Salo Kristiina 2013. Neuromuscular adaptation of conscripts during an 8-week military basic training period in Finnish winter conditions. Department of Biology of Physical Activity, University of Jyväskylä. Master’s Thesis of Biomechanics, 70 p.

The purpose of this study was to evaluate the development of neuromuscular adaptation, whether aerobic capacity (VO2max) is related to the neuromuscular adaptation and possible overreaching of the neuromuscular system during 8 weeks of basic training (BT) in winter conditions. 24 male conscripts (18-21 years) participated in the study. They were divided into three groups (G1 good; G2 moderate; G3 low) based on their VO2max values at the beginning of BT. Good > 46.0 ml/kg/min for group 1; moderate 40.0 -45.9 ml/kg/min for group 2; low < 39.9 ml/kg/min for group 3.

H–reflex (Hmax/Mmax) at rest and V–wave (V/Mmax) during maximal isometric plantar flexion were measured from the soleus muscle in the beginning, after 5 weeks and at the end of 8 week long BT. Maximal leg extension force (MVC) and rapid force production as a peak force (0-500ms) were measured from quadriceps muscle group during maximal isometric leg bench press at the same time points. Muscle activity was measured in the same leg bench press with EMG signals from Vastus Lateralis and Vastus Medialis muscles. Passive twitch response was measured from Triceps Surae muscle.

The major finding of this study was the differences observed in the neural activity (H–reflex and V–wave) in conscripts of different aerobic fitness level. This was seen especially in V-wave responses which were significantly lower in the poor aerobic capacity group during the entire BT period. H–reflex was significantly higher in G1 after 5 weeks of training compared to both G2 and G3 (G2 -39% p < 0.01, G3 -32% p < 0.05). H-reflex increased significantly only within G2 and during the last 3 weeks of training (38 % p < 0.05). V–wave was significantly lower in G3 compared to G1 in the beginning, after 5 weeks and at the end of BT (from -40 % to -57 % p < 0.01 - 0.05). There were no significant differences between the groups or changes in any of the groups in leg extension MVC or in rapid force production during the basic training. Passive twitch showed significantly higher values in G1 than in G2 during the fifth week of training (-28% p < 0.05). aEMG500 / aEMGmax – ratio from right leg extension showed significant findings within G3 between training weeks 1 and 5 (-35 % p < 0.05). The aerobic capacity measures (VO2max) showed a rising development for all groups during the entire basic training period. Statistically significant differences of VO2max were observed in groups G1 and G2 which were significantly higher compared to group G3 after 5 weeks (G1 -27% p < 0.01, G2 -20% p < 0.01) and after 8 weeks (G1 -25% p < 0.01, G2 -21% p < 0.01).

In conclusion significantly better H – reflex response for good level conscripts indicates weaker neuromuscular adaptation of those with poor in aerobic fitness level. This conclusion is also supported by the higher V – wave responses of the good level conscripts versus low in every week the measurements were made. Weaker central activity can lead to overall decrease in performance and therefore even to interruptions from military training. Certain type of training could improve these results while even 3-4 weeks of strength training has been shown to increase the neuromuscular responses. Passive twitch showed no significant change which indicates the muscle properties did not weaken but did not develop either. There were also no signs of overreaching in any group based on the measured force production and aerobic capacity values.

Keywords: Aerobic capacity, neuromuscular fatigue, overreaching