Research Participants
Eleven female handball players (mean age, 21.5 ± 0.9 years; mean height, 163.1 ± 5.2 cm; mean weight, 57.8 ± 3.8 kg) participated in the present study, which was approved by the University of Tsukuba Physical Education Research Institutional Review Board (Task No. PE 24-82). Each participant provided written or oral consent after receiving a thorough explanation of the study. All procedures followed during the conduct of this study were in accordance with the Declaration of Helsinki. We gave a direct advertisement to the female handball players in the Kanto College Handball League and recruited a sample of volunteers. Players were excluded from participating if they had any injury-related pain or disorder.
Experiment Protocol
Surface electromyography was used to record lower limb muscle activity while participants performed FFS and RFS cutting. We calculated knee angle changes using a three-dimensional motion analysis, which permitted us to examine the relationship between muscle activity and knee joint angle. A force plate was synchronized with electromyography to compare muscle activity immediately before and after foot strike.
Task
Each participant stood 80 cm to one side of the force plate and began moving towards it. After striking the centre of the force plate with one foot, the participant cut in a direction 60° lateral to the strike foot, relative to the direction of movement; the other foot then reached the goal mark (Fig. 1). Each participant was instructed to conduct this cutting movement as quickly as possible. A carpet was spread on the floor for safety. Each participant performed three trials; the average of the trials was calculated prior to analysis.
Surface Electromyography
Muscle activity was recorded using surface electromyography (K800, Biometrics, Newport, UK) for the rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), semitendinosus (ST), biceps femoris (BF), tibialis anterior (TA), peroneus longus (PL), and the lateral head of the gastrocnemius (GL). Electromyography amplifiers (SX230-1000, Biometrics; inter-electrode distance 2 cm), which function as combined surface electrodes and amplifiers, were placed parallel to the direction of the muscle fibres; a grounding electrode was placed on the left wrist. Induced myogenic potential was converted to a digital display (sampling rate, 1000 Hz) and saved on a personal computer using the TRIAS data importing and analysis system (Biometrics). Raw electromyography signals were converted to root mean square values and used to compare strike forms. To examine muscle activity prior to and early in the foot strike, we determined muscle activity 50 ms prior to and 50 ms after toe contact with the strike plate.
Assessment of Knee Valgus and Lower Leg External Rotation Angles
To assess knee and lower leg angles, we used a three-dimensional motion analysis system (Motive:Tracker; OptiTrack, Corvallis, OR, USA), a force plate (9268B; Kistler Group, Winterthur, Switzerland), and six infrared cameras (V100:R2; OptiTrack) with a sampling rate of 100 Hz. The jump landing point was on the force plate; GRF data were measured (sampling rate of 1000 Hz) and used to calculate the GRF at the jump landing point. Referring to a report by Kifuji [17], we placed reflective markers (diameter, 14 mm) on the tips of each participant’s shoes, lateral malleolus, inside and outside margins of the knee, greater trochanter, and anterior superior iliac spine. The inside and outside margins of the knee were defined based on the midpoint of their anteroposterior diameter, except the patella, which was defined at the height of the knee joint space. The centre of the hip joint was defined as the point one third of the distance from the greater trochanter on the line connecting the greater trochanter with the anterior superior iliac spine.
In the left femur coordinate system, the vector from the anterior superior iliac spine towards the midpoint between the outside and inside of the knee (knee midpoint, MK) was defined as LFz. The vector obtained from the cross product of the vector from the centre of the knee towards the outside of the knee and LFz was defined as LFx, and the vector obtained from the cross product of LFz and LFx was defined as LFy. In the left lower leg coordinate system, the vector from MK towards the midpoint between the medial malleolus and lateral malleolus (ankle midpoint, MA) was defined as LTz. The vector obtained from the cross product of the vector from the medial malleolus towards the lateral malleolus and LTz was defined as LTx, and the vector obtained from the cross product of LTz and LTx was defined as LTy. In the right femur coordinate system, the vector from the MK towards the anterior superior iliac spine was defined as RFz and the vector obtained from the cross product of the vector from the outside to the inside of the knee and RFz was defined as RFx; the vector obtained from the cross product of RFz and RFx was defined as RFy. In the right lower leg coordinate system, the vector from MK towards MA was defined as RTz; the vector obtained from the cross product of the vector from the lateral malleolus to the medial malleolus and RTz was defined as RTx, and the vector obtained from the cross product of RTz and RTx was defined as RTy. The left (right) knee angle was defined as the Euler angle in conversion from the left (right) femur coordinate system to the left (right) lower leg coordinate system using the rotation order (Y-axis → X-axis → Z-axis). These angles were used to calculate the knee valgus and lower leg rotation angles.
Statistical Analysis
Data are represented as medians (25 and 75 %). To compare RFS and FFS cutting, we used SPSS Statistics for Windows, Version 19 (IBM, Armonk, NY, USA), to perform the Wilcoxon signed-rank test; the level of statistical significance was set as p < 0.05. We also calculated the effect size (r = z value/sqrt (number of samples) as part of the post hoc power analysis.