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Changes in reflex excitability following isometric contraction in humans
A Gollhofer1, A Schöpp, W Rapp
1Department of Sport Science, University of Stuttgart, Germany.
European Journal of Applied Physiology and Occupational Physiology
|February 12, 1998
Summary
Post-isometric reflex modulation occurs with both electrical and mechanical stimulation. Mechanical stimulation shows faster recovery, suggesting it may not be solely due to inhibitory reflex mechanisms for efficient stretching.
Area of Science:
- Neuroscience
- Human Physiology
- Biomechanics
Background:
- Muscle stretch responses post-isometric contraction are traditionally attributed to inhibitory reflex mechanisms.
- Electrical stimulation (H-reflex) shows maximal suppression during force relaxation, with gradual recovery over 20 seconds.
- It remains unclear if mechanical and electrical stimulation elicit similar post-contraction reflex behaviors.
Purpose of the Study:
- To investigate and compare post-isometric reflex modulation following both mechanical and electrical stimulation modalities.
- To determine the influence of contraction intensity and stimulus parameters on reflex responses.
- To elucidate the underlying mechanisms of post-isometric reflex modulation.
Main Methods:
- Ten subjects performed isometric plantarflexion at 30% and 60% maximal voluntary contraction (MVC).
- Dorsiflexion stimuli (varying speed and amplitude) were applied to evoke mechanical responses.
- Electrically evoked responses (H-reflex) were measured concurrently.
- Modulation of mechanically and electrically evoked responses during force relaxation was analyzed.
Main Results:
- Both mechanical and electrical stimulation resulted in depressed reflex responses during force relaxation.
- Mechanical stimulation exhibited a significantly faster recovery (<400 ms) compared to electrical stimulation, which showed delayed recovery.
- Increasing stretch velocity enhanced the magnitude of mechanical responses without altering the modulation shape.
- Response modulation was independent of contraction intensity and stimulus amplitude.
Conclusions:
- Post-isometric reflex modulation is likely influenced by presynaptic inhibition and potentially peripheral mechanisms like alpha-gamma coactivation affecting the stretch receptor.
- Differences in recovery kinetics between mechanical and electrical stimulation suggest distinct underlying mechanisms.
- The rapid recovery of stretch responses contradicts the notion that reflex suppression post-contraction aids in efficient muscle stretching.