Heavy-load resistance exercise is not normally associated with stretch shortening muscle activation. Rapid lengthening, and a decreased period of time between lengthening and shortening phases of contraction (amortisation phase) has been associated with increased energy transfer from passive muscle structures (series elastic component, SEC), and gains in muscle efficiency during stretch shortening muscle activation. In this investigation, we sought to determine the effect of limb velocity on the amortisation phase and electromyographic (EMG) activity during heavy load (85%1RM) resistance exercise. Twelve healthy males performed 6RM unilateral elbow flexion-extension exercise during; rapid lengthening-shortening, rapid shortening and slow lengthening-shortening muscle activation. The amortisation phase and velocity of lengthening and shortening were determined using a shaft encoder and muscle activity was recorded at the biceps brachii using intramuscular and surface EMG. A decrease (P<0.05) in time spent in amortisation was observed during rapid lengthening-shortening (0.10±0.00 ms), compared to rapid shortening (0.14±0.00 ms) and slow lengthening-shortening (0.19±0.00 ms). Intramuscular EMG was ∼25% (P<0.05) greater during the first, second and third repetition of rapid lengthening-shortening exercise compared to the other two conditions. A trend towards increased surface EMG activity was also noted during rapid lengthening-shortening, however significance (P<0.05) was only observed in the second repetition. In this investigation, the characteristics typical of a stretch shortening muscle activation were observed alongside elevated electromyographic activity during heavy-load rapid lengthening-shortening resistance exercise.
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