Mixing Up Muscle Lengths: The Effects of Manipulating Peak Torque At Different Muscle Lengths in the Elbow Flexors

Abstract

PURPOSE: This repeated measures within-participant unilateral design study investigated the effect of resistance training with the peak torque occurring at only longer muscle lengths (LONG) to training with the peak torque occurring at a mixture of long and short muscle lengths (MIXED). METHODS: 7 recreationally trained males (n=5) and females (n=2) trained for eight weeks, with limbs randomized to either LONG or MIXED conditions. The repeated measures design required participants to repeat the 8 week training after a 6-8 week washout period, which effectively created observations equivalent to  sample size of 14. In the LONG condition, participants performed six sets of seated lengthened cable curls to momentary failure with 3 minutes of interset rest each session. The MIXED condition performed three sets of seated lengthened cable curls and three sets of standing shortened cable curls to momentary failure with 3 minutes of interset rest each session. Elbow flexor cross-sectional area (CSA) and regional muscle thickness (MT) were assessed via panoramic b-mode ultrasonography at Pre and Post testing. Dynamic strength, isometric strength, and arm circumference were also assessed at Pre and Post testing. Session rating of perceived exertion (sRPE) and perceived soreness were assessed before and after every session. To compare changes in the primary training outcomes between conditions (i.e., elbow flexor cross-sectional area, muscle thickness, isometric force, dynamic strength, and arm circumference), Bayesian linear mixed effect models were constructed to mimic an analysis of covariance (i.e., ANCOVA) with an adjustment for the baseline score of the dependent variable. To compare longitudinal trends in the subjective perceptions of fatigue (i.e., session RPE and perceived elbow flexor soreness) between conditions Bayesian linear mixed effect models with a Gaussian response distribution were fit. For parameters of interest from each model (i.e., marginal effects for condition or the two-way interaction between condition and site, exercise, or session), draws were taken from the posterior distribution to construct a probability density function (i.e., mode and associated highest density intervals) that was used to make probabilistic inferences. The probability density functions related to the primary research questions were also compared to a region of practical equivalence (ROPE) defined by the typical error of measurement. RESULTS: Changes in total CSA for both LONG (6.01 cm2 [95% HDI: -1.64, 14.66], 80.12% probability > ROPE) and MIXED (3.18 cm2 [95% HDI: -5.66, 12.69], 55.51% probability > ROPE) were considered meaningful, but the contrast between conditions (2.58 cm2 [HDI: -4.1, 9.01], 48.96% probability > ROPE) was not meaningful. Contrast estimates in arm circumference were meaningfully greater for the LONG condition at proximal (0.33 mm [95% HDI: -0.92, 1.95], 52.19% probability > ROPE), middle (0.46 mm [95% HDI: -1.06, 1.9], 54.39% probability > ROPE), and distal (0.89 mm [95% HDI: -0.74, 2.4], 74.26% probability > ROPE) regions of the upper arm. Changes in dynamic strength for both conditions in the shortened cable curl (LONG=3.67 kg [95% HDI: -0.77, 7.65], 76.99% probability > ROPE; MIXED=5.44 kg [95% HDI: 0.59, 9.78], 92.31% probability > ROPE) and lengthened cable curl (LONG=6.93 kg [95% HDI: 2.37, 11.86], 97.86% probability > ROPE; MIXED=6.57 kg [95% HDI: 0.35, 11.77]; 93.86% probability > ROPE) were considered meaningful, but the contrast between conditions was not meaningful. Changes in regional MT, regional isometric force, total isometric strength, sRPE and perceived soreness were not considered meaningful for either condition. CONCLUSIONS:  Given the lack of meaningful differences between conditions and the questionable relevance of circumference measurements, It seems that neither training condition provides clearly superior outcomes for hypertrophy, strength, or perceptual fatigue.