31/12/2025
https://www.facebook.com/share/184Pms5fWe/
🧩 𝐓𝐡𝐞 𝐌𝐲𝐬𝐭𝐞𝐫𝐲 𝐨𝐟 𝐭𝐡𝐞 𝐒𝐭𝐫𝐞𝐭𝐜𝐡-𝐒𝐡𝐨𝐫𝐭𝐞𝐧𝐢𝐧𝐠 𝐂𝐲𝐜𝐥𝐞: 𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐜𝐚𝐥 𝐨𝐫 𝐍𝐞𝐮𝐫𝐚𝐥?
■ In human movement, muscles often perform better when they are actively stretched immediately before they shorten. This phenomenon is known as the Stretch-Shortening Cycle (SSC).
■ A common example is the way we naturally dip down (stretch) before jumping up (shorten).
■ This sequence produces higher force, work, and power compared to a shortening movement that starts from a standstill—a boost known as the SSC effect.
■ While the existence of the SSC effect is well-documented, the exact physiological reasons behind it are debated.
■ Scientists generally group the causes into two categories:
■ ⚙️ Mechanical factors: Changes within the muscle fibers themselves, such as elastic energy return or the engagement of the protein titin.
■ 🧠 Neural factors: Changes in the nervous system, such as reflexes or increased excitability in the spinal cord and motor cortex.
■ A 2025 study by Rissmann et al. sought to determine if the brain and spinal cord modulate their excitability during the shortening phase of the SSC to contribute to this performance boost.
🔬 The Study Design
■ The researchers studied the plantar flexor muscles (calf muscles) of 18 healthy adults.
■ Participants performed two types of movements on a dynamometer, carefully matched to have the same level of muscle activity (EMG):
■ ➡️ Pure Shortening (SHO): The muscle shortened without a prior stretch.
■ 🔁 Stretch-Shortening Cycle (SSC): The muscle was actively stretched immediately before shortening.
■ To measure neural excitability during these movements, the researchers used advanced stimulation techniques:
■ 🧠 Transcranial Magnetic Stimulation (TMS): To measure cortical excitability (how responsive the motor cortex in the brain is).
■ 🧠 Cervicomedullary Electrical Stimulation (CES): To measure spinal excitability (how responsive the spinal cord is).
■ 📈 Electromyography (EMG): To detect stretch reflexes.
📊 Key Findings
■ 1. The SSC Effect is Real but Not Neural
■ As expected, the participants produced significantly more torque (about 12% more) during the SSC contractions compared to the pure shortening contractions.
■ However, when the researchers looked at the nervous system, they found no corresponding boost:
■ 🚫 No Cortical Change: There was no difference in cortical excitability between the SSC and pure shortening conditions.
■ 🚫 No Spinal Change: Similarly, spinal excitability remained unaltered during the shortening phase of the SSC.
■ 🚫 Absent Reflexes: The researchers found almost no stretch reflex activity during the active stretch phase (observed in only 1 out of 15 participants).
■ 2. The Mechanism is Mechanical
■ Because the performance increased (the 12% boost) while the neural drive from the brain and spine remained constant, the authors concluded that the SSC effect in this context is driven by mechanical mechanisms rather than neural ones.
■ This supports the theory that intrinsic properties of the muscle sarcomeres—such as "residual force enhancement" (rFE) and altered cross-bridge kinetics—are responsible for the extra power.
■ 3. Residual Force Depression (rFD)
■ The study also examined what happened after the movement, during a steady isometric hold.
■ Muscles typically produce less force after shortening, a phenomenon called Residual Force Depression (rFD).
■ The study found that steady-state torque was significantly lower following the SSC compared to the reference contractions.
■ Interestingly, just like the shortening phase, this depression in force was not correlated with any changes in cortical or spinal excitability.
🧠 Conclusion
■ The results of this study indicate that the performance benefits of the stretch-shortening cycle—at least during submaximal, controlled movements—are not associated with modulations in cortical or spinal excitability.
■ The nervous system does not appear to "ramp up" its responsiveness to facilitate the movement.
■ Instead, the boost is derived from mechanical factors triggered during the active stretch, which persist to enhance force during the subsequent shortening phase.
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