DOUBLE TWITCH CHIARACTERIST1CS OF THE TRICEPS SURAE MUSCLES-RELATIONSHIP TO MAXIMAL BOUNCE JUMPING
AbstractThe predominantly fast twitch gastrocnemius and the slower soleus muscle act in synergy around the ankle joint. Earlier studies have demonstrated the preferential involvement of soleus in postural tasks whereas gastrocnemius is more activated when demands of force and speed of contraction increase. The aim of this study was to relate mechanical characteristics and fatigue properties of the triceps surae muscles to the performance of jumping in subjects involved in sports with different demands on endurance andlor power. Eight physically active males with mixed background in sports participated in the study. Subjects performed maximal Counter Movement Jumps (CMJ) and maximal hopping on a force plate for 80 s with the instruction to maintain a minimal ground contact time. Electromyographic activity (EMG) was recorded with surface electrodes from soleus and medial head of gastrocnemius. Maximal twitch responses were recorded with the right foot tightly strapped to a mechanical device instrumented with a strain gauge. The tibia1 nerve in the popliteal fossa was electrically stimulated at supramaximal level with either a single or a double (2.5-200 Hz) pulse (1 ms pulse duration). First sign of fusion between consecutive muscle twitches was seen between 4-5 Hz. Maximal twitch fusion force ranged 1.89-2.31 of the single twitch peak force for the different subjects. Subjects with a high degree of fusion at 200 Hz stimulation (5 ms between stimuli) showed a higher degree of fatiguability (decrease in hopping height) during the test (r=-0.73, pc0.05). EMG level of the soleus and gastrocnemius muscles remained similar in the preactivation and eccentric phases of contraction during 80 s of maximal hopping. However, in the concentric contraction phase there was a marked decrease in EMG level for both SOL and MG (to 59217% and 59220% of initial value, respectively). This was parallelled by a significant decrease in hopping height (to 74215% of initial height). Mean values of maximal jumping height was 0.2h0.05 m (range 0.17-0.30 m) during hopping and 0.3820.04 m (0.32-0.45 m) during CMJ. A significant relationship was found between the relative amount of preactivation of the soleus muscle and hopping height (r=0.92 pc0.01), i. e. lower hopping subjects showed the least amount of preactivation of the soleus muscle. The corresponding relationship for the gastrocnemius muscle was not significant. Subjects involved in long term endurance training demonstrated a relative inability to preactivate the soleus muscle during maximal hopping. They also showed a lower degree of fusion of the muscle twitch in a double twitch stimulus protocol. These results support the notion of task specificity in the activation of muscles within the triceps surae synergy and further imply that training history may influence this activation pattern.
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