Differential association between localized thigh muscle activity and whole-body substrate use between overweight and normal-weight young men

This study examined how localized thigh-muscle activation relates to whole-body substrate use in men with normal weight (NW) versus overweight/obesity (OW). Thirty-six participants ([21.4 ± 1.9] years), categorized into NW (BMI: [22.4 ± 1.8] kg∙m -2 , n = 17) and OW (BMI: [28.7 ± 2.2] kg∙m -2 , n = 19), engaged in 15 activity tasks ranging from lying to a maximal incremental treadmill test. Respiratory quotient (RQ), an index that quantifies the proportion of fat versus carbohydrate oxidation during activity, was measured via indirect calorimetry. Average electromyography amplitude (aEMG) quantified quadriceps, hamstrings, and gluteal activity relative to maximal voluntary contraction (%EMG MVC ). Relative intensity in different activities was expressed as a percentage of peak oxygen consumption (%VO 2 peak). Linear mixed models examined the associations between %VO 2 peak and both RQ and aEMG across three muscle groups, and between aEMG and RQ, with weight status as a moderator. RQ and aEMG for all muscle groups were positively associated with increasing %VO 2 peak, and these associations did not differ by weight status group. Each unit increase in hamstrings aEMG ( β = -0.005 16, 95% CI [-0.009 59, -0.000 732], p = 0.025, Cohen's f 2 = 0.28) and in gluteal muscles aEMG ( β = -0.002 07, 95% CI [-0.003 59, -0.000 551], p = 0.01, Cohen's f 2 = 0.30) was associated with a smaller increase in RQ in OW than in NW, while this was not observed for the quadriceps. Collectively, young men with OW showed that greater hamstrings and gluteal muscles activation corresponded to a slower increase in whole-body RQ than those with NW, suggesting different substrate utilization strategies associated with specific muscle groups.