Changes in the contractile properties of motor units in functionally overloaded medial gastrocnemius muscle of the rat

The study aimed at examining the contractile properties of motor units (MUs) in medial gastrocnemius muscle subjected to 3 months compensatory overload, induced by bilateral tenotomy of its synergists (lateral gastrocnemius, plantaris and soleus). To assure that operated muscles were regularly voluntarily activated, surgical procedures were followed by keeping animals in wheelequipped cages and treadmill exercise. The final electrophysiological experiments were carried out on 106 MUs of the overloaded medial gastrocnemius muscle (OMG) and 88 MUs of the untreated, healthy gastrocnemius muscle (MG). Functionally isolated MUs were examined by electrical stimulation of thin filaments of the L4 – L5 ventral roots. MUs classification was based on 20 Hz tetanus index which divided MUs into fast and slow, whereas the fatigue index distinguished fast fatigable (FF) and fast resistant (FR) MUs. Results showed that both OMG mass and its relation to the body weight were higher in comparison to MG. MUs proportion was modified in response to altered functional demand and in OMG higher percentage of S and FF simultaneously with lower contribution of FR MUs were observed. Changes in MUs contractile properties of OMG in comparison to MG included: shorter half-relaxation time and lower twitch force in all types of MUs and in parallel higher (FR and S) or unchanged (FF) tetanus force. Changes in force parameters in OMG resulted in lower values of the twitch-to-tetanus ratio in all three types of MUs. Higher posttetanic potentiation was also noted for all MUs types in OMG. Changes in fatigue resistance were observed only in fast MUs: for FF type the mean value of the fatigue index was lower in OMG in comparison to MG, but for FR type this value was higher in OMG. In conclusion, the adaptation of the medial gastrocnemius muscle to overload included transformation of some MUs accompanied by changes in MUs contractile properties.

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