Anatomical and Neuromuscular Determinants of Strength Change in Previously Untrained Men Following Heavy Strength Training

Abstract

This study examined whether changes in strength following a moderate-duration strength training program were associated with changes in specific combinations of anatomical and neuromuscular variables. 36 men (18-40 y) completed 10 weeks of lower-limb heavy resistance (6-RM) strength training. Measurements included cross-sectional area (CSA), fascicle length (l_f) and fascicle angle (θ_f) from proximal, middle and distal regions of the four quadriceps components; agonist (EMG:M), antagonist (EMG) muscle activities and percent voluntary quadriceps activation (%VA; interpolated twitch technique); patellar tendon moment arm distance; and maximal isometric, concentric and eccentric (60° s^-1) torque. Multiple regression models were developed to quantify the relationship between the change in maximum torque and the changes in combinations of anatomical and neuromuscular variables. The best model for each contraction mode was determined using Akaike's Information Criterion (AIC_c), an information-theoretic approach for model selection. Strength increased significantly following training (mean range = 12.5-17.2%), and moderate relationships were observed between modeled data (using best-fit prediction models) and the change in torque for each contraction mode. The change in isometric torque was best (although weakly) predicted by the linear combination of the change in proximal-region vastus lateralis (VL) CSA and fascicle angle (R ² = 0.27, p < 0.05; AIC_c w_i = 0.52, i.e., the probability the model would be selected as the "best model"). The models best predicting the change in concentric and eccentric torque both included the combination of the change in quadriceps (i.e., mean of all muscles) EMG:M and the change in vastus intermedius fascicle angle combined with either a change in proximal-region VL (R ² = 0.40, p < 0.001; AIC_c w_i = 0.15) or whole quadriceps (R ² = 0.41, p < 0.001; AIC_c w_i = 0.30) CSA (concentric and eccentric, respectively). Models incorporating the change in proximal CSA typically received substantial support (AIC_C < 2) for concentric torque prediction models, and the change in % VA and pre-training moment arm distance had substantial support for use in eccentric torque prediction models. In conclusion, adaptations varied between individuals, however strength training programs targeted to improve a group of variables that particularly includes agonist muscle activation might yield the greatest improvements in concentric and eccentric knee extension strength, whereas proximal muscle size and fascicle angle appear most important for isometric torque improvements.

Keywords: cross-sectional area; fascicle angle; linear models; muscle activity; strength training.

FIGURE 1

ACSA of individual quadriceps components at distal (A), middle (B) and proximal (C) regions of the thigh, identifying rectus femoris (RF), vastus medialis (VM) vastus lateralis (VL) and vastus intermedius (VI).

FIGURE 2

Predicted change in torque (ΔT) was modeled based on the AIC_C rankings using the best-fit model for the change in maximal isometric (A), and isokinetic concentric (B) and eccentric (C) torque prediction. Figures show the mean (± SE) for each model. (ln) = the natural log of the change in torque. CSA,Q_PROX and CSA,VL_PROX = proximal cross-sectional area of whole quadriceps, or of vastus lateralis (VL) in isolation, respectively; EMG:M_AVEQ = normalized average quadriceps (AVEQ) amplitude; θ_fVL_PROX and θ_fVI = fascicle angle of VL obtained at proximal region, and vastus intermedius (VI), respectively; R² = adjusted R².