Higher legs muscle mass reduces gross mechanical efficiency during moderate intensity cycling in young healthy men

Abstract

Twelve healthy untrained men (age 22 ± 1 years; body mass (BM) 76.8 ± 14.4 kg; height 180 ± 8 cm, (mean ± SD)), participated in this study. The subjects performed an incremental exercise test on a cycloergometer with an increase of power output (PO) by 30 W every 3 min - until exhaustion. Gross mechanical efficiency (GE) and delta efficiency (DE) during exercise of moderate-intensity (below lactate threshold - < LT) was calculated. Both legs muscle mass (LMM) (determined using 3T MRI) amounted to 14.1 ± 2.1 kg (i.e., 18.6% of body mass). Pulmonary oxygen consumption (V̇O₂) at rest (sitting position) was 391 ± 42 mL min^-1. The slope of the V̇O₂(PO) relationship (at the PO's < LT) amounted to 10.25 ± 0.99 mL O₂ min^-1 W^-1 and the intercept 501 ± 130 mL min^-1. Pulmonary maximal oxygen uptake (V̇O_2max) was 3198 ± 458 mL O₂ min^-1, 42.2 ± 5.7 mL O₂ min^-1 kg^-1 BM and 187 ± 30 mL O₂ min^-1 kg^-1 of LMM. The LMM was positively correlated with the V̇O₂ at rest (p = 0.01). No relation between the LMM and the DE was found, whereas GE at the PO of 30-90 W was negatively correlated with the LMM (p ≤ 0.05). We concluded that greater muscle mass is not favorable when performing moderate-intensity cycling, since it results in poorer gross muscle mechanical efficiency.

Keywords: Delta efficiency; Energy expenditure; Exercise; Maximal oxygen uptake; Muscle mass; Power output.

Fig. 1

Legs muscle mass in relation to body mass and pulmonary oxygen consumption at rest in the studied group of subjects (n = 12). The correlations between the body mass (BM) and the muscle mass of both legs (LMM) (panel A) and between LMM and the pulmonary oxygen uptake at rest ( at rest) (at sitting position) (panel B).

Fig. 2

Oxygen uptake – power output relationship during moderate-intensity cycling in the studied group of subjects. Pulmonary oxygen uptake () obtained during moderate-intensity cycling i.e., at the power outputs corresponding to 30, 60, 90, 120 W (mean ± SD, panel A). Individual values of the pulmonary at rest and at the applied moderate-intensity power outputs (30–120 W) (panel B). Data are presented for 12 subjects at the power outputs in the range of 30–90 W and for 8 subjects at 120 W (the individuals with LT > 90 W).

Fig. 3

Legs muscle mass and pulmonary oxygen uptake in the studied group of subjects (n = 12). The correlation between the muscle mass of both legs (LMM) and the pulmonary oxygen uptake ( ) measured during cycling at 30 (panel A), 60 (panel B), 90 (panel C) and 120 W (panel D). The correlation between the LMM and the intercept value of the (PO) relationship during moderate-intensity cycling (30–120 W) (panel E).

Fig. 4

Legs muscle mass and gross efficiency in the studied group of subjects. Relationship between the muscle mass of both legs (LMM) and gross efficiency (GE) during cycling at power output corresponding to 30 W (panel A); 60 W (panel B); 90 W (panel C); 120 W (panel D). Data are presented for 12 subjects at the power outputs in the range of 30–90 W and for 8 subjects at 120 W (the individuals with LT > 90 W).

Fig. 5

Estimated leg oxygen consumption in the studied group of subjects based on the formulas presented by Poole et al.,. Estimation of the leg at a given power output in the present study was made by using the (PO) relationship formulas for the pulmonary and the legs , as present by Pool et al. (see Fig. 1 therein) and divided by the measured subjects legs muscle mass (both legs). Data are presented for 12 subjects, except for the power output of 120 W, where data are shown for 8 individuals, whose LT > 90 W.

Fig. 6

A typical example of MRI scans of the leg. Frontal plane image of the upper leg (panel A) and lower leg (panel B). Image in the transverse plane of the upper leg (panel C) and lower leg (panel D).