Running and Jumping After Muscle Fatigue in Subjects with a History of Knee Injury: What Are the Acute Effects of Wearing a Knee Brace on Biomechanics?

nee is one of the most frequently injured joints, involving various structures. To prevent reinjury after rehabilitation, braces are commonly used. However, most studies on knee supports focus on subjects with anterior cruciate ligament (ACL) injuries and do not account for muscle fatigue, which typically occurs during prolonged intense training and can significantly increase the risk of injury. Hence, this study investigates the acute effects of wearing a knee brace on biomechanics in subjects with a history of various unilateral knee injuries or pain under muscle fatigue. In total, 50 subjects completed an intense fatigue protocol and then performed counter-movement jumps and running tests on a force plate while tracking kinematics with a marker-based 3D motion analysis system. Additionally, subjects filled out a visual analog scale (VAS) to assess knee pain and stability. Tests were conducted on the injured leg with and without a knee brace (Sports Knee Support, Bauerfeind AG, Zeulenroda-Triebes, Germany) and on the healthy leg. Results indicated that

and running tests on a force plate while tracking kinematics with a marker-based 3D motion analysis system. Additionally, subjects filled out a visual analog scale (VAS) to assess knee pain and stability. Tests were conducted on the injured leg with and without a knee brace (Sports Knee Support, Bauerfeind AG, Zeulenroda-Triebes, Germany) and on the healthy leg. Results indicated that wearing the knee brace stabilized knee movement in the frontal plane, with a significant reduction in maximal medio-lateral knee acceleration and knee abduction moment during running and jumping. The brace also normalized loading on the injured leg. We observed higher maximal knee flexion moments, which were associated with increased vertical ground reaction forces, segment velocities, and knee flexion angles. Subjects reported less pain and greater stability while wearing the knee brace. Therefore, we confirm that wearing a knee brace on the injured leg improves joint biomechanics by enhancing stability and kinematics and reducing pain during running and jumping, even with muscle fatigue. Consequently, wearing a knee brace after a knee joint injury may reduce the risk of reinjury. Keywords:knee joint injury; muscle fatigue; knee brace; running; counter-movement jump; knee joint moment; knee joint stability; pain; rehabilitation 1. Introduction The knee is one of the most frequently affected joints in the human body, with injuries involving various structures, including ligaments, menisci, cartilage, and bones [1–4]. In fact, knee injuries account for up to approx. 40% of all joint injuries in sports such as soccer, tennis, basketball, volleyball, and running [5–9]. Once the knee has been injured, the risk of reinjury increases significantly, with up to 30% of individuals sustaining a second injury, depending on the conditions [1,3,10–12]. Reinjuries commonly occur during prolonged or Bioengineering2025,12, 661

Bioengineering2025,12, 661 2 of 13 intense physical activities, as fatigue compromises muscle performance and neuromuscular control [2,11,13,14]. As a result, joint stability and function are compromised, increasing the risk of injury even during common movements such as running or jumping [15–19]. To reduce the risk of reinjuries and further damage to the knee joint, knee supports such as braces are frequently used. They provide external mechanical support through the elastic material and integrated lateral and medial rubber bands. Therefore, they promote better joint alignment, enhance joint stability and muscular function, reduce pain, and might improve proprioception through compression of the underlying musculoskeletal structures [15,16,20,21]. However, most studies investigating the effects of knee sup- ports on running and jumping have either focused on healthy subjects [14,15,17–20,22–28] or predominantly on subjects with anterior cruciate ligament (ACL) injuries [16,29–36]. Therefore, the effect of knee supports on the biomechanics of subjects with injured knee structures other than the ACL remains largely unclear. Moreover, none of these studies investigated subjects under conditions of muscle fatigue, which typically occurs dur- ing prolonged intense physical activities [2,11,13]. Most studies also used treadmills for walking or running [15,16,19,22,32,37,38], instead of overground walking or running on a force plate [20,21,30]. Although using a treadmill for biomechanical investigations mostly offers methodological advantages, it does not represent the natural running move- ment and shows various differences in kinetic and kinematic parameters compared to overground running [39–42]. Hence, in this study, we aimed to investigate the acute effects of wearing a knee brace on knee joint biomechanics in subjects with a history of various unilateral knee injuries or pain under conditions of muscle fatigue. We hypothesize that wearing a knee brace on the injured leg improves joint biomechanics by enhancing stability and kinematics (e.g., knee flexion angle), normalizing knee joint loading (e.g., knee flexion and abduction moments), and reducing pain during running and jumping. 2. Materials and Methods 2.1. Subjects In summary, 50 subjects with various unilateral knee joint injuries were recruited for this study (Table). The injuries must have occurred between 1 and 10 years prior to the examination. Subjects should

kinematics (e.g., knee flexion angle), normalizing knee joint loading (e.g., knee flexion and abduction moments), and reducing pain during running and jumping. 2. Materials and Methods 2.1. Subjects In summary, 50 subjects with various unilateral knee joint injuries were recruited for this study (Table). The injuries must have occurred between 1 and 10 years prior to the examination. Subjects should experience discomfort, such as mild to moderate pain or the feeling of instability or giving way in the knee joint during activities like running or jumping. Subjects should be between 18 and 50 years old, should have received a verbal or written return-to-sport recommendation from a medical doctor, and, therefore, should still be actively engaged in sports for at least 2 h per week. Exclusion criteria were acute injuries of the knee joint or any other joint of the leg within the last 3 months, use of prostheses or endoprostheses, inflammatory joint diseases, neurological diseases or dysfunctions, cardiovascular diseases, and any other conditions affecting motor performance. Before the examination, all subjects were informed about the study's purpose and provided written informed consent. All procedures were conducted in accordance with the Declaration of Helsinki and received approval from the Ethics Committee of Chemnitz University of Technology (reference number #101508546).

Bioengineering2025,12, 661 3 of 13 Table 1.Demographic and clinical data; presented as mean±SD. Age [years] Height [cm] Weight [kg] Gender [Male/Female] Physical Activity [Hours per Week] Side of Injured Leg [Left/Right] Time Since Injury [years] Therapeutic Intervention [Conserva- tive/Surgical] 33.5±9.6 178.5±9.7 74.4±12.8 31/19 6.4 ±4.0 28/22 6.1 ±3.7 27/23 Types of Injuries of the Knee Joint n (% of n) Ligament Injuries: e.g., partial or complete tear of anterior and/or posterior cruciate ligaments, medial and/or lateral collateral ligaments 16 (32) Meniscus Injuries: e.g., contusion or tear of the medial and/or lateral meniscus 16 (32) Cartilage Damage: e.g., Patellofemoral and/or femoral-tibial cartilage damage or osteoarthritis (level 1) 13 (26) Fractures and bone injuries: e.g., fractures of the patella or femur condyles 5 (10) Inflammations and others: e.g., nonspecific load pain, tendinopathy, plica syndrome, edema 13 (26) 2.2. Experimental Setup and Data Acquisition All anthropometric data were collected, and 16 reflective markers (Plug-in Gait lower body marker set) were attached to the subjects'pelvis and both legs and feet for motion capturing (Figure). Subsequently, subjects performed a standardized fatigue protocol to induce muscle fatigue in the lower extremities. The fatigue protocol consisted of three consecutive sets, each performed at an intensity level adjusted to each subject’s indi- vidual fitness. Each set included 60 s of jumping jacks, 20–30 repetitions of calf raises, 10–20 repetitionsof step-ups and side lunges, and 15–25 squat jumps. After completing the three sets, subjects performed a wall sit until physical exertion. Before and after performing the fatigue protocol, subjects rated their level of exertion using a Borg scale, which ranged from 6 (no exertion) to 20 (high exertion) [43]. Immediately after the fatigue protocol, subjects performed overground running and counter-movement jumps in randomized order on a force plate (Kistler, Winterthur, Switzerland; dimensions 0.6×0.9 m; sampling frequency 1000 Hz), while their kinematics were tracked using a 3D motion analysis system with 10 cameras (Vicon, Oxford, UK; sampling frequency 200 Hz, Nexus 2.10.2). For both running and jumping tests, five consecutive trials were conducted in a randomized order for each of the three measurement conditions: healthy leg, injured leg, and injured

Winterthur, Switzerland; dimensions 0.6×0.9 m; sampling frequency 1000 Hz), while their kinematics were tracked using a 3D motion analysis system with 10 cameras (Vicon, Oxford, UK; sampling frequency 200 Hz, Nexus 2.10.2). For both running and jumping tests, five consecutive trials were conducted in a randomized order for each of the three measurement conditions: healthy leg, injured leg, and injured leg with a knee brace. After completing both the running and jumping tests, subjects filled out a visual analog scale (VAS) to assess knee pain and stability for each of the three measurement conditions, with ratings ranging from 0 (no pain, very stable) to 10 (extreme pain, very unstable), respectively (Table). All tests were conducted using the subjects 'regular sports shoes. The knee brace used in this study was the “Sports Knee Support” brace (Bauerfeind AG, Zeulenroda-Triebes, Germany). According to the manufacturer, the brace is made of ultralight elastic knit material, which provides an alternating pressure massage during movement. Integrated lateral and medial rubber bands offer stability, while the embedded knitted elastic silicone pad securely positions and guides the kneecap, ensuring optimal force distribution within the knee joint.

Bioengineering2025,12, 661 4 of 13 Figure 1.Subject performing the running (A) and jumping (B) tests on the force plate. For motion tracking 16 reflective markers (Plug-in Gait lower body marker set) were attached bilaterally to the pelvis (spina iliaca anterior superior and spina iliaca posterior superior), thighs, knee joints, tibiae, ankle joints, and toes. From the force plate and Vicon data, biomechanical parameters such as maximal vertical ground reaction force, ground contact time, maximal knee flexion angle, and maximal knee flexion moment were extracted. Note that for the knee moment, positive values represent flexion moments, while negative values indicate extension moments. Table 2.Effects of the fatigue protocol on subjective rating and jumping height (mean±SD). Statistically significant differences between conditions are indicated with a. Parameter Before Fatigue Protocol After Fatigue Protocol p-Value d Fatigue Protocol Fatigue (6–20) 7.2 ±1.6 a 12.0 ±2.7 a a < 0.001 a = 0.73 Maximal Jumping Height [cm] 35.2±7.9 a 33.4 ±7.9 a a < 0.001 a = 0.23 Running and Jumping Tests Before the running test, the individual running speed was determined for each subject. For this purpose, each subject ran five times over the force plate, starting approximately

Bioengineering2025,12, 661 5 of 13 7 m before it, while the running speed was measured using light barriers (ALGE-TIMING, Lustenau, Austria) (FigureA). From these five runs, the average speed was calculated, with±5% as the upper and lower speed limits. If a subject exceeded their individual running speed limits or if the force plate was missed with the leg to be tested, the trials were repeated until five valid trials were collected for each measurement condition. For the counter-movement jumps, subjects were instructed to stand in front of the force plate with a hip-width stance, keeping their arms crossed on the chest to prevent the reflective markers on the hips from being obscured during measurement. Upon an acoustic signal, the subjects stepped onto the force plate with the leg to be tested. Imme- diately afterward, they performed the counter-movement jump by flexing the knee joint to approximately 90 ◦ , followed by an explosive push-off, fully straightening the legs and jumping as high as possible while keeping their arms still. After executing the jump, the subjects were required to stand upright again, wait for a few seconds, and then step off the force plate (FigureB). Trials were considered invalid and had to be repeated if the legs were bent during the jump, the jump went forward instead of upward, or if the foot of the tested leg did not land on the force plate completely. For each measurement condition, five trials were collected. 2.3. Statistical Analysis Biomechanical parameters were extracted from force plate and Vicon data using a custom routine written in MATLAB R2023b (MathWorks™, Natick, MA, USA). Depending on the task, this included the maximal vertical ground reaction force, ground contact time, maximal knee flexion angle, maximal knee flexion moment, maximal knee abduction mo- ment, maximal medio-lateral knee acceleration, and maximal jump height. All parameters were calculated for the ground contact phase. For statistical analysis, the means and stan- dard deviations (mean±SD) were calculated for each biomechanical parameter based on the five trials for each subject. The Shapiro–Wilk test was used to assess normal distribution. To investigate subjects'fatigue induced by

maximal knee abduction mo- ment, maximal medio-lateral knee acceleration, and maximal jump height. All parameters were calculated for the ground contact phase. For statistical analysis, the means and stan- dard deviations (mean±SD) were calculated for each biomechanical parameter based on the five trials for each subject. The Shapiro–Wilk test was used to assess normal distribution. To investigate subjects'fatigue induced by the fatigue protocol, as well as subjects'stability and pain, a sign test was used. Jumping heights before and after completing the protocol were compared using at-test. To evaluate differences between measurement conditions (healthy leg, injured leg, injured leg with knee brace), a one-way ANOVA for repeated measures followed by Bonferroni post hoc tests was conducted for normally distributed data, while the Friedman test was used for non-normally distributed data. Statistical significance for all tests was set atα= 0.05. Effect sizes were calculated using Cohen's d and categorized as trivial (<0.2), small (<0.5), medium (<0.8), or large (≥0.8). Subjective data from the ratings were analyzed using the t-test for independent samples if normally distributed and the Mann–Whitney test if not normally distributed. 3. Results 3.1. Demographic, Clinical, and Subjective Data Table included 50 participants, with a predominance of males (n= 31) over females(n= 19).The most frequently injured structures were ligaments and menisci, followed by cartilage damage, inflammations, and, lastly, fractures and bone injuries. These injuries occurred more frequently in the left leg (n= 28) than in the right leg (n= 22), and the mean time since injury was 6.1±3.7 years. Most injuries were treated conservatively (n= 27) rather than surgically (n= 23). Table exhibited a significantly higher subjective level of physical exertion compared to before, which was also reflected by a significant reduction in jumping height.

Bioengineering2025,12, 661 6 of 13 The subjective ratings on the VAS indicated a significant reduction in pain and greater stability when wearing the knee brace on the injured leg while running. Subjects also reported significantly increased knee stability during jumping (Table). Table 3.Effects of the knee brace on subjective ratings of pain and stability for the running and jumping tests (mean±SD). Statistically significant differences between conditions are indicated with a. Parameter Injured Leg Injured Leg with Knee Brace p-Value d Running Pain (0–10) 2.1 ±1.4 a 1.9 ±1.3 a a = 0.004 a = 0.15 Stability (0–10) 2.0±1.4 1.8 ±1.3 0.125 - Jumping Pain (0–10) 2.2 ±1.4 2.2 ±1.4 1.000 - Stability (0–10) 2.0±1.4 a 1.9 ±1.3 a 0.070 - 3.2. Motor Performance 3.2.1. Running The average running speed across all subjects and measurement conditions was 3.3±0.4 m/s. Several statistically significant differences were found between the measure- ment conditions for the running test. When comparing the healthy leg with the injured leg, the injured leg primarily showed lower maximal knee abduction moments, whereas the other parameters did not show statistically significant differences (Figure). The effect of the brace resulted in significantly increased maximal knee flexion moments, decreased maximal knee abduction moments, and significantly reduced maximal medio- lateral knee accelerations when comparing the injured leg with and without the knee brace (Figure). Moreover, wearing the brace on the injured leg resulted in significantly less ground contact time during running compared to the injured leg without the brace (Table). Figure 2.Comparison of the three measurement conditions (healthy leg, injured leg, and injured leg with knee brace) for the biomechanical parameters: (A) maximal knee flexion moment, (B) maximal knee abduction moment, and (C) maximal medio-lateral knee acceleration for the running test.

Bioengineering2025,12, 661 7 of 13 Table 4.Comparison of the three measurement conditions (healthy leg, injured leg, and injured leg with knee brace) for the biomechanical parameters: maximal knee flexion angle, maximal vertical ground reaction force, and ground contact time for the running test (mean±SD). Statistically significant differences between the three measurement conditions are indicated with a, b. Parameter Healthy Leg Injured Leg Injured Leg with Knee Brace p-Value d Maximal Knee Flexion Angle [ ◦ ] 43.7±6.5 42.1 ±6.4 43.6 ±6.0 0.107 - Maximal Vertical Ground Reaction Force [Body Weight] 2.43±0.37 2.38 ±0.34 2.43 ±0.36 0.232 - Ground Contact Time [ms] 258.2±32.2 a 257.1±33.1 b253.8±34.0 a; b 0.023 a = 0.038 a = 0.13 b = 0.015 b = 0.10 3.2.2. Counter-Movement Jump For the jumping test, only the maximal knee flexion angle revealed statistically sig- nificant differences, with lower values for the injured leg compared to the healthy leg (Table). When comparing the injured leg without and with the brace, the effect of the brace was associated with significantly higher maximal knee flexion moments, reduced maximal medio-lateral knee acceleration (Figure), as well as higher maximal knee flexion angles (Table). Figure 3.Comparison of the three measurement conditions (healthy leg, injured leg, and injured leg with knee brace) for the biomechanical parameters: (A) maximal knee flexion moment, (B) maximal knee abduction moment, and (C) maximal medio-lateral knee acceleration for the jumping test.

Bioengineering2025,12, 661 8 of 13 Table 5.Comparison of the three measurement conditions (healthy leg, injured leg, and injured leg with knee brace) for the biomechanical parameters: maximal knee flexion angle, maximal vertical ground reaction force and jumping height for the jumping test (mean±SD). Statistically significant differences between the three measurement conditions are indicated with a, b. Note that the parameter jumping height was only compared between the measurement conditions of the injured leg without and with brace. Parameter Healthy Leg Injured Leg Injured Leg with Knee Brace p-Value d Maximal Knee Flexion Angle [ ◦ ] 77.1±23.2 a 74.4±22.2 a; b 79.2±22.9 b a = 0.015 b < 0.001 a = 0.12 b = 0.21 Maximal Vertical Ground Reaction Force [Body Weight] 2.17±0.87 2.04 ±0.77 2.03 ±0.85 0.162 - Maximal Jumping Height [cm] - 33.7 ±6.8 a 33.2 ±6.8 a a = 0.007 a = 0.07 4. Discussion In this study, we investigated the acute effects of wearing a knee brace on knee joint biomechanics in subjects with a history of various unilateral knee injuries or pain under conditions of muscle fatigue. We hypothesize that wearing a knee brace on the injured leg improves joint biomechanics by enhancing stability and kinematics (e.g., knee flexion angle), normalizing knee joint loading (e.g., knee flexion and abduction moments), and reducing pain during running and jumping. 4.1. Effects of the Injuries The effect of the injury resulted in several differences between the healthy and injured legs during running and jumping. These included significantly lower maximal knee abduction moments and reduced vertical ground reaction forces. Since our subjects reported slight pain during the tests on the VAS scale (2.1±1.4), this reduction in the maximal load may have been due to increased awareness and caution [16,21,29–31]. This could also explain why the injured leg exhibited shorter ground contact times during the stance phase of running. The injured leg also showed significantly reduced flexion during the ground contact phase, particularly during the landing phase of jumping. Besides pain, this may have been caused by the sensation of instability, as reported by the subjects on the VAS scale

The study examines how knee braces affect biomechanics during running and jumping in individuals with knee injuries.