This study examined the interaction between rotational foot positioning during leg press and individuals' habitual foot progression angle (FPA) phenotype on the acute contractile response of the vastus medialis muscle

Participants A total of 24 male athletes (age: 27.5 ± 3.76 years) who were licensed and actively competing in bodybuilding with a minimum of five years of regular resistance training experience volunteered to participate. Prior to the study, participants were provided with detailed information regarding the research protocol and written informed consent was obtained.

Inclusion criteria were aged between 20 and 35 years, a minimum of five years of regular resistance training experience, and currently active participation in training. Additionally, measurements from both feet were required to meet the established threshold values for participants to be classified according to their FPA. Exclusion criteria were a history of lower extremity surgery or serious musculoskeletal injury within the past year, the presence of congenital or acquired foot deformity, and a history of neurological, cardiovascular, or metabolic disease.

Assessment of Demographic and Physical Characteristics A personal information form was used to obtain demographic data. Body height and mass were measured using standard procedures.

Assessment of Muscle Contractile Properties Muscle contractile properties were assessed using tensiomyography (TMG-BMC Ltd., Ljubljana, Slovenia) on the vastus medialis muscle of the dominant leg. Participants were positioned in supine with the knee supported at approximately 30° of flexion. The muscle belly was identified by palpation, and surface electrodes were placed proximal and distal to the motor point. The measurement probe was positioned perpendicular (90°) to the skin surface. Electrical stimulation was incrementally increased until maximal radial displacement of the muscle was obtained. Participants were instructed to keep their muscles fully relaxed throughout the measurements.

The following TMG parameters reflecting muscle contractile properties were analyzed: maximal radial deformation (Dm), contraction time (Tc), and delay time (Td). These parameters have been reported to demonstrate high relative reliability, with intraclass correlation coefficients of 0.97 for Dm, 0.92 for Tc, and 0.86 for Td, along with low measurement error (CV ≈ 2.7-4.7%) (Loturco et al., 2016; Tous-Fajardo et al., 2010). All measurements were performed before the exercise protocol and within 90 seconds following completion of the final repetition.

TMG Measurement Reliability Prior to the main experimental sessions, test-retest measurements were performed on 12 participants with a 48-hour interval. Reliability was assessed using the intraclass correlation coefficient [ICC(3,1)] calculated with a two-way mixed-effects model. ICC values for TMG parameters were 0.91 for Dm, 0.88 for Tc, and 0.83 for Td, indicating high reliability. The standard error of measurement (SEM) and minimal detectable change (MDC95) were calculated to quantify measurement error. SEM and MDC95 values were 0.31 mm and 0.86 mm for Dm, 0.49 ms and 1.36 ms for Tc, and 1.17 ms and 3.24 ms for Td, respectively.

Assessment of Foot Progression Angle Habitual FPA values were assessed during dynamic gait analysis using a baropodometric walkway platform (BTS P-Walk, BTS Bioengineering, Italy; 2.4 m). The system has been reported to be reliable for the assessment of gait parameters. Measurements were performed under standardized laboratory conditions. Participants were instructed to walk across the platform at their natural walking speed, and data collection was initiated without prior notification to preserve the naturalness of the gait pattern. Data recording began before the participant fully stepped onto the platform. FPA values were automatically calculated by the device's integrated software as the angle between the longitudinal axis of the foot and the direction of progression during gait. FPA values for both feet were obtained separately and the values provided by the device were used in analyses. A minimum of three valid walking trials were collected per participant, and mean FPA values across trials were used in analyses.

FPA Measurement Reliability Test-retest reliability of FPA measurements was assessed in 12 participants with a 48-hour interval. The two-way mixed model intraclass correlation coefficient (ICC 3,1) was 0.92. The minimal detectable change (MDC95) was calculated as 1.8°.

Leg Press Foot Positioning and Exercise Protocol In the neutral protocol, participants' feet were placed on the platform at shoulder-width apart with foot rotation in a neutral (0°) position. In the toe-out protocol (30°), feet were placed at the same shoulder-width distance with the forefoot rotated 30° outward. Foot positions were measured using a standard goniometer at each session and verified by angle markings drawn on the footplate.

All procedures were performed on a 45° inclined leg press machine (Technogym, Italy). Range of motion was restricted from 90° of knee flexion to full extension. One-repetition maximum (1RM) values were determined 48 hours prior to the experimental sessions. The 1RM was defined as the maximum load the participant could complete with technically correct form through the full range of motion.

In the experimental session, participants performed the leg press exercise following a standardized general warm-up. The exercise protocol consisted of 2 × 15 repetitions at 40% 1RM followed by 4 × 10 repetitions at 75-80% 1RM. Inter-set rest was standardized at 2 minutes. Tempo was controlled using a metronome with a 2-second eccentric phase and 1-second concentric phase, and technical compliance was monitored by an experienced coach. Participants were required to complete all repetitions; sets were terminated in the event of technical breakdown or failure to complete a repetition.

Inclusion Criteria:

• Age between 20 and 35 years
• Minimum five years of regular resistance training experience
• Currently active in training
• Foot progression angle measurements of both feet meeting the defined threshold values for group classification

Exclusion Criteria:

• History of lower extremity surgery or serious musculoskeletal injury within the past one year
• Congenital or acquired foot deformity
• History of neurological, cardiovascular, or metabolic disease

Individual participant data will not be shared due to ethical restrictions and participant confidentiality agreements.

The investigator performing TMG measurements was blinded to participants' phenotype group and exercise condition (single-blind).