Inter-individual variability in the adaptation of human muscle specific tension to progressive resistance training

Abstract

Considerable variation exists between people in the muscle response to resistance training, but there are numerous ways muscle might adapt to overload that might explain this variable response. Therefore, the aim of this study was to quantify the range of responses concerning the training-induced change in maximum voluntary contraction (MVC) knee joint torque, quadriceps femoris (QF) maximum muscle force (F), physiological cross-sectional area (PCSA) and specific tension (F/PCSA). It was hypothesized that the variable change in QF specific tension between individuals would be less than that of MVC. Fifty-three untrained young men performed progressive leg-extension training three times a week for 9 weeks. F was determined from MVC torque, voluntary muscle activation level, antagonist muscle co-activation and patellar tendon moment arm. QF specific tension was established by dividing F by QF PCSA, which was calculated from the ratio of QF muscle volume to muscle fascicle length. MVC torque increased by 26 ± 11% (P < 0.0001; range -1 to 52%), while F increased by 22 ± 11% (P < 0.0001; range -1 to 44%). PCSA increased by 6 ± 4% (P < 0.001; range -3 to 18%) and specific tension increased by 17 ± 11% (P < 0.0001; range -5 to 39%). In conclusion, training-induced changes in F and PCSA varied substantially between individuals, giving rise to greater inter-individual variability in the specific tension response compared to that of MVC. Furthermore, it appears that the change in specific tension is responsible for the variable change in MVC.

Heterogeneity in resistance training-induced muscle strength and mass responses in men and women of different ages

“…Similar inter-individual variation has been observed also previously in muscle strength responses to RT (Erskine et al 2010). Additionally, aging did not influence the present changes in muscle strength although a sarcopenic phenotype (i.e., lower muscle strength per body mass, see Table 1) was observed in older subjects.…”

“…The confidence intervals (CI) of changes in muscle size and strength in the control group were determined by Univariate GLM, and the upper 95 % CI was used as the lower limit for a significant individual training-induced change in the training group (Hopkins 2000;Karavirta et al 2011b). Individuals with gains in muscle size and strength beyond 1 SD from the mean of the training group were defined as high responders (Erskine et al 2010). Statistical significance was accepted when p ≤ 0.05.…”

“…Also, other RT studies have reported that, in some subjects, muscle size gains are either minimal or non-existent following a training period Davidsen et al 2011;Raue et al 2012;Mitchell et al 2013;Phillips et al 2013). Similarly to muscle size responses, gains in muscle strength during RT are also highly individual (Hubal et al 2005;Erskine et al 2010). However, the range of individual responses to RT in people of different ages has not yet been elucidated.…”

Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training

“…The relationship between the individual changes in strength following training and the putative underpinning neural and morphological adaptations has previously been investigated primarily using bivariate correlations. These studies have reported changes in strength to be both significantly related and unrelated to the changes in neural [significant: agonist electromyography (EMG): r = 0.66-0.74 (Häkkinen et al 1985;Häkkinen and Komi 1986;Shima et al 2002); non-significant: agonist activation assessed with the interpolated twitch technique (ITT): r = −0.124-0.47 (Erskine et al 2010b), antagonist EMG: r = 0.09 (Erskine et al 2010b) or morphological variables [significant: muscle volume assessed via magnetic resonance imaging (MRI): r = 0.48-0.53 (Erskine et al 2014); non-significant: muscle volume assessed via MRI: r = 0.15 (Erskine et al 2010b); muscle fascicle pennation angle (θ p ) assessed via ultrasonography: r = −0.33-0.26 (Nimphius et al 2012)]. Therefore, there is considerable confusion about the contribution of specific physiological variables to the individual changes in strength.…”

“…Only a few studies have examined two or more predictor variables to assess neural and morphological factors, typically finding only one variable to make a significant contribution to the explained variance (Erskine et al 2010b(Erskine et al , 2014, negating the need for a more comprehensive multi-factorial analysis. The exception is Higbie et al (1996) who found a remarkable 65% of the variability in concentric strength gains to be due to the combination of neural (agonist activation assessed with EMG) and hypertrophic (muscle cross-sectional area) adaptations.…”