LEG EXTENSIONS INNOVATIONS
Strength equipment that optimizes muscle function can help members achieve the results that maintain loyalty to their facility.
Members Want Strength & Muscle Surveys confirm that building strength and muscle is a top priority for many gym members. More than one in three ¹ identify muscle growth as their primary goal, and over two-thirds ² strength train regularly. Facilities have an opportunity to better support these members by providing more effective routines for strength and muscle growth. Beyond training volume and progression, equipmentinnovations can improve muscle activation and growth by leveraging biomechanics and research-driven design.
Balanced Quadricep Development Matters Strengthening the quadriceps improves athletic performance and supports common aesthetic goals. More importantly, as the primary stabilizers of the knee, balanced quadricep growth reduces the risk of knee pain, 3 injury, 4 and osteoarthritis, 5 leading causes of mobility decline that often cause members to scale back or stop training. Balanced quadricep growth helps members stay healthy and pain-free, supporting consistent facility visits and lowering the risk of membership attrition.
The Quadricep Muscles
• Rectus Femoris( RF): A biarticular muscle( crosses both the hip and knee joint) that contributes to hip flexion and knee extension.
• Vastus Medialis( VM), Vastus Intermedius( VI), and Vastus Lateralis( VL): Three monoarticular muscles that cross the knee jointand contribute to knee extension.
What is Active Insufficiency? Muscles that cross two joints are not fully activated in movements that involve shortening from both ends. Most quad-strengthening exercises involve hip flexion, shortening RF and preferentially recruiting vasti muscles.
Thigh Cross Section 6
Leg Extension Hypertrophy Results
RF + 4.6 % VL + 9.7 %
RF + 10.9 % VL + 11.8 %
Conventional Seated Position
RF + 12.4 % VL + 10.8 %
RF + 15.8 % VL + 9.9 %
Recline at 40 ° Hip Flexion
Values 11 reflect the proximal and distal change in RF and VL muscle thickness after 10 week
Addressing VL Dominance The vasti muscles( VM, VI, VL) cross the knee, but RF crosses both the hip and knee. As a result, common leg exercises fail to deliver balanced quad development. In fact, the back squat disproportionately grows the vasti with minimal RF hypertrophy, 7 and the leg press increases VL size by five times that of the RF. 8 This imbalance reflects RF active insufficiency; hip flexion shortens RF, restricting effective force generation and limiting RF improvements in strength and size.
Solutions: Targeting RF More Effectively Sports scientists 9 emphasize that“ the RF is a very difficult muscle to target and strengthen,” yet recent research identifies a promising solution. Activation of the RF muscle differs based on whether leg extensions are performed in either conventional seated or reclined positions, with reclining positions improving RF activation by reducing active insufficiency. 10
While these findings confirm that hip flexion influences RF recruitment, the next question is whether this extends to superior RF muscle growth. To answer this, researchers compared changes in RF and VL muscle thickness after ten weeks of leg extension training in a conventionalor 40 ° hip flexion( reclined) position.
Compared to conventional seated positions, the reclined position produced a 2.7-fold and 1.45-fold greater increase in proximal and distal RF thickness, respectively, as shown in the anatomical image. 11 The researchers concluded that“ when the goal is to increase overall quadriceps hypertrophy, we suggest training with reduced hip flexion.” 11 This clarifies that while both positions increase RF and VL growth, conventional seated positions preferentially target VL while reclining to 40 ° or more preferentially targets RF.
Innovation for Balanced Quad Development The evidence supporting reclined positions for balanced quad development led to the creation of the first commercially available leg extension equipment with the option of adjusting incrementally to 50 ° hip flexion, the MGPL73. A 50 ° reclined position was selected to improve RF recruitment while maintaining a supportive base of support. By reducing active insufficiency, this reclining adjustment enhances RF activation while still engaging the VL for balanced quadriceps development.
Member Advantages This significantly advancement in leg extension equipment design addresses a long-standing limitation in RF activation and growth. Because the recline feature is adjustable, those who prefer a more upright seat position can continue to train as before. Members who use the recline function gain balanced quad strength and size more efficiently while also benefiting from improved knee stability and long-term joint health, both essential for member retention. In addition, customizable hip positioning and a wide seat surface improve accessibility for various body shapes and sizes.
Innovation Backed by Science As strength training evolves, Matrix continues to lead with research-backed innovation. By integrating the latest biomechanics and sports science, our engineering team develops solutions that improve training effectiveness, helping facilities deliver smarter, more efficient strength and hypertrophy results to their members.
References
1. Sports & Fitness in the US 2024; Consumer Insights Data for US-based Adults.
2. 2024 Fitness Survey Findings: Workouts are Me-Time … Interest in Strength Training Continues to Grow. Jan 3, 2024. Life Time Investor Relations.
3. Gawda et al. Bioelectrical activity of vastus medialis and rectus femoris muscles in recreational runners with anterior knee pain. J Hum Kinet 2019; 66: 81-88.
4. Alentorn-Geli et al. Assessment of neuromuscular risk factors for anterior cruciate ligament injury through tensiomyography in male soccer players. Knee Surg Sports Traumatol Arthrosc 2015; 23: 2508-13.
5. Segal et al. Effect of quadriceps strength and proprioception on risk for knee osteoarthritis. Med Sci Sports Exerc 2010; 42( 11): 2081-8.
6. Image modified from Jones et al. Muscles of the lower limb at Radiopaedia. org
7. Kojic et al. Does back squat exercise lead to regional hypertrophy among quadriceps femoris muscles? International Journal of Environmental Research and Public Health 2022; 19( 23): 16226.
8. Mangine et al. Resistance training does not induce uniform adaptations to quadriceps. PLOS ONE 2018; 13: e0198304.
9. Dr. Mitsuya; lead author in research studies on hip flexion angle and rectus femoris activation. Quote received via email personal communication, 2025.
10. Mitsuya et al. Hip flexion angle affects longitudinal muscle activity of the rectus femoris in leg extension exercise. Eur J Appl Physiol 2023; 123( 6): 1299-1309.
11. Larson et al. The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise. SportRxiv 2024; 19: 1-12.
Karlie Intlekofer, Ph. D. GLOBAL WELLNESS RESEARCHER