Energy Expenditure Analysis and Prediction in Smith Machine Squats Integrating Mechanical Loads and Sex

ABSTRACT: Qiao, Z, Guo, B, Gao, Y, Wang, Y, Jiang, M, Yu, J, Yi, L, Yan, B, Qiu, J, and Girard, O. Energy expenditure analysis and prediction in Smith machine squats integrating mechanical loads and sex. J Strength Cond Res 40(5): 519-527, 2026-Calculating energy expenditure (EE) and metabolic equivalents (METs) from oxygen uptake (V̇o2) alone often underestimates resistance training intensity because of its intermittent, brief, and high-intensity nature. This study aims to refine EE and METs estimation during Smith machine squats by integrating mechanical work, glycolytic contribution, and postexercise EE, with particular emphasis on weight-dependent and sex-specific metabolic responses. Fifty-one subjects (31 men) performed weight-bearing squats (4 kg-80% 1 repetition maximum) for 5 repetitions across 3 sets with 4-minute rest intervals. Aerobic, glycolytic, and postexercise EE were estimated from V̇o2, blood lactate, and excess postexercise oxygen consumption, respectively. Mechanical work was measured using a linear-position transducer system. Training load (p < 0.05) and sex (p < 0.01) significantly influenced total EE. Mechanical efficiency was significantly influenced by training load (p < 0.001). Energy expenditure-based METs increased with training weight (p < 0.05) and were higher than V̇o2-based METs (p < 0.001). Training weight and mechanical work strongly correlated with total EE (r ≥ 0.80, p < 0.001). The optimal model, including body weight, training load, peak heart rate, and movement distance, demonstrated high accuracy (R2 = 0.885, cross-validated R2 = 0.803, MAE = 10.067). A simplified model using only body weight and training weight achieved comparable accuracy (R2 = 0.847, cross-validated R2 = 0.750, MAE = 10.883). Total EE increased with training weights, but low-to-moderate-weight squats showed superior mechanical efficiency. Women tended to expend less energy than men during squats, but no difference in mechanical efficiency was observed. V̇o2-based METs underestimated squat intensity. Both the simplified and optimal models accurately estimated squat EE, with the optimal model being more accurate.