Surface EMG Characterization of the Jefferson Curl at a Submaximal Load Matched to the Romanian Deadlift: A Cross-Sectional Study in Resistance-Trained Adults

Background The Jefferson curl, a loaded spinal flexion exercise, has gained popularity in strength and rehabilitation practice but lacks empirical characterization. Whether it elicits different posterior-chain activation than the Romanian deadlift (RDL), a well-documented hip-hinge exercise, at a matched submaximal load is unknown. Hypothesis/Purpose: To characterize erector spinae activation during the Jefferson curl at a submaximal load and test the hypothesis that, at the same absolute load, the Jefferson curl elicits greater erector spinae activation than the RDL; and to describe gluteus maximus and biceps femoris activation without a directional hypothesis. Study Design Cross-sectional observational study. Methods Twenty-one resistance-trained adults performed five repetitions of the Jefferson curl and of the RDL on a 30-cm platform. Load was adjusted to a Borg RPE of 16 during the Jefferson curl, and the same absolute load was used for the RDL. Surface EMG was recorded from the upper, middle, and lower erector spinae, gluteus maximus, and biceps femoris bilaterally and normalized to maximal voluntary isometric contraction (%MVIC). Left and right values were averaged. A linear mixed-effects model with exercise*muscle fixed effects and participant as a random intercept was used. Benjamini-Hochberg procedure controlled the false discovery rate. Results The Jefferson curl elicited higher mean activation than the RDL in the upper erector spinae (62.84 vs 44.52 %MVIC, adjusted p=0.003, Hedges’ g=0.83), lower erector spinae (60.07 vs 47.85 %MVIC, adjusted p=0.035, g=0.49), and biceps femoris (54.18 vs 41.54 %MVIC, adjusted p=0.035, g=0.60). The middle erector spinae and gluteus maximus activation did not differ significantly (adjusted p≥0.45). Conclusion In resistance-trained adults, the Jefferson curl at a submaximal load produced greater upper- and lower-erector spinae and biceps femoris activation than the RDL at the same absolute load. The Jefferson curl can be considered when designing targeted recruitment of the erector spinae and hamstrings through controlled spinal flexion. Level of Evidence 3.