1 Introduction
Competitive sports like basketball involve jump-landings that involve quick, frequent, and intense movements during practice and games. An athlete’s fatigue and asymmetric postures cause increased joint torque, resulting in greater chances of injuries and deteriorated performance [
16][
8].
In competitive sports, characterising and evaluating an athlete’s fatigue and readiness non-invasively is crucial for helping coaches and fitness professionals tailor training programs. Vertical jumping is an essential motor skill in basketball [
9] and can be used to characterise fatigue. The Countermovement Jump (CMJ) test is an efficient, non-invasive method for assessing jumping performance, neuromuscular fatigue, and readiness. The Reactive Strength Index modified (
RSImod) is a CMJ-driven metric that reflects lower extremity explosiveness, combining jump height and contact time to evaluate an athlete’s capacity for generating maximum vertical impulse quickly [
4] [
10]. Studies show
RSImod’s positive association with strength and endurance performance and a negative association with acceleration and change of direction speed [
12]. Research also indicates that
RSImod is correlated with net impulse and jump height [
20], with performance worsening when countermovement depth is limited (less) [
19]. Biomechanical analyses of drop jumps and simulations highlight the importance of knee flexion [
13], knee kinematics, stance width [
22], trunk stability [
7], and foot rotation [
6], all contributing to enhanced jump performance.
Traditional methods adopted by sports coaches for evaluating athlete jump performance involve manual monitoring and correcting them during training sessions. However, this approach is labour-intensive, error-prone, non-scalable, non-flexible, opaque, and has inter-expert variability [
11]. In contrast, recent progress in Machine Learning (ML) and Computer Vision (CV) technologies has transformed sports analytics by enabling non-intrusive monitoring of athletes and developing data analytic models and their analysis through collaborative efforts involving coaches, biomechanical researchers, and data scientists [
24]. CV techniques can accurately measure joint movements and forces, providing precise kinematic data crucial for performance analysis and injury prevention without the need for intrusive markers [
2]. ML models integrated with CV significantly enhance the detection of movement patterns and prediction of potential injuries by identifying subtle asymmetries often missed by traditional methods [
3]. Real-time evaluation systems offer immediate feedback and comprehensive data analysis, optimising training regimens and enabling quick adjustments during sessions [
17]. Additionally, integrating artificial intelligence (AI) in CV frameworks allows for the rapid and accurate analysis of large volumes of video data, improving the prediction of injury risks and developing personalised training programs [
5]. For instance, a study on sports assist systems collected and analysed sensor data from athletes’ limbs during training and incorporated ML models to identify errors or suggest improvements in athletic movements [
24]. Another study utilised cameras to record jump exercises performed by athletes and designed a CV-based framework that analysed lower-limb asymmetries during jump-landing for injury risk assessment [
21].
Current studies primarily focus on quantitative fatigue analysis in team sports [
20][
19][
13]. However, no research has assessed athletes’ fatigue and game readiness regarding biomechanical features - lower-extremity joint mechanics in motion and load. This research helps characterise the relationship between the biomechanical features (kinetic-kinematic) extracted during jump-landings and
RSImod. The quantitative analysis helps predict athletes’ readiness in
RSImod. As shown in Figure
1, we employ a CV-based framework for biomechanical assessment of athletes’ jump landings to predict athletic readiness. The framework uses motion to analyse jump-landing biomechanics and it looks at overall movement quality, symmetry, coordination, and fluidity during jump-landing. As an outcome, it predicts an athlete’s
RSImod score for the following week. This comprehensive evaluation aids in developing effective training schedules and game strategies that optimise performance while minimising the risk of injury.
Thus, this study’s primary aims (PAs) are
(PA-I): design of a CV-based framework for biomechanical assessment of jump landings to extract lower-extremity kinetic-kinematic features,
(PA-II): examining the impact of fatigue on these features and their relationship with the CMJ-derived
RSImod score of athletes,
(PA-III): prediction of the
RSImod score for the following week based on the biomechanical feature using ML algorithms,
(PA-IV): plotting SHapley Additive explanations (SHAP) values to offer coaches detailed insights into the specific impact of each biomechanical feature for individual athletes.
2 Proposed Framework
Figure
1 illustrates a detailed workflow for analysing athletic performance through video data to provide actionable feedback to coaches. The process begins with capturing frontal and lateral videos of an athlete performing the countermovement jumps. These videos are processed using Google’s MediaPipePose [
14] with the BlazePose model, which detects 33 key joint coordinates, generating a 2D joint coordinate skeleton for the athlete. The skeleton is then analysed to identify key frames, specifically focusing on critical moments such as initial contact and maximum knee flexion in both the frontal and lateral views.
Once these key frames are identified, feature extraction is performed, yielding distinct kinetic and kinematic biomechanical features that capture various aspects of the athlete’s movements. These features are then subjected to a relationship assessment to determine their correlation with the RSImod. An XGBoost regressor, an ensemble ML algorithm, predicts RSImod values based on the extracted features.
2.1 Subjects
This study involved 17 female basketball athletes of the women’s basketball team at SHU University, with the following demographic characteristics: mean age of 21.00 years (±3.00), height of 174.21 cm (±19.27), and body mass of 73.98 kg (±11.52), competing in AABB Division I. The research received approval from the Institutional Review Board (IRB), and all subjects were provided with information about the study and consented before participating (IRB approval number 170720A on 9/14/2020).
2.2 Procedures
We used two Panasonic LUMIX FZ80 - 4K digital cameras (with an 18.1-megapixel sensor, 60X zoom DC VARIO lens with a focal range of 20-1200mm and an aperture range of F2.8-5.9), offering a high resolution and stabilisation for recording the videos from frontal and lateral planes. The athletes performed CMJ over dual force plates (FDlite, Vald Performance, Brisbane, QL, AUS) with a sampling rate of 1000 Hz), as a part of their regular training routine. These cameras were placed 136 inches in front of and to the side of the jump spot. Figure
2 shows the camera setup for video recording. Each CMJ session was scheduled on the same day of the week, typically on Monday, to maintain consistency. Athletes were directed to maximise jump height and minimise ground contact time. After placing their hands on their sides and executing a deep squat, they performed a maximal jump. Three CMJs were performed following a low-intensity warm-up. The highest
RSImod score from the three CMJs in a session was considered for analysis.
2.3 Pose Estimation
Initially, pose estimation is performed using Google’s MediaPipe Pose [
14]. This lightweight model runs on GPU and CPU devices, ensuring flexibility and high performance. The model generates 33 pose landmarks, each characterized by two coordinates—
x and
y determine its position in the frame and visibility, indicating the confidence of the estimated key point. Figure
3 shows the 33 body landmarks detected. We utilised 16 body landmarks (11-16 and 23-32) for biomechanical feature extraction.
We applied normalisation and scaling techniques to ensure that the model remains impartial to subject size and appearance variations, facilitating broad applicability without reliance on specific details of the input skeleton. The normalised values are calculated once and applied across all frames.
•
Vertical axis (y) normalisation: Divide the given value by the sum of vertical distances between landmarks (11-left shoulder, 23-left knee), (23-left knee, 25-left hip), and (27-left ankle, 25-left knee)—representing the height of the subject’s torso and leg.
•
Horizontal axis (x) normalisation: The values are divided by the sum of horizontal distances between landmarks (15 -right waist, 13-right elbow), (13-right elbow, 11-right shoulder), (11-right shoulder, 12-left shoulder), (12-left shoulder, 14-left elbow), and (14-left elbow, 16-left wrist)—representing the width of the torso and both upper extremities.
2.4 Key Frame Identification
The initial contact (IC) frame is critical in CMJ analysis as it marks the moment of ground contact, enabling precise timing measurements, jump height calculation, and delineation of jump phases. On the other hand, the frame with maximum knee flexion KFmax is crucial for biomechanical analysis. In the next step, we used Algorithm 1 and 2 to extract these key (IC and KFmax) video frames.
Figure
4 demonstrates
KFmax identification over a sample video. It shows two plots, one for each knee. The line in blue marks the original values, while the orange marks the rolling average (size = 3). For example, in both cases, we observe that the two consecutive intersection points pair between these lines with the highest distance in frame numbers 6 and 8. The distance between point 8 and its consecutive points (7, 9) is longer than 6 and its consecutive points (5, 7). Hence, opting for the point with the lowest
x value (of 8’s neighbours), we conclude frame 7 as the
KFmax frame.
2.5 Biomechanical Feature Extraction
After contact with the ground, the reaction to the load adds to the body weight. Thus, the kinematic results are described by the lower angular movements of the body. In the next stage, we performed lower extremity kinetic-kinematic feature extraction. (
PA-I). Equation
1 shows the formula for cosine angle calculation (for knee flexion angle) and
2 for Euclidean distance calculation (for stance width).
where:
•
\(\overrightarrow{L_{23, 25}}\) denotes the vector between the landmarks 23 (left hip) and 25 (left knee).
•
\(\overrightarrow{L_{25, 27}}\) denotes the vector between the landmarks 25 (left knee) and 27 (left ankle).
2.5.1 Knee Flexion Angle (frontal and lateral):
(1)
Compute the left knee flexion angle using the cosine of coordinates for points (23-left hip, 25-left knee, 27-left ankle).
(2)
Compute the right knee flexion angle using the cosine of coordinates for points (24-right hip, 26-right knee, 28-right ankle).
2.5.2 Countermovement Depth (lateral):
Determine the countermovement depth by calculating the hip keypoint’s vertical displacement (23-left knee or 24-right knee) from the start to the lowest point during the countermovement.
2.5.3 Hip Flexion Angle (frontal and lateral):
(1)
Compute the left hip flexion angle using the cosine of coordinates for points (11-left shoulder, 23-left hip, 25-left hip) (refer equation
1).
(2)
Compute the right hip flexion angle using the cosine of coordinates for points (12-right shoulder, 24-right hip, 26-right knee).
2.5.4 Trunk Flexion Angle (frontal and lateral):
(1)
Compute the left trunk flexion angle using the cosine of coordinates (shoulder, hip, knee) for points (11, 23, 25).
(2)
Compute the right trunk flexion angle using the cosine coordinates for points (12, 24, 26).
2.5.5 Lateral Trunk Flexion Angle (frontal):
Determine the angle formed by two lines: one from the hip midpoint to the top of the frame and the other connecting the hip midpoint to the shoulder midpoint.
2.5.6 Ankle Plantar Flexion (frontal and lateral):
Assess the angle between a reference line from the heel to the bottom of the frame and a line connecting the heel (29 or 30) and footindex (31, 32) key points.
2.5.7 Stance Width (frontal):
Calculate the ratio of the Euclidean distance between the left and right shoulder key points (11, 12) to the distance between the left and right ankle key points (27, 28) as shown in equation
2. Stance width could be normal
\((1)\), narrow (> 1), and wide (< 1).
2.5.8 Foot Symmetry (frontal and lateral):
Examine the y-coordinate of the left and right footindex (31 or 32) to assess whether the feet landed simultaneously. If the feet land simultaneously, we assign the value 1 else 0.
2.5.9 Foot Orientation (frontal):
Assess the angle between a reference line from the heel to the bottom of the frame and a line connecting the heel and footindex key points. The orientation can be toe-in (internal rotation), toe-out (external rotation), or neutral (no rotation).
2.6 Visualisation over Kinovea
Figure
5 visually represents some of the features. Figure
5(a) depicts the calculation of foot rotation (internal or external), which reflects foot orientation. The right foot of the athlete appears externally rotated, while the left, however slightly internally rotated, is in a near-neutral orientation position. Figure
5(b) demonstrates the knee and trunk flexion angles. Figure
5(c) shows lateral trunk flexion, where the athlete has turned towards the right, resulting in a 3° flexion. Figure
5(d) demonstrates the hip flexion angle and relative countermovement depth (297.71px) as the difference between the hip and the reference keypoint on the force plate. We used the software Kinovea (www.kinovea.org, version 0.8.25) [
18] to visualise angles and distances.
2.7 Athletic Readiness Prediction
Next, we conducted statistical analysis to assess and visualise the marginal effects of the extracted features on the
RSImod score. It was observed that there was an imbalance in the distribution of data records, with only 38% attributed to high and very high
RSImod scores. In comparison, the remaining 62% belonged to low and moderate
RSImod scores. Hence, we augmented the dataset using the data balancing technique - Synthetic Minority Over Sampling with Gaussian Noise (SMOGN) [
1] and then trained an XGBoost regressor over the dataset for
RSImod prediction based on these extracted features.
2.8 Model Interpretations
We generated SHAP [
15] values for these features to provide detailed individual insights into the specific impact of each biomechanical feature for athletes to coach. SHAP excels in identifying joint effects and interactions, providing a comprehensive view of how biomechanical elements collaborate, which is valuable for capturing complexity in biomechanical analyses, and contributing to individualised and global understandings of feature contributions to
RSImod scores.
3 Results and Discussion
A total of 84 countermovement jumps videos were subjected to analysis. The RSImod score was quantified in mm/ms (from force plate) as an outcome of the CMJ assessments conducted weekly over a duration spanning from week 1 to week 26. The videos were recorded at a resolution of 1920×1080 pixels. Each jump session was approximately 30 seconds long. The frame rate set was 70 fps, resulting in approximately 210 frames (average 200) per session.
The mean
RSImod score was 0.37, with a standard deviation (SD) of 0.08. To validate the effectiveness of our developed framework, we computed the
RSImod score from the CMJ videos (addressing
PA-I). Specifically, we quantified the jump height by measuring the vertical displacement of the hip keypoint (23 or 24) from the starting position to the peak of the jump. Frames corresponding to take-off and landing moments were pinpointed by analyzing body posture or velocity changes, with timestamps recorded for precise timing. The contact time was then computed by subtracting the take-off timestamp from the landing timestamp. Using the following equation, we computed the
RSImod scores.
This resulted in a mean
RSImod score of 0.34 with 0.05 SD. Notably, the correlation (
r) between force plate-measured and framework-measured
RSImod scores was a high 0.94, affirming the framework’s accuracy. Table
1 details the statistical significance of the numeric features for the
RSImod score in terms of Pearson’s correlation coefficient (
r) and the effect size (ES). ES is a quantitative measure of the strength or magnitude of a relationship or difference between variables in a study. It was used to quantify the real-world impact of biomechanical features on athletic performance regarding the magnitude of association [
23].
3.1 Inferential Analysis (addressing PA-II):
Knee Flexion Angle:
Figure
6 shows the relationship between knee flexion angle and RSImod. The analysis reveals a positive correlation between increased knee flexion and higher
RSImod scores. Optimal knee flexion, implying a deeper squat position during landing, positively influences jump height. It is observed that, on average, Initial knee flexion typically ranges from 20° to 30°, with the majority of data points falling within the +1SD range of
RSImod scores. At maximum flexion, data points predominantly range from 60° to 70°.
RSImod increases with knee flexion, although a decrease is above 85°.
Countermovement Depth.
Countermovement depth positively correlates with
RSImod as shown in Figure
7. Deeper countermovement allows for increased potential energy storage, enhancing force production and leading to a more robust and explosive upward movement during the jump.
Hip Flexion Angle.
We observe a positive correlation between increased Hip Flexion and elevated
RSImod scores in Figure
8. Adequate hip flexion is identified as a contributing factor to efficient energy storage and release, ultimately leading to a more powerful jump. The initial hip flexion typically ranges from 30° to 45°, with most data points falling within the +1SD range of
RSImod scores. At
KFmax, an increase in
RSImod is noted with higher hip flexion (160°-170°). However, above 1SD, some cases exhibit below-average RSI scores, suggesting the need for optimal hip flexion.
Trunk Flexion Angle.
We observe a positive correlation between increased Trunk Flexion and elevated
RSImod scores in Figure
9. The observation suggests that bending forward is crucial in engaging key muscle groups, facilitating elastic element preloading, and enhancing the force for upward propulsion during the jump. Data analysis indicates that initial trunk flexion typically ranges from 0° to 20°. Both values above and below 1SD result in low RSI scores. At
KFmax, the observed range extends from 60° to 80°, with values below and above this range associated with low RSI scores. Figure
9 shows the relationship between trunk flexion angle and RSImod.
Ankle Plantar Flexion.
Ankle plantar flexion negatively correlates with
RSImod (Figure
10). Excessive ankle plantar flexion negatively impacts force production efficiency during the push-off phase, emphasising the importance of optimal ankle plantar flexion for power generation.
Lateral Trunk Flexion.
Figure
11 shows that lateral trunk flexion negatively correlates with
RSImod. There must be minimal lateral movement during jump-landings, ideally close to zero degrees. Excessive lateral movement disrupts force distribution, compromising the coordinated engagement of muscles and elastic elements involved in the countermovement.
Stance Width.
A shoulder width to a slightly wider stance enhances force transfer, optimizing force production and biomechanical balance for an improved
RSImod (Figure
12). A narrow stance may hinder
RSImod, limiting the body’s capacity for efficient force generation and transfer in the countermovement, potentially resulting in a low
RSImod score.
Foot Symmetricity.
In Figure
13, We observed a positive correlation with
RSImod. Symmetric landing with equal force distribution on both feet contributes to balanced force distribution, optimizing force production and transmission mechanics during the countermovement.
Foot Orientation.
It is observed in Figure
14 that individuals with a neutral or slightly outward foot orientation tend to achieve higher RSI scores. Optimal foot orientations positively impact force transmission, while less ideal foot orientations, such as toe-in positions, may be associated with disruptions in force generation and transmission, potentially leading to poor RSI scores.
3.2 RSImod Prediction (addressing PA-III )
As a preprocessing step, we applied the SMOGN [
1] technique to the dataset to address the imbalance issue, resulting in an augmented overall sample size of 219 records. After the rebalancing process, 50% of the records were now associated with high and very high levels of athlete readiness, and the remaining 50% represented low and moderate levels. We employed a stratified 10-fold cross-validation approach for model generalization. The dataset was partitioned into training (70%) and testing (30%) sets. Specifically, 154 records were utilised for training, and 65 were reserved for testing. The performance evaluation of the XGBoost regressor (with hyperparameter values
colsamplebytree = 0.8,
gamma = 0.1,
maxdepth = 5,
minchildweight = 1,
nestimators = 200; as identified using grid search), trained using
RSImod as the target feature, yielded a Mean Squared Error (MSE) of 0.020 (ideal 0) and adjusted
R2 of 0.892.
Table
2 summarises a comparison of proposed work with state-of-the-art in terms of qualitative and quantitative outcomes. The proposed study achieved a mean RSImod score of 0.34 with a strong correlation (r = 0.94) between force plate and framework measurements, surpassing the inter-rater reliability of 0.85 reported by Padua et al. (2009). Significant effect sizes for biomechanical features, particularly countermovement depth (0.781) and lateral trunk flexion (-0.721), underscore the framework’s robustness in evaluating jump mechanics. Additionally, the application of machine learning to predict RSImod, with an adjusted R² of 0.892, introduces a powerful new method for assessing athlete readiness in real time.
3.3 Interpretability and Explanation (addressing PA-IV)
We plotted SHAP values to break down the model’s prediction into contributions from each feature, quantifying each feature’s impact on athletic readiness prediction [
15]. Figure
15 presents model-average SHAP values. Significant features, such as trunk flexion angle at maximum knee flexion, hip flexion angle, countermovement depth, and ankle plantar flexion, correlate with biomechanical principles widely recognised in sports science. The emphasis on trunk flexion at
KFmax is crucial as it reflects the forward bending of the upper body, engaging essential muscle groups and enhancing elastic element preloading for potent upward propulsion. The negative impact of excessive trunk flexion after landing aligns with coaching principles that discourage such post-landing movements. Adequate hip flexion is recognised for efficient energy storage, supporting optimal power generation.
In contrast, the caution against excessive hip flexion echoes established knowledge on avoiding inefficiencies in force production. The positive correlation of countermovement depth with RSImod resonates with the understanding that a deeper countermovement allows for increased potential energy storage, facilitating a more potent upward movement. The inverse correlation of ankle plantar flexion with RSImod emphasises the significance of maintaining proper ankle positioning for efficient force production during the push-off phase of a jump.
3.4 Feedback to coach
The study presents outcomes to coaches using visual dashboards and individual athlete profiles, featuring SHAP value plots and historical RSImod trends for quick overviews. Comparative charts highlight performance patterns, and video overlays show biomechanical adjustments during jumps. These tools support the creation of individualized training programs, aid in identifying features affecting RSImod scores for injury prevention, and enable efficient allocation of training resources by focusing on critical biomechanical factors that influence athletic readiness.
4 Conclusion
This CV-driven semantic analysis of videos is a significant improvement, providing an innovative and holistic approach to sports performance evaluation compared to conventional, manual-centric techniques. By deciphering the intricate relationship between biomechanical features and RSImod, athletes and coaches can fine-tune training regimens to enhance reactive strength. It optimises performance and plays a pivotal role in injury prevention. With RSImod prediction and SHAP explanations, the individualised nature of this predictive approach ensures that training programs are tailored to an athlete’s unique characteristics, fostering targeted improvements.
The framework, on average, can analyse a single video in approximately 90 seconds (with the potential for further reduction on more advanced hardware configurations) and predict an RSImod score with an MSE of 0.020 and R2 of 0.892. Jump tests, a standard practice in sports like basketball, volleyball, and soccer for assessing athlete readiness, can benefit from this framework. The suggested two-camera arrangement is convenient for implementation in athletic training facilities, expanding the system’s applicability.
We believe our approach of continuously monitoring the biomechanical features and their impact on the RSImod (weekly) facilitates real-time adjustments, enabling athletes to track progress and make informed decisions about their training journey. Coaches gain valuable insights into biomechanical features, empowering them to provide precise feedback and interventions. As the framework becomes more robust with the coming seasons’ data, it could replace force plates as we could predict athletic readiness (RSImod score) for athletes only based on their biomechanical features.