Heterogeneity of the strength response to progressive resistance exercise training in older adults: Contributions of muscle contractility

Abstract

Background: Older adults display wide individual variability (heterogeneity) in the effects of resistance exercise training on muscle strength. The mechanisms driving this heterogeneity are poorly understood. Understanding of these mechanisms could permit development of more targeted interventions and/or improved identification of individuals likely to respond to resistance training interventions. Thus, this study assessed potential physiological factors that may contribute to strength response heterogeneity in older adults: neural activation, muscle hypertrophy, and muscle contractility.

Methods: In 24 older adults (72.3 ± 6.8 years), we measured the following parameters before and after 12 weeks of progressive resistance exercise training: i) isometric leg extensor strength; ii) isokinetic (60°/sec) leg extensor strength; iii) voluntary (neural) activation by comparing voluntary and electrically-stimulated muscle forces (i.e., superimposed doublet technique); iv) muscle hypertrophy via dual-energy x-ray absorptiometry (DXA) estimates of regional lean tissue mass; and v) intrinsic contractility by electrically-elicited twitch and doublet torques. We examined associations between physiological factors (baseline values and relative change) and the relative change in isometric and isokinetic muscle strength.

Results: Notably, changes in quadriceps contractility were positively associated with the relative improvement in isokinetic (r = 0.37-0.46, p ≤ 0.05), but not isometric strength (r = 0.09-0.21). Change in voluntary activation did not exhibit a significant association with the relative improvements in either isometric or isokinetic strength (r = 0.35 and 0.33, respectively; p > 0.05). Additionally, change in thigh lean mass was not significantly associated with relative improvement in isometric or isokinetic strength (r = 0.09 and -0.02, respectively; p > 0.05). Somewhat surprising was the lack of association between exercise-induced changes in isometric and isokinetic strength (r = 0.07).

Conclusions: The strength response to resistance exercise in older adults appears to be contraction-type dependent. Therefore, future investigations should consider obtaining multiple measures of muscle strength to ensure that strength adaptations are comprehensively assessed. Changes in lean mass did not explain the heterogeneity in strength response for either contraction type, and the data regarding the influence of voluntary activation was inconclusive. For isokinetic contraction, the strength response was moderately explained by between-subject variance in the resistance-exercise induced changes in muscle contractility.

Keywords: Aging; Dynapenia; Resistance exercise; Sarcopenia; Variability.

Figure 1.

Before resistance exercise training, baseline isometric and isokinetic torque measures of leg extensor strength were strongly associated (r=0.892, p<0.001).

Figure 2.

There was substantial heterogeneity in response to resistance exercise training in leg extensor isometric (A) and isokinetic (B) torque, respectively. Filled bars represent women, and open bars represent men. The small closed circle above selected individual data points represent data from participants in the DART Study. These are shown to illustrate that subjects across both studies demonstrated substantial variability in the strength response to resistance exercise training. C. The % change in isokinetic torque and % change in isometric torque were not associated (r=0.07, p=0.731).

Figure 3.

Associations between % change in voluntary activation and % change in isometric (A) and isokinetic (B) strength were not statistically significant (r=0.35, p=0.145 and r=0.33, p=0.076, respectively).

Figure 4.

Associations between % change in thigh lean mass and % change in isometric (A) and isokinetic (B) strength were not statistically significant (r=0.09, p=0.671 and r=−0.02, p=0.937, respectively).

Figure 5.

Associations between % change in quadriceps twitch (A) and doublet torque (B) and % change in isometric torque were not statistically significant (r=0.09, p=0.694, and r=0.21, p=0.359, respectively). C. The association of % change in the twitch-to-doublet ratio with % change in isometric torque was not statistically significant (r=0.06, p=0.794). D. The correlation between % change in twitch torque and % change in isokinetic torque was significant (r=0.46, p=0.023). E. The correlation of % change in doublet torque and % change in isokinetic torque was not statistically significant (r=0.37; p=0.096) F. The association of % change in the twitch-to-doublet ratio with % change in isokinetic torque was significant (r=0.48, p=0.031).