Author(s): TERZIS, G., MPAMPOULIS, T.1, VEKAKI, E.1, PAPADIMAS, G.2, PAPADOPOULOS, C.2, BITZANAKIS, E.1, PLATIAS, G.1, SAKELLARIOU, O.1, MINOPOULOU, E.1, BOGDANIS, G.1, Institution: NATIONAL AND KAPODISTRIAN UNIVERSITY OF ATHENS, Country: GREECE, Abstract-ID: 593

INTRODUCTION:
Resistance training (RT) induces large increases in muscle strength and hypertrophy but with great variability among individuals, despite a similar initial level of strength and muscle mass. The muscle fiber type composition has been claimed as one of the critical biological factors dictating individual RT-induced changes in muscle mass and strength [1], with individuals with type II fiber predominance having an advantage. However, a recent study challenged this outcome showing that the fiber type composition does not dictate changes in muscle strength and hypertrophy [2]. The discrepancy between these results might be due to the application of inappropriate resistance training loads in individuals with different muscle fiber type composition. Aim of the present study was to compare the effects of resistance training with high and low loads on muscle strength and hypertrophy in individuals with different muscle fiber type composition.
METHODS:
Eighteen young females (age 20.7±3.0yrs, height 1.64±0.06m, mass 59.9±7.0kg), unaccustomed to RT, were selected from a larger group of female physical education students according to their right vastus lateralis fiber type composition (needle muscle biopsies) and assigned to two different groups: G1 with 60,3±7.2% type I muscle fibers (range 50.2%-73.0%), and G2 with 59.8±5.8% type II muscle fibers (range 51.2%-66.1%), p<0.01. All participants followed 10 weeks (2/week) of progressive single-leg press RT, with one leg performing 4-5 sets x 6-8 reps at 80-85% maximum strength (1RM) and the opposite leg 3 sets x 20 repetitions at 55-60% 1RM, with equal training volume between legs and similar rate of perceived exertion. The training sequence between legs was alternated in every session. Single leg press 1RM and quadriceps cross sectional area (CSA, ultrasonography) were measured before and after the intervention. Two-way repeated measures ANOVA was mainly used to analyze the data at p≤0.05.
RESULTS:
The initial 1RM was similar between G1 and G2 and between the lower extremities training with either low or high load (p>0.05). Leg press 1RM increased more with high load RT compared to low load RT, both in G1 and G2 (p<0.05). Leg press 1RM increased more with high load RT in G2 compared to G1 (91.1±26.8% vs. 62.7±17.6%, p<0.05). Quadriceps CSA increased more with high load RT in G2 compared to G1 (14.7±9.1% vs. 4.0±4.8%, p<0.05), but similarly between G1 and G2 after low load RT (8.7±6.4% vs. 11.9±9.9%, p>0.05).
CONCLUSION:
These results suggest that young females with type II muscle fibers predominance, unaccustomed to RT, may experience a higher increase in muscle strength and hypertrophy when exposed to high load RT compared to females with type I muscle fibers predominance. Low load RT induces similar increases in muscle strength and hypertrophy regardless of the muscle fiber type composition.

1.Haun et al. (2009), Front Physiol 10, 1-17.
2.Van Vossel et al. (2023), J Physiol 601.12, 2307-2327.