Strength-Duration (SD) Curve in Electrotherapy
- 1. Prepared by Dr. SOMA BALAJI PT MSK & SPORTS Strength-Duration (SD) Curve An Electrodiagnostic Tool in Electrotherapy
- 2. • Understand the concept and physiology behind the SD curve • Learn how to plot and interpret the SD curve • Differentiate between normal and abnormal curves • Explore clinical uses and electrodiagnostic relevance • Identify limitations and precautions Learning Objectives
- 3. • SD Curve is a graphical representation of the relationship between: • Intensity (strength) of current and • Duration (time) required to elicit a muscle contraction • Important in assessing nerve and muscle excitability • Commonly used in denervation testing Introduction
- 4. • Motor nerve or muscle fibers need a certain minimum intensity and duration of current to depolarize and cause contraction • With increased duration, required intensity decreases (up to a limit) • Based on rheobase and chronaxie values Physiological Basis
- 5. • Definition: Minimum current intensity required to produce a minimal muscle contraction using a long-duration stimulus (usually 1000 ms) • Unit: Milliamps (mA) • Indicates nerve or muscle excitability Rheobase
- 6. • Definition: Minimum duration of current needed to cause a muscle contraction when twice the rheobase current is applied • Unit: Milliseconds (ms) • Clinical interpretation: • <1 ms = normal • 1 ms = suggests partial or complete denervation Chronaxie
- 7. “A strength-duration curve is a plot of the minimum current strength (intensity) required to elicit a threshold response (contraction) against the duration of stimulus applied.” SD Curve – Definition
- 8. • SD curve tester or diagnostic stimulator • Surface electrodes • Stopwatch/timer • Graph paper/software • Patient in relaxed position Equipment Required
- 9. 1.Place electrodes over muscle motor point 2.Apply rectangular pulses at varying durations (1 ms, 0.5 ms, 0.1 ms… down to 0.01 ms) 3.Adjust current strength for each duration to just elicit contraction 4.Record threshold intensity for each duration 5.Plot values on graph: X-axis = duration, Y-axis = strength Procedure
- 10. • Gradual curve • Lower intensity needed with increasing duration • Chronaxie <1 ms • Shape: exponential downward slope Normal SD Curve
- 11. 1.Complete Denervation: • No response to short-duration stimuli • Only long-duration and high-intensity pulses elicit response • Chronaxie >10 ms • Shape: Almost flat at short durations Abnormal SD Curves
- 12. 2. Partial Denervation: • ‘Kink’ or ‘notch’ seen in curve • Mixed innervated and denervated fibers • Chronaxie mildly elevated
- 13. Interpreting the Curve Curve Type Rheobase Chronaxie Diagnosis Normal Normal < 1 ms Healthy nerve Partial Denervation Normal or ↑ > 1–10 ms Partial nerve injury Complete Denervation ↑↑ > 10 ms Severe axonal damage
- 14. • Diagnose nerve injuries (neuropraxia, axonotmesis, neurotmesis) • Assess denervation and reinnervation • Select appropriate electrical stimulation parameters • Monitor progress in nerve regeneration • Guide rehabilitation planning Clinical Uses of SD Curve
- 15. • Stimulated using long-duration pulses (100–1000 ms) • Response takes 2–4 weeks post-injury to appear • Helps decide whether faradic or galvanic stimulation is needed SD Curve in Denervated Muscle
- 16. • Rheobase: Minimum intensity for long pulse (1000 ms) • Chronaxie: Duration required with 2× rheobase intensity • Accommodation: Nerve’s ability to adapt to slow rise in intensity • Non-accommodating tissues: Denervated muscle fibers Key Terms
- 17. • Non-invasive • Early identification of nerve injuries • Quantitative data for diagnosis • Tracks recovery process • Guides electrotherapy dosage Advantages of SD Curve
- 18. • Not reliable in early stages (<2 weeks of nerve injury) • Not useful in upper motor neuron lesions • Requires patient cooperation and skilled examiner • Subjective visual assessment of contraction Limitations
- 19. • Digital SD curve plotting with EMG integration • Software-assisted interpretation • Use in robotic neuro-rehabilitation planning • Integration with portable diagnostic stimulators Recent Advances
- 20. SD Curve vs EMG Parameter SD Curve EMG Focus Muscle excitability Electrical activity Cost Low High Skill needed Moderate High Interpretation Visual/manual Complex (waveforms) Use Peripheral lesions Both central & peripheral
- 21. • Patient: 30-year-old male with radial nerve injury • SD curve plotted at 3 weeks • Chronaxie = 4.5 ms Partial denervation → • Treatment: Galvanic stimulation + monitoring • Follow-up curve showed improvement Case Study
- 22. Viva / Exam Questions
- 24. What is the significance of a ‘kink’ in the SD curve?
- 25. Why is SD curve not useful in UMN lesions?
- 26. Draw and label a normal SD curve.
- 27. What are the clinical uses of the SD curve?
- 28. References • Jagmohan Singh. Textbook of Electrotherapy • Kahn & Low. Electrotherapy Explained • Nussbaum E. (2020). Evidence-Based Electrotherapy • PubMed articles on electrodiagnostic testing and SD curve • APTA Clinical Guidelines (Electrotherapeutics)
- 29. Thank You