Loaded inter-set stretch may selectively enhance muscular adaptations of the plantar flexors

Abstract

The purpose of this study was to evaluate differences in changes in muscle strength and muscle thickness (MT) of the plantar flexor muscles between traditional resistance training (RT) involving passive rest and RT combined with inter-set stretch in the calf raise exercise. Employing a within-subject design, 21 young, healthy men performed plantar flexion exercises twice per week in both a traditional RT (TRAD) format and combined with a 20-second inter-set stretch (STRETCH). One leg was randomly assigned to the TRAD condition and the contralateral leg performed the STRETCH condition throughout the 8-week study period. Dependent variables included MT of the lateral gastrocnemius (LG), medial gastrocnemius (MG) and the soleus (SOL), and isometric strength of the plantar flexors. Results indicated a potential beneficial hypertrophic effect of STRETCH compared to TRAD for the SOL [0.7 mm, CI90% = (0, 1.6)], while the LG had more ambiguous effects [0.4 mm (-0.4, 1.3)] and MG effects were equivocal [0 mm (-0.6, 0.7)]. In general, LG demonstrated greater standardized growth [z = 1.1 (1, 1.3)] as compared to MG [z = 0.3 (0.2, 0.5)] and SOL [z = 0.3 (0.2, 0.5)]. Measures of isometric strength showed a modest advantage to STRETCH. In conclusion, loaded inter-set stretch may enhance MT of the soleus but effects on the gastrocnemii appear uncertain or unlikely in untrained men; plantar flexor strength appears to be modestly enhanced by the interventional strategy.

Fig 1. Study timeline.

After being accepted into the study, all participants went through a 1-week acclimation phase. Thereafter, the limbs of participants were randomized to their respective conditions and underwent preintervention testing. The intervention lasted 8 weeks, after which participants underwent postintervention testing.

Fig 2. Example of an ultrasound image: A representative image illustrating the measurement of muscle thickness.

Fig 3. Consort diagram.

Flow chart illustrating the data collection process.

Fig 4. Model-adjusted individual outcomes for hypertrophy and strength in the traditional (TRAD) and stretch (STRETCH) conditions.

Each data point represents an individual’s model-predicted outcome for the TRAD (x-axis) and STRETCH (y-axis) conditions. The black, diagonal line is the identity line; a data point on the line indicates an identical expected outcome in TRAD and STRETCH for that individual. (A) Model-adjusted post-intervention outcomes in muscle thickness (mm). (B) Model-adjusted post-intervention outcomes in isometric plantar flexion strength (N⋅m). Individual lines (translucent) were demeaned within each participant and then summed with the grand mean for that exercise to stress the variability in trends rather than intercepts. Bold, opaque lines depict LOESS curves fit to the entire sample. LG = lateral gastrocnemius; MG = medial gastrocnemius; SOL = soleus; EXT = knee extended; FLEX = knee flexed.