Discriminative validity of summarized hip and knee angular accelerations for lower extremity training load quantification in male soccer players during a standardised training drill.

This study assessed the discriminative validity of summarized hip and knee angular accelerations during a standardized training drill. Twenty-eight soccer players performed a standardized training drill that mimics game demands. Discriminative validity was examined by assessment of between-group differences of summarized preferred kicking leg hip and knee angular accelerations, and Playerload between national and regional soccer players for the full training drill, and parts based on locomotor intensity, or additional pass and jumping header activities. Furthermore, relationships were assessed between the summarized hip and knee angular accelerations and conventional load indicators derived from a local positioning measurement system, such as high-intensity running distance and Playerload. National players had higher summarized hip (Mean difference: 62.7 A.U. ES = 0.77, p = 0.049) and knee (Mean difference: 137.1 A.U. ES = 1.06, p = 0.008) angular accelerations. Significant interaction effects were observed during high-intensity running (Hip: 0.2 A.U./m, ES = 0.98, p = 0.005; Knee: 0.61 A.U./m, ES = 1.52, p < 0.001), and sprinting (Hip: 0.3 A.U./m, ES = 1.01, p < 0.02; Knee: 0.56 A.U./m, ES = 1.57, p < 0.001). Between-group differences were not present for additional passing or jumping header activities. Compared to summarized hip and knee angular accelerations, Playerload had less ability to discriminate between players and activities. Moreover, the lower extremity training load indicators were unrelated to conventional load indicators. Together these results confirm discriminative validity of summarized hip and knee angular acceleration training load indicators during a standardised training drill.

The results in this study substantiate the use and validity of summarized hip and knee angular accelerations as external training load indicators for the lower extremities in soccer. Based on the known-group difference technique, expected differences in summarized hip and knee angular accelerations between national and regional soccer players were observed during a standardized soccer training drill. These differences were present during high intensity running and sprint activities, but not for passing or jumping header activities. The limited ability of Playerload to discriminate between players and tasks, and insignificant relationships between measurement systems in this study demonstrate that summarized hip and knee angular accelerations can be used complementary to whole-body training load indicators for evaluating soccer training and match play. The presented methodology provides insights in the training load of individual limbs, which potentially can be used to assess the effectiveness of specific training drills to under- or overload specific limbs. Trainers, coaches, and sport practitioners can use this information to balance training load and recovery in soccer, aiming to improve performance and prevent overload.

Construct Validity and Test-Retest Reliability of Hip Load Compared With Playerload During Football-Specific Running, Kicking, and Jumping Tasks.

PURPOSE: To determine the test-retest reliability of the recently developed Hip Load metric, evaluate its construct validity, and assess the differences with Playerload during football-specific short-distance shuttle runs. METHODS: Eleven amateur football players participated in 2 identical experimental sessions. Each session included 3 different shuttle runs that were performed at 2 pace-controlled running intensities. The runs consisted of only running, running combined with kicks, and running combined with jumps. Cumulative Playerload and Hip Loads of the preferred and nonpreferred kicking leg were collected for each shuttle run. Test-retest reliability was determined using intraclass correlations, coefficients of variation, and Bland-Altman analyses. To compare the load metrics with each other, they were normalized to their respective values obtained during a 54-m run at 9 km/h. Sensitivity of each load metric to running intensity, kicks, and jumps was assessed using separate linear mixed models. RESULTS: Intraclass correlations were high for the Hip Loads of the preferred kicking leg (.91) and the nonpreferred kicking leg (.96) and moderate for the Playerload (.87). The effects (95% CIs) of intensity and kicks on the normalized Hip Load of the kicking leg (intensity: 0.95 to 1.50, kicks: 0.36 to 1.59) and nonkicking leg (intensity: 0.96 to 1.53, kicks: 0.06 to 1.34) were larger than on the normalized Playerload (intensity: 0.12 to 0.25, kicks: 0.22 to 0.53). CONCLUSIONS: The inclusion of Hip Load in training load quantification may help sport practitioners to better balance load and recovery.

Biomechanical Load Quantification Using a Lower Extremity Inertial Sensor Setup During Football Specific Activities.

Training load monitoring systems in football do not focus on lower extremities and therefore potentially neglect important information to optimise performance or reduce injury risk. The current study aims to present joint and segment angular accelerations as novel indicators to quantify lower extremity biomechanical load measured by a new inertial sensor setup. Relationships were explored with commonly used whole-body training load indicators using principal component analysis (PCA). Sixteen male amateur football players performed a linear sprint and an agility T-test. An inertial sensor setup, and local position measurement system were used to collect training load data. Hip Load, Knee Load, Thigh Load and Shank Load were introduced to quantify lower extremity biomechanical load. Three principal components were identified for both tests, explaining 91% and 86% of the variance. The indicators for the lower extremities contributed to the second principal component for both tests and provide distinct information compared to whole-body load indicators. The results show the potential to use an inertial sensor setup combined with common monitoring systems to evaluate training load, which may help optimise future performance and reduce injury risk. These relationships should be further examined during other football specific activities such as shooting or jumping.

Associations between Hamstring Fatigue and Sprint Kinematics during a Simulated Football (Soccer) Match.

PURPOSE: Neuromuscular fatigue is considered to be important in the etiology of hamstring strain injuries in football. Fatigue is assumed to lead to decreases in hamstring contractile strength and changes in sprinting kinematics, which would increase hamstring strain injury risk. Therefore, the aim was to examine the effects of football-specific fatigue on hamstring maximal voluntary torque (MVT) and rate of torque development (RTD), in relation to alterations in sprinting kinematics. METHODS: Ten amateur football players executed a 90-min running-based football match simulation. Before and after every 15 min of simulated play, MVT and RTD of the hamstrings were obtained in addition to the performance and lower body kinematics during a 20-m maximal sprint. Linear mixed models and repeated measurement correlations were used to assess changes over time and common within participant associations between hamstring contractile properties and peak knee extension during the final part of the swing phase, peak hip flexion, peak combined knee extension and hip flexion, and peak joint angular velocities, respectively. RESULTS: Hamstring MVT and sprint performance were significantly reduced by 7.5% and 14.3% at the end of the football match simulation. Unexpectedly, there were no indications for reductions in RTD when MVT decrease was considered. Decreases in hamstring MVT were significantly correlated to decreases in peak knee angle (R = 0.342) and to increases in the peak combined angle (R = -0.251). CONCLUSIONS: During a football match simulation, maximal voluntary isometric hamstring torque declines. This decline is related to greater peak knee extension and peak combined angle during sprint running, which indicates a reduced capacity of the hamstrings to decelerate the lower leg during sprint running with fatigue.

An Inertial Measurement Unit Based Method to Estimate Hip and Knee Joint Kinematics in Team Sport Athletes on the Field.

Current athlete monitoring practice in team sports is mainly based on positional data measured by global positioning or local positioning systems. The disadvantage of these measurement systems is that they do not register lower extremity kinematics, which could be a useful measure for identifying injury-risk factors. Rapid development in sensor technology may overcome the limitations of the current measurement systems. With inertial measurement units (IMUs) securely fixed to body segments, sensor fusion algorithms and a biomechanical model, joint kinematics could be estimated. The main purpose of this article is to demonstrate a sensor setup for estimating hip and knee joint kinematics of team sport athletes in the field. Five male subjects (age 22.5 ± 2.1 years; body mass 77.0 ± 3.8 kg; height 184.3 ± 5.2 cm; training experience 15.3 ± 4.8 years) performed a maximal 30-meter linear sprint. Hip and knee joint angles and angular velocities were obtained by five IMUs placed on the pelvis, both thighs and both shanks. Hip angles ranged from 195° (± 8°) extension to 100.5° (± 8°) flexion and knee angles ranged from 168.6° (± 12°) minimal flexion and 62.8° (± 12°) maximal flexion. Furthermore, hip angular velocity ranged between 802.6 °·s-1 (± 192 °·s-1) and -674.9 °·s-1 (± 130 °·s-1). Knee angular velocity ranged between 1155.9 °·s-1 (± 200 °·s-1) and -1208.2 °·s-1 (± 264 °·s-1). The sensor setup has been validated and could provide additional information with regard to athlete monitoring in the field. This may help professionals in a daily sports setting to evaluate their training programs, aiming to reduce injury and optimize performance.

Inertial Sensor-Based Motion Tracking in Football with Movement Intensity Quantification.

Inertial sensor-based measurements of lower body kinematics in football players may improve physical load estimates during training sessions and matches. However, the validity of inertial-based motion analysis systems is specific to both the type of movement and the intensity at which movements are executed. Importantly, such a system should be relatively simple, so it can easily be used in daily practice. This paper introduces an easy-to-use inertial-based motion analysis system and evaluates its validity using an optoelectronic motion analysis system as a gold standard. The system was validated in 11 football players for six different football specific movements that were executed at low, medium, and maximal intensity. Across all movements and intensities, the root mean square differences (means ± SD) for knee and hip flexion/extension angles were 5.3° ± 3.4° and 8.0° ± 3.5°, respectively, illustrating good validity with the gold standard. In addition, mean absolute flexion/extension angular velocities significantly differed between the three movement intensities. These results show the potential to use the inertial based motion analysis system in football practice to obtain lower body kinematics and to quantify movement intensity, which both may improve currently used physical load estimates of the players.