©Journal of Sports Science and Medicine (2019) 18, 101-108 http://www.jssm.org ` Research article Comparison of Quadriceps and Hamstring Muscle Activity during an Isometric Squat between Strength-Matched Men and Women Sophia Nimphius 1, Jeffrey M. McBride 2, Paige E. Rice 1,2, Courtney L. Goodman-Capps 2 and Christopher R. Capps 2 1 Centre for Exercise and Sports Science Research, School of Medical and Health Sciences, Edith Cowan University, Joondalup, Australia; 2 Neuromuscular and Biomechanics Laboratory, Department of Health and Exercise Science, Ap- palachian State University, Boone, North Carolina, USA hamstring to quadriceps muscle activity ratios than men (Harput et al., 2014). However, strength was not controlled Abstract The primary purpose of this investigation was to determine for in the aforementioned investigations. As such, it was whether strength-matched men and women exhibit a different hypothesized that strength might be a confounding variable magnitude and ratio of leg muscle activity during a maximal vol- not often discussed or accounted for in research comparing untary isometric squat. The secondary purpose was to assess the neuromuscular function in men and women. Such hypoth- effect of normalization method on differences in strength between esis is partially supported by recent findings demonstrating men and women. Thirty-two men (n = 16) and women (n = 16) that matching men and women for strength may negate were successfully strength-matched (≤10% difference) by maxi- some of the previously observed differences in neuromus- mal force produced during an isometric squat (IS) when normal- cular performance (Hatzikotoulas et al., 2004; Hunter et al., ized to body weight. Subjects first performed a maximal isometric 2004; Rice et al., 2017). Thus, strength disparities com- knee extension (IKE) and knee flexion (IKF) followed by the IS monly observed between men and women, but not a per- and muscle activity (EMGmax) was recorded for the vastus medi- alis (VMO), vastus lateralis (VL), semitendinosus (ST) and bi- manent attribute of men and women (e.g., trainable), may ceps femoris (BF). Muscle activity during the IS was expressed be a confounding factor influencing neuromuscular func- relative to the maximums observed during the IKE and IKF tion more than the sex differences sought to be evaluated (%EMGmax). The results indicate that VMO, VL, ST and BF in prior neuromuscular research. %EMGmax were not significantly different (p > 0.05) between Several comparisons surrounding athletic perfor- men and women during the IS (Men VMO = 136.7 ± 24.9%, mance of men and women propose that major neuromus- Women VMO = 157.1 ± 59.8%, Men VL = 126.2 ± 38.2%, cular and strength incongruities exist between sexes Women VL = 128.1 ± 35.5%, Men ST = 25.5 ± 13.6%, Women (Hanson et al., 2008; Rice et al., 2017). However, previous ST = 25.2 ± 21.8%, Men BF = 46.1 ± 26.0%, Women BF = 42.2 ± 24.8%). Furthermore, the VMO:VL and hamstring to quadri- research has shown that resistance training interventions elicit similar levels of improvement in muscular strength as ceps (H:Q) %EMGmax ratio were not significantly different be- tween groups in the IS (Men VMO:VL = 1.15 ± 0.28, Women well as neural adaptations in men and women (Staron et al., VMO:VL = 1.22 ± 0.26, Men H:Q = 0.28 ± 0.14, Women H:Q = 1994). A strength-matched participant approach has 0.24 ± 0.20). This investigation indicates that the magnitude of scarcely been implemented (Hatzikotoulas et al., 2004; muscle activity and the ratios examined are not significantly dif- Hunter et al., 2004; Rice et al., 2017) but provides a strong ferent between men and women in a maximal voluntary isometric research design to elucidate actual differences between squat when matched for normalized strength. Future investiga- men and women versus modifiable differences attributed tions should consider subject strength and normalization proce- to training history or in this specific example, muscular dures in the experimental design to elucidate possible sex differ- ences in neuromuscular performance capabilities. strength. Although controlling factors amongst groups in this domain of research can be challenging, analysis of all Key words: Knee extension, knee flexion, agonist, antagonist. modifiable variables must be considered. Therefore, the purpose of the current research was to determine whether strength-matched men and women Introduction exhibit a different magnitude and ratio of leg muscle activ- ity during a maximal voluntary isometric squat. An isomet- Several investigations have sought to determine sex differ- ric squat movement was utilized in efforts to first isolate ences in neuromuscular function between men and women whether strength differences between males and females (Hannah et al., 2015; Harput et al., 2014; Myer et al., 2005; might be a confounding variable in the study of basic neu- Spiteri et al., 2014). Imbalances in ratio or magnitude of romuscular function between sexes. A secondary purpose muscle activity are suggested to result in poor performance of this research was to examine the effect of different nor- in physical activities and potentially increase risk of injury, malization procedures [e.g., relative to absolute, body particularly in women (Hewett et al., 2005). Prior evidence weight (or body mass) and lean weight (or lean mass)] on suggests that women have a greater imbalance in medial to differences in strength between men and women. It was hy- lateral leg muscle activity (Myer et al., 2005), agonist to pothesized that there would be no significant differences in antagonist muscle activity in comparison to males during magnitude or ratio of muscle activity in strength-matched dynamic tasks (Ebben, 2009), and greater disproportional men and women, and there would be an effect of normali- Received: 16 October 2018 / Accepted: 17 December 2018 / Published (online): 11 February 2019 102 Strength-match men and women zation procedure when comparing the magnitude of Participants laid in a supine position on the scanning bed strength in men and women. Controlling for covariates with both arms pronated by their side and internally rotated such as strength will allow for research to identify more thighs with feet fixed to hold position (Hart et al., 2014). accurately the factors or variables that indeed underpin the Segmental analysis was performed using the inbuilt analy- differences between the sexes with respect to physical per- sis software (Version 12.4; QDR for Windows, Hologic, formance or risk for injury. Waltham, WA) to assess the thigh mass to normalize knee extension and flexion torque. Length of the femur and tibia Methods were assessed using the previously described inbuilt soft- ware as the distance between the most prominent aspect of Participants the greater trochanter to the lateral epicondyle (femur The institutional ethics committee approved the study, and length) and from the tibale mediale to medial malleolus written consent was obtained from each participant before (tibia length). The length of the tibia (corrected for force commencement of testing. Participants were recruited if transducer cuff location) was used as the moment arm for they were currently performing resistance training a mini- calculation of torque during both the IKE and IKF. The mum of two times per week, but no other physical activity segmental analysis has been previously described and reli- or sporting experience were targeted. All participants were ability (intraclass correlation [ICC] and coefficient of var- free of any musculoskeletal injuries within the paster year, iation [CV]) previously assessed (ICC ≥ 0.94; CV ≤ 2.6%) including any prior anterior cruciate ligament injuries. An by our lab (Hart et al., 2014). a priori power analysis based on previous research (Ebben, 2009) results (α = 0.05; β = 0.80; d = 0.64) indicated that 22 participants (11 pairs) would provide an actual power of 0.82. Thirty-two men (n=16; age: 21.1 ± 1.8 yrs; resistance training age: 4.1 ± 2.5 yrs) and women (n = 16; age: 22.0 ± 1.7 yrs; resistance training age: 2.4 ± 2.4 yrs) were success- fully matched (≤10% difference) for maximal force pro- duced during an isometric squat when normalized to body- weight as seen in Figure 1. Prior to further analysis, success of matching was evaluated using Pearson’s correlation co- efficient (r = 0.988; p ≤ 0.001) and paired comparisons (p = 0.89; d = 0.01). All other physical characteristics are in- cluded in Table 1. Study design Participants completed a single 60-90 minute testing ses- Figure 1. Strength-matched men (grey) and women (black) sion. All participants’ anthropometric measures and body pairs based upon force produced during an isometric squat composition were assessed before a standardized warm-up. normalized to body weight (BW). Participants then completed submaximal and maximal tri- als of isometric knee extension (IKE), isometric knee flex- Standardised warm-up and maximal effort trial proce- ion (IKF) and isometric squat (IS) while force produced dures and EMG activity of the vastus lateralis (VL), vastus me- Each participant performed a five-minute warm-up on dialis (VMO), semitendinosus (ST) and biceps femoris Monarch bicycle at 50W with a 60-80 rpm cadence before (BF) were measured. physical testing. To ensure maximal voluntary contrac- tions, the following procedures were undertaken as previ- Anthropometrics and body composition ously recommended (Gandevia, 2001). Before each exer- Height and body mass (BM) were first measured.