The purpose of this systematic review was to summarize the associations of eccentric force variables during jumping and eccentric lower-limb strength with vertical jump performance. A literature search was conducted in September 2022 using PubMed, Web of Science, and Scopus. Thirteen cross-sectional studies investigating the relationship between eccentric force and strength variables, such as force, rate of force development (RFD), power, time, and velocity, and vertical jump performance, including the jump height, reactive strength index (RSI), and reactive strength index-modified (RSImod), were included in this systematic review. As eccentric strength, variables during the unloading-to-braking phase of countermovement jump (CMJ) (force, RFD, etc.) and the eccentric force of the squat movement and knee joint were included. The CMJ height, RSImod, and drop jump RSI were included to analyze the vertical jump performance. The modified form of the Downs and Black checklist was used to evaluate quality. Associations between the force and RFD during the descending phase of the CMJ and jump height were observed in some studies but not in others, with differences between the studies. Some studies reported associations between the force and/or RFD during the descending phase of the CMJ and RSImod of the CMJ, with no differences among their results. In addition, there are associations of the eccentric forces during squatting and knee extension with the CMJ and the drop jump heights and RSI of the drop jump. The eccentric force variables in the CMJ and RSImod are related; however, their relationship with jump height remains unclear. Furthermore, improved eccentric muscle strength may contribute to vertical jump height because of the associations of the eccentric strength during knee extension and squatting with jump height.

In this study, we aimed to clarify the characteristics of neuromuscular function, kinetics, and kinematics of the lower extremity during sprinting in track and field athletes with a history of strain injury. Ten male college sprinters with a history of unilateral hamstring injury performed maximum effort sprint on an athletic track. The electromyographic (EMG) activity of the long head of the biceps femoris (BFlh) and gluteus maximus (Gmax) muscles and three-dimensional kinematic data were recorded. Bilateral comparisons were performed for the EMG activities, pelvic anterior tilt angle, hip and knee joint angles and torques, and the musculotendon length of BFlh. The activity of BFlh in the previously injured limb was significantly lower than that in the uninjured limb during the late-swing phase of sprinting (p < 0.05). However, the EMG activity of Gmax was not significantly different between the previously injured and uninjured limbs. Furthermore, during the late-swing phase, a significantly more flexed knee angle (p < 0.05) and a decrease in BFlh muscle length (p < 0.05) were noted in the injured limb. It was concluded that previously injured hamstring muscles demonstrate functional deficits during the late swing phase of sprinting in comparison with the uninjured contralateral muscles.

Hirose, N and Tsuruike, M. Differences in the electromyographic activity of the hamstring, gluteus maximus, and erector spinae muscles in a variety of kinetic changes. J Strength Cond Res 32(12): 3366–3372, 2018—This study aimed to clarify the differences in the electromyographic (EMG) activity of the semitendinosus (ST), semimembranosus (SM), biceps femoris long head (BFl), gluteus maximus (GM), and erector spinae (ES) muscles during leg curl and bridge exercises across different knee angles and isometric contraction outputs. Sixteen male volunteers participated in this study. The EMG of all targeted muscles was measured at 20 and 40% of the maximal voluntary isometric contraction (MVIC) in the leg curl and during bilateral and unilateral bridge exercises. The knee flexion angle was randomly set at 30, 60, 90, and 120° during each of the exercises. The obtained data were normalized by the MVIC of the corresponding muscle, and each of the normalized values was compared with that of the ST. The EMG activity of the ST was significantly greater at 120° of knee flexion than that of 30 and 60° of knee flexion during leg curl regardless of intensity (p < 0.05), in contrast with that of the SM and BFl. However, bridge exercises diminished this inverse relationship. The ES activity changed similarly to that of the hamstrings, and no difference was observed in the activity of the GM regardless of different knee angles during bridge exercise(s). The strength and conditioning professionals should alter the knee joint angle and load during bridge and leg curl exercises according to which hamstring muscle they want to strengthen because ST, SM, and BFl EMG activity varies depending on the intensity and knee angles during these exercises.

Yanaoka, T, Hamada, Y, Kashiwabara, K, Kurata, K, Yamamoto, R, Miyashita, M, and Hirose, N. Very-short-duration, low-intensity half-time re–warm up increases subsequent intermittent sprint performance. J Strength Cond Res 32(11): 3258–3266, 2018—This study investigated the effect of very-short-duration, low-intensity half-time re–warm up (RW) on subsequent intermittent sprint performance. Using a randomized cross-over design, 11 healthy men performed 3 trials. In the experimental trials, participants performed the first 40-minute intermittent exercise followed by a 15-minute half-time. The interventions at half-time were 15 minutes of seated rest (control), 3 minutes of moderate-intensity RW (cycling at 60% of maximal oxygen uptake [V̇o ₂max]; [60% RW]), and 3 minutes of low-intensity RW (cycling at 30% of V̇o ₂max; [30% RW]). After half-time, participants performed the Cycling Intermittent-Sprint Protocol (CISP), which consisted of 10 seconds of rest, 5 seconds of maximal sprint, and 105 seconds of active recovery at 50% of V̇o ₂max, with the cycles repeated over the 20-minute duration. The mean work and electromyogram amplitude during the sprint in the CISP were higher in both RW trials than in the control trial (p < 0.05). Muscle temperature, estimated from the skin temperature, at 60 minutes was higher in the 60% RW trial than in the control and 30% RW trials (p < 0.05). The mean change in oxygenated hemoglobin concentration during active recovery at 55–65 minutes tended to be higher in both RW trials than in the control trial (60% RW trial: p = 0.06, 30% RW trial: p = 0.06). In conclusion, very-short-duration, low-intensity RW increased intermittent sprint performance after the half-time, in comparison with a traditional passive half-time practice, and was as effective as a moderate-intensity RW when matched for total duration.

The ideal timing to implement anterior cruciate ligament injury prevention programs with respect to maturation is unclear. The purpose of this study was to investigate the effects of an injury prevention program on knee mechanics in early-, late-, and post-pubertal females. In the study, 178 adolescent female basketball players were assigned to six groups: early-pubertal training, early-pubertal control, late-pubertal training, and late-pubertal control, post-pubertal training, and post-pubertal control. The training groups performed an injury prevention program for six months. Medial knee displacement, knee flexion range of motion, and the probability of high knee abduction moment were assessed before and after the training period. After the six-month training period, medial knee displacement was significantly increased in the early-pubertal control group whereas it was unchanged in the early-pubertal training group. Knee flexion range of motion was significantly decreased in the early-pubertal control group whereas it did not change in the early-pubertal training group. The probability of high knee abduction moment was increased in the early-pubertal control group whereas it was unchanged in the earl-pubertal training group. The probability of high knee abduction moment was also decreased in the post-pubertal training group whereas it did not change in the post-pubertal control group. The program limited the development of high-risk movement patterns associated with maturation in early puberty while improving the knee mechanics in post-pubertal adolescents. Therefore, an injury prevention program should be initiated in early puberty and continue through the post-puberty years.

The semitendinosus (ST) muscle is primarily used during Nordic hamstring exercise (NHE), which is often prescribed for preventing hamstring injury, though the biceps femoris long head (BFlh) muscle that is more susceptible to injuries. Thus, this study aimed to identify the modulation of BFlh muscle activity with different knee flexion angles during NHE using an inclined platform. Fourteen male athletes performed NHE and maintained their position at maximum inclination (NH). Subjects also performed isometric NHE using a platform inclined to 50° (ICL) and 40° (ICH), and the knee flexion angle was controlled to 50° and 30°. The electromyography (EMG) activity of the BFlh, ST, semimembranosus, gluteus maximus, elector spinae, and rectus abdominus muscles was determined during each exercise. The EMG of the ST was higher than that of the BFlh during NHE and the highest of all muscles in all exercises (p < 0.05). Moreover, the activity of the BFlh tended to be higher than that of the ST for ICH than for ICL, regardless of the knee joint angle. The activity of the BFlh becomes equivalent to that of the ST during NHE at a knee flexion angle of less than 50°. These results indicate that performing NHE at a shallow knee flexion angle will enhance the activity of the BFlh muscle.