Isotonic resistance exercise outperforms eccentric quasi-isometric resistance exercise for increasing elbow flexor muscle thickness and estimated one-repetition maximum in untrained individuals: exploring the influence of sex and volume

As a novel, low-velocity resistance exercise method, eccentric quasi-isometric resistance exercise (EQI-RE) results in greater time under tension than traditional isotonic resistance exercise (TRD-RE) and is surmised to increase muscle mass and strength. However, females may be more fatigue-resistant than males when performing acute EQI-RE, which could lead to long-term differences in time under tension and resistance exercise volume. At present, studies have yet to compare muscle hypertrophy or strength improvements following TRD-RE and EQI-RE training, and whether sex differences exist in these outcomes. Twenty-two (<i>n</i> = 13 females) untrained individuals completed ∼8 weeks of effort matched unilateral TRD-RE and EQI-RE of the elbow flexors. Muscle thickness and estimated one-repetition maximum (E-1RM) were evaluated before and after training. TRD-RE produced significantly larger relative increases in muscle thickness (6.7% ± 3.9% vs. 4.0% ± 3.3%, <i>p</i> = 0.004) and E-1RM (19.6 ± 8.5% vs. 12.8 ± 6.2%, <i>p</i> = 0.001) than EQI-RE. Although females accrued greater resistance exercise volume than males across the TRD-RE and EQI-RE training, there were no relative sex differences in muscle thickness or E-1RM improvements (<i>p</i> > 0.25). Sex differences in fatiguability may therefore manifest in differences in resistance exercise volume between males and females after 8 weeks of TRD-RE and EQI-RE of the elbow flexors, but this does not lead to relative differences in muscle thickness or E-1RM improvements. Although EQI-RE did produce significant increases, TRD-RE of the elbow flexors appears more effective at increasing muscle thickness and E-1RM.