconds rest were obtained.  To measure soleus RTD and EMD participants were instructed to isometrically plantarflex the ankle as fast and as hard as they could.  Three trials with 60 seconds rest were measured.  EMD was determined by calculating the time between the onset of soleus EMG activity and onset of soleus torque production.  RTD was calculated by determining the slope of the torque-time curve from the onset of torque production to the maximal torque production.

RESULTS
The Wilks Lambda multivariate test revealed differences between sexes on the linear combination of motor neuron pool and functional neuromuscular variables (P = 0.001).  Follow-up univariate tests revealed that males had significantly greater RI (males = 0.86 ± 0.21, females = 0.68 ± 0.30; P = 0.042; See Figure 1).  Males also had greater RTD (males = 387.93 ± 180.90 n·m·s-1, females = 263.89 ± 85.15 n·m·s-1; P= 0.033; See Figure 2).  No other univariate tests were significant.

CONCLUSIONS
The sexes differ on modulation of spinal control of movement and activation of the neuromuscular system.  Males were able to produce torque more quickly than females.  This is potentially important during injurious situations when rapid activation of the muscle is needed due to the length of time needed to attain maximal torque.  Additionally, RI was greater in males. RI is considered to be a variable gain control by modulating motor unit firing frequency. Both of these variables are modifiable with training.  However, no differences were observed between the sexes on motor neuron excitability or presynaptic inhibition.

Figure 1: Recurrent inhibition between the sexes
Males demonstrated significantly more recurrent inhibition compared to females (P = 0.042).

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