Effects Of Very Short-term Isokinetic Training On Peak Torque, Muscle Thickness, And Co-activation In Women

Document Type

Article

Publication Title

Medicine & Science in Sports & Exercise

Abstract

The early-phase increases in strength following resistance training (within 2-4 days) are primarily attributed to neural adaptations rather than hypertrophy. There is limited research, however, on whether decreases in antagonist co-activation (Co-Act) may contribute to these gains in strength.

PURPOSE: The present study examined the effects of 4 days of isokinetic forearm flexion (FF) and forearm extension (FE) training at 180°·s-1 in untrained women on FF and FE peak torque (PT); biceps brachii (BB) and triceps brachii (TB) muscle thickness (MT); and FF and FE electromyographic (EMG) Co-Act.

METHODS: Ten untrained women (Mean ± SD age = 21.6 ± 1.0 years; height = 168.7 ± 4.9 cm; body mass = 65.2 ± 6.8 kg) completed 4 days of maximal, reciprocal, concentric, isokinetic FF and FE training with 6 sets of 10 repetitions at 180°·s-1 of the non-dominant arm. The subjects also completed a pre-training (PRE) and post-training (POST) visit that consisted of Ultrasounds for BB and TB MT; FF and FE PT at 180°·s-1; and FF and FE EMG Co-Act during the PT repetition. The EMG amplitudes were normalized to their respective PT repetition, and EMG Co-Act was expressed as a percentage of antagonist divided by agonist. Separate two-way repeated-measures ANOVAs were used to assess changes in PT and EMG Co-Act (Movement × Time), as well as MT (Muscle × Time). Post-hoc pairwise comparisons with Bonferroni corrections were used when appropriate. An alpha of p ≤ 0.05 was used to determine statistical significance.

RESULTS: There was no significant (p = 0.494) interaction for PT, but there was a significant (p = 0.040) main effect for Time collapsed across Movement, which indicated a 15.1% increase from PRE (19.53 ± 5.45 N⋅m) to POST (22.47 ± 5.93 N⋅m). There was no significant (p = 0.743) interaction or main effect of Time (p = 0.106) for MT. There was a significant (p = 0.031) interaction for EMG Co-Act, which indicated a decrease (p = 0.011) and no change (p = 0.598) in FF EMG Co-Act (-7.78%) and FE EMG Co-Act (+2.32%), respectively, from PRE to POST.

CONCLUSION: The training-induced increases of FF and FE PT in untrained women following 4 days of isokinetic training were likely the result of neural adaptations since there were no increases in BB or TB MT. A decrease in Co-Act may have contributed to the training-induced increase in PT for FF, but not for FE.

Research Highlights

  • The Problem: The study investigates whether decreases in antagonist electromyographic co-activation contribute to early-phase strength gains occurring within 2 to 4 days of resistance training, independent of muscle hypertrophy.

  • The Method: Ten untrained women (age: 21.6 ± 1.0 years; height: 168.7 ± 4.9 cm; body mass: 65.2 ± 6.8 kg) completed 4 days of maximal reciprocal concentric isokinetic forearm flexion and forearm extension training (6 sets of 10 repetitions at 180°·s⁻¹) on the non-dominant arm, with pre- and post-training measurements of peak torque, ultrasound muscle thickness of the biceps brachii and triceps brachii, and electromyographic co-activation.

  • Quantitative Finding: Peak torque collapsed across movement increased by 15.1% from pre-training (19.53 ± 5.45 N⋅m) to post-training (22.47 ± 5.93 N⋅m, p = 0.040); muscle thickness showed no significant main effect of time (p = 0.106); forearm flexion electromyographic co-activation decreased significantly by -7.78% (p = 0.011); forearm extension electromyographic co-activation showed no significant change (+2.32%, p = 0.598).

  • Finding: Increases in forearm flexion and forearm extension peak torque after 4 days of isokinetic training resulted from neural adaptations rather than muscle hypertrophy, with decreased antagonist co-activation contributing to strength gains in forearm flexion but not forearm extension.

DOI

10.1249/01.mss.0001262224.87731.f8

Publication Date

10-2026

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