Several investigations have sought to determine sex differences in neuromuscular function between men and women (Hannah et al., 2015; Harput et al., 2014; Myer et al., 2005; Spiteri et al., 2014). Imbalances in ratio or magnitude of muscle activity are suggested to result in poor performance in physical activities and potentially increase risk of injury, particularly in women (Hewett et al., 2005). Prior evidence suggests that women have a greater imbalance in medial to lateral leg muscle activity (Myer et al., 2005), agonist to antagonist muscle activity in comparison to males during dynamic tasks (Ebben, 2009), and greater disproportional hamstring to quadriceps muscle activity ratios than men (Harput et al., 2014). However, strength was not controlled for in the aforementioned investigations. As such, it was hypothesized that strength might be a confounding variable not often discussed or accounted for in research comparing neuromuscular function in men and women. Such hypothesis is partially supported by recent findings demonstrating that matching men and women for strength may negate some of the previously observed differences in neuromuscular performance (Hatzikotoulas et al., 2004; Hunter et al., 2004; Rice et al., 2017). Thus, strength disparities commonly observed between men and women, but not a permanent attribute of men and women (e.g., trainable), may be a confounding factor influencing neuromuscular function more than the sex differences sought to be evaluated in prior neuromuscular research.
Several comparisons surrounding athletic performance of men and women propose that major neuromuscular and strength incongruities exist between sexes (Hanson et al., 2008; Rice et al., 2017). However, previous research has shown that resistance training interventions elicit similar levels of improvement in muscular strength as well as neural adaptations in men and women (Staron et al., 1994). A strength-matched participant approach has scarcely been implemented (Hatzikotoulas et al., 2004; Hunter et al., 2004; Rice et al., 2017) but provides a strong research design to elucidate actual differences between men and women versus modifiable differences attributed to training history or in this specific example, muscular strength. Although controlling factors amongst groups in this domain of research can be challenging, analysis of all modifiable variables must be considered.
Therefore, the purpose of the current research was to determine whether strength-matched men and women exhibit a different magnitude and ratio of leg muscle activity during a maximal voluntary isometric squat. An isometric squat movement was utilized in efforts to first isolate whether strength differences between males and females might be a confounding variable in the study of basic neuromuscular function between sexes. A secondary purpose of this research was to examine the effect of different normalization procedures [e.g., relative to absolute, body weight (or body mass) and lean weight (or lean mass)] on differences in strength between men and women. It was hypothesized that there would be no significant differences in magnitude or ratio of muscle activity in strength-matched men and women, and there would be an effect of normalization procedure when comparing the magnitude of strength in men and women. Controlling for covariates such as strength will allow for research to identify more accurately the factors or variables that indeed underpin the differences between the sexes with respect to physical performance or risk for injury.
The institutional ethics committee approved the study, and written consent was obtained from each participant before commencement of testing. Participants were recruited if they were currently performing resistance training a minimum of two times per week, but no other physical activity or sporting experience were targeted. All participants were free of any musculoskeletal injuries within the paster year, including any prior anterior cruciate ligament injuries. An a priori power analysis based on previous research (Ebben, 2009) results ([alpha] = 0.05; [beta] = 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 [+ or -] 1.8 yrs; resistance training age: 4.1 [+ or -] 2.5 yrs) and women (n = 16; age: 22.0 [+ or -] 1.7 yrs; resistance training age: 2.4 [+ or -] 2.4 yrs) were successfully matched ([less than or equal to] 10% difference) for maximal force produced during an isometric squat when normalized to bodyweight as seen in Figure 1. Prior to further analysis, success of matching was evaluated using Pearson's correlation coefficient (r = 0.988; p [less than or equal to] 0.001) and paired comparisons (p = 0.89; d = 0.01). All other physical characteristics are included in Table 1.
Participants completed a single 60-90 minute testing session. All participants' anthropometric measures and body composition were assessed before a standardized warm-up. Participants then completed submaximal and maximal trials of isometric knee extension (IKE), isometric knee flexion (IKF) and isometric squat (IS) while force produced and EMG activity of the vastus lateralis (VL), vastus medialis (VMO), semitendinosus (ST) and biceps femoris (BF) were measured.
Anthropometrics and body composition
Height and body mass (BM) were first measured. Dual-energy x-ray absorptiometry (DXA) (QDR-1500, Hologic Discovery A, Waltham, MA) was used to assess the magnitude and quality of body composition (fat, lean and total).
Participants laid in a supine position on the scanning bed with both arms pronated by their side and internally rotated thighs with feet fixed to hold position (Hart et al., 2014). Segmental analysis was performed using the inbuilt analysis software (Version 12.4; QDR for Windows, Hologic, Waltham, WA) to assess the thigh mass to normalize knee extension and flexion torque. Length of the femur and tibia were assessed using the previously described inbuilt software as the distance between the most prominent aspect of the greater trochanter to the lateral epicondyle (femur length) and from the tibale mediale to medial malleolus (tibia length). The length of the tibia (corrected for force transducer cuff location) was used as the moment arm for calculation of torque during both the IKE and IKF. The segmental analysis has been previously described and reliability (intraclass correlation [ICC] and coefficient of variation [CV]) previously assessed (ICC [greater than or equal to] 0.94; CV [less than or equal to] 2.6%) by our lab (Hart et al., 2014).
Standardised warm-up and maximal effort trial procedures
Each participant performed a five-minute warm-up on Monarch bicycle at 50W with a 60-80 rpm cadence before physical testing. To ensure maximal voluntary contractions, the following procedures were undertaken as previously recommended (Gandevia, 2001). Before each exercise (IKF, IKE and IS), participants were given specific instructions and practice efforts including submaximal contractions at their perceived ~50% and ~75% followed by three maximal effort trials (Hannah et al., 2012). All participants were provided with feedback during each trial and asked if their efforts were considered maximal allowing those considered not maximal to be discarded in replacement for another maximal effort. Values were also checked prior to removal. Two minutes of rest was provided between all trials. The ICC [greater than or equal to] 0.98; CV [less than or equal to] 3.2% demonstrated high reliability for peak force during the IKF, IKE and IS.
Electromyography (EMG) of VL, VMO, ST and BF was assessed using standardized protocols as described by SENIAM (Hermens et al., 2000) and described for open-source access at http://seniam.org/. Each site was abraded and cleaned using rubbing alcohol followed by fixation of the electrode (Delsys Trigno Wireless System, Natlick, Massachusetts, USA) directly to the skin using double-sided adhesive tape. EMG data were sampled at 2000Hz and collected using a custom LabVIEW program (National Instruments Version 14, Austin, TX). EMG data was offline bandpass filtered between 6 and 500Hz using a 2nd order Butterworth filter. Maximal EMG ([EMG.sub.max]) of all isometric tasks were analyzed using root mean square with an averaging window of 250 ms over an epoch of 500 ms (250 ms on either side) of the time when either maximum isometric force or torque occurred, as recommended by both SENIAM guidelines and...