Author(s): VÅRVIK, F., BJØRNSEN, T.2, TORSTVEIT, M.K.1, RAASTAD, T.3, Institution: UNIVERSITY OF AGDER, Country: NORWAY, Abstract-ID: 2747

INTRODUCTION:
Heavy-load and power resistance training are widely used to develop strength and power in team sports. Whether these modalities induce distinct in-season physiological adaptations remains unclear, and understanding the role of key cellular regulators of adaptation is crucial, yet direct comparative data are scarce. This study compared the effects of heavy-load and power training on muscle fiber adaptations, performance outcomes, and acute cellular responses in handball players.
METHODS:
Thirty-one resistance-trained female handball players (tier 2) were randomized to HEAVY (n=16; 20±3 yrs) or POWER (n=15; 21±3 yrs). HEAVY performed high-load resistance training (10 sets/week), whereas POWER performed explosive jump exercises (200 jumps/week). Two weekly sessions were conducted during 12 weeks of the competitive season. In the final session (6 heavy sets vs 80 jumps), acute signaling and neuromuscular fatigue were measured. Vastus lateralis biopsies were obtained pre- and post-intervention and 1.5h post-acute session. Outcomes included fiber type distribution, cross-sectional area (CSA), myonuclei, satellite cells, squat 1RM, leg-press power, countermovement jump (CMJ) height, plantar flexor power and stiffness. Acute outcomes included quadriceps isometric force, cellular stress as αB-crystallin immunostaining intensity and protein content in cytosolic and cytoskeletal fractions, and signaling as p-p70S6K and p-ERK1/2. Data were analyzed using two-way repeated-measures ANOVA, with between-group effect sizes (ES).
RESULTS:
Training attendance was 90% in both groups. Type IIX/IIA fiber proportions decreased by 7 percentage-points in both groups (time p<0.01). Fiber CSA increased more in HEAVY than POWER (IIA: 19% vs -4%, p=0.03; IIX/IIA: 28% vs 3%, p=0.04). Squat 1RM increased more in HEAVY than POWER (15% vs 6%, p<0.01). Leg-press power (10% vs 9%) and CMJ height (6% vs 8%) improved for HEAVY and POWER over time (p<0.01), with no interactions (p>0.41), whereas plantar power and stiffness metrics favored POWER (ES: -0.5 to -0.3). Acute force loss was 10% in HEAVY and 6% in POWER (time p=0.04, no interaction p=0.66, ES=0.2). αB-crystallin in type I fibers was unchanged but increased in type II fibers (time p<0.01; 48% vs 39%, no interaction p=0.51, ES=0.4). αB-crystallin translocation to the cytoskeletal fraction increased more in HEAVY than POWER (p=0.04, ES=0.7). Phosphorylation increased for p70S6K (7-fold vs 4-fold; ES=1.1) and ERK1/2 (2-fold vs 4-fold; ES=–0.6) for HEAVY and POWER respectively (time p<0.01, no interactions (p>0.36).
CONCLUSION:
During the competitive season, HEAVY training elicited greater type II fiber hypertrophy and strength, while both HEAVY and POWER improved power performance. Acutely, both sessions induced fatigue and cellular stress, with indications of greater response following HEAVY. Effect sizes for p-p70S6K favored HEAVY, consistent with greater hypertrophy, whereas p-ERK1/2 favored POWER, suggesting possible modality-specific signaling responses.