Morphological Adaptations Following a Strength-Endurance or Strength-Power Resistance Training Block with Concurrent Speed and Change-of-Direction Training

Regional and whole muscle adaptations in lower body skeletal muscle were investigated following a four-week strength-endurance (G10) or strength-power (G2) training block with concurrent speed and change of direction training. Fourteen recreationally trained individuals (age = 24.1 + 4.1 yrs) completed the study (10 males, height = 1.76 + 0.05 m, body mass = 86.7 + 8.0 kg, and 4 females, height = 1.58 + 0.09 m, body mass = 62.8 + 5.8 kg). Subjects were assigned to either G10 or G2 using match-pair randomization based on sex and initial relative back squat strength. The training protocols differed only in set-repetition configuration and associated loading-intensities during resistance training. Morphological assessments of the right vastus lateralis muscle were conducted using ultrasonography before (PRE) and after (POST) the training intervention. Regional measurements were taken at 33, 50, and 66% of femur length to investigate changes following S-E or S-P training. Multiple repeated measures mixed analysis of variance (ANOVA) were conducted for changes in summated and regional anatomical cross-sectional area (ACSA) and muscle thickness (MT), as well as regional changes in size, thickness, pennation angle (PA), and fascicle length (FL). Statistically significant interaction effects were only observed for ACSA and MT distally and PA proximally, while FL did not change significantly. Findings indicate that high volume, moderate intensity resistance training (i.e. G10) elicits superior hypertrophic adaptations when compared to low volume, high intensity training (i.e. G2). Furthermore, size and architectural adaptations diverge within the muscle and between groups, likely resulting from distinct peaks and volumes of mechanical tension. Our observations posit that specific resistance training stimuli create specific and inhomogeneous alterations of muscle morphology, which can positively or adversely affect sport performance, depending on the sport or training goal. Moreover, current findings support the existence and robustness of a strength-endurance continuum as a repetition paradigm.