Maximize muscle mechanical output during the stretch-shortening cycle--the contribution of preactivation and stretch load

It is well documented that the stretch-shortening cycle (SSC), the most common muscle behavior, enhances muscle mechanical output. Stretch load and muscle preactivation level have been suggested as the two important factors regulating mechanical output. The purpose of this series studies is to systematically examine influences of the preactivation and the stretch load on muscle mechanical output during SSC. In the First study, a two dimensional lower extremity dynamic model was used to evaluate the influence of the approach on mechanical output. The peak summed power during the push-off phase demonstrated a quadratic trend across heights and appeared to be driven primarily by the ankle joint response. When an approach was used summed peak power was approximately 10% greater, regardless of the number of steps. In the Second study, we investigated muscle activity of seven major low extremity muscles during drop jumps. The surface EMG activities were full-wave rectified and averaged (aEMG) during the pre-activation (50ms before touchdown), downward and pushoff phases. The results showed that the aEMG of most tested muscles during the preactivation phase and the downward phase increased with more steps of the approach. This increase did not change the antagonist-agonist coactivation ratio, therefore would not attribute to knee joint injury. On the other hand, no aEMG changes were found with different drop heights. In the Third study, stretch load and preactivation were used as inputs for a muscle model to calculate muscle force, muscle velocity and muscle power. This model quantified how the different preactivation level and stretch load (velocity) affect the muscle mechanical output. Results showed that for low preactivation levels, increasing preactivation level could significantly increase gain in height for all stretch velocities we tested, but increasing stretch velocity may decrease the gain in height; for high preactivation levels, further increasing preactivation level may not increase gain in height. Over all, increasing preactivation enhances mechanical output due to increased active state level during SSCs; when preactivation is high, increasing stretch load enhances mechanical output due to increased positive work. Stretch load needs a high preactivation level to maximize the mechanical output

THE INFLL;ENCES OF APPROACH RUN ON MUSCLE ACTIVITY DURING DROP JUMP

Drop jump, as one of the most popular plyometric exercise that involves stretch shortening cycle, has been proved very effective to improve vertical jump performance due to greater lower extremity power output. However, beyond optimal height, further increasing dropping height could not increase or even decrease power output. Our previous study has showed the approach run preceding the drop jump increased the power output further and decreased downward duration time and downward movement. The muscular activities associated with these changes were not clear. Therefore, the purpose of this study was to investigate the effects of approach run on the muscle activity of low extremity muscles

Stretch Could Reduce Hamstring Injury Risk During Sprinting by Right Shifting the Length-Torque Curve

Stretch could reduce hamstring injury risk during sprinting by right shifting the length-torque curve. J Strength Cond Res 32(8): 2190–2198, 2018—It was hypothesized that static stretch would shift the length-torque curve to the right, which may reduce the risk of muscle strain injuries. The purpose of this study was to evaluate the acute effects of static stretching of hamstring (SSH) on the risk of hamstring injury during sprinting indicated by the shift of the length–torque relationship. Twelve female college athletes (age: 20.8 ± 0.7 years; height: 1.61 ± 0.05 m; body mass: 54.25 ± 4.22 kg) participated in this study. Subjects performed overground sprinting under 2 conditions: after warm-up with 4 × 30 seconds SSH or after warm-up without SSH. Three-dimensional kinematic and kinetic data and electromyography of biceps femoris long head (BFlh), rectus femoris, and vastus medialis were collected during testing. The maximum length of BFlh during late swing phase increased after SSH with large effect size and close to statistically significant (p = 0.05, d = 1.22), but the knee flexion torque at the peak length did not change significantly. Static stretching of hamstring significantly reduced peak values of both horizontal (d = 1.46) and vertical (d = 1.79) ground reaction forces, and BFlh\u27s activation level during the preactivation (late swing) phase (p = 0.05, d = 2.16). The results indicated that the length of BFlh–knee torque relationship and the length of BFlh–hip torque relationships during the late swing phase and initial stance phase were shifted to the right after SSH, which may reduce risk of hamstring strain injuries. We suggest that preactivity static stretching should not be simply removed and participators should give priority to stretch muscles that are vulnerable to strain injuries

The Relationship Between the Contact Force at the Ankle Hook and the Hamstring Muscle Force During the Nordic Hamstring Exercise

A novel device has been developed to assess eccentric hamstring strength during the Nordic hamstring exercise (NHE) by measuring the contact force at the ankle hook (brace). The purpose of this study was to determine the correlation between the force measured at the ankle hook and the hamstring force estimated by a low extremity model. Thirteen male college sprinters were recruited to perform NHE on an instrumented device Nordbord (Vald Performance, Australia). Contact forces were measured at a sampling rate of 50 Hz at the hooks using the uniaxial load cells. 3D kinematics were measured simultaneously at a sampling rate of 200 Hz using a 16-camera motion analysis system (Vicon Motion Analysis, Oxford, United Kingdom) during the NHE. The data were processed with Visual 3D (C-Motion, Germantown, MD, United States) and OpenSim (NCSRR, Stanford, CA, United States) to calculate the knee joint center’s coordinates and hamstring moment arms during NHE. A static low extremity model was built to estimate the hamstring force during NHE. We have observed a significant but not very high correlation (r2 = 0.58) between peak hamstring force and the peak contact force at the ankle hook. The peak contact force measured at the ankle hook can only explain a little more than half of the variations in peak hamstring muscle forces during NHE. Caution must be exercised when assessing the eccentric hamstring strength using the ankle contact force during NHE

Nordic Exercise Should Not Be Used for Predictive Modeling of Hamstring Injuries

Work published in Medicine and Science in Sports and Exercise

Differences in strength and landing biomechanics between female jumpers and swimmers

BACKGROUND: It remains unclear if plyometric training as a single component could improve landing mechanics that are potentially associated with lower risk of ACL injury in the long term OBJECTIVE: The purpose of this study was to investigate the influence of experience undertaking plyometrics on landing biomechanics in female athletes. METHODS: Non-jumpers with little experience in plyometric training (12 female college swimmers) and jumpers with five years of experience in plyometric training (12 female college long jumpers and high jumpers) were recruited to participate in two testing sessions: an isokinetic muscle force test for the dominant leg at 120∘ /s and a 40-cm drop landing test. An independent t test was applied to detect any significant effects between cohorts for selected muscle force, kinematic, kinetic, and electromyography variables. RESULTS: While female jumpers exhibited greater quadriceps eccentric strength (P = 0.013) and hamstring concentric strength (P = 0.023) during isokinetic testing than female swimmers, no significant differences were observed in kinematics, kinetics, and muscle activities during both drop landing and drop jumping. CONCLUSIONS: The results suggest that the female jumpers did not present any training-induced modification in landing mechanics regarding reducing injury risks compared with the swimmers. The current study revealed that plyometric training as a single component may not guarantee the development of low-risk landing mechanics for young female athletes.ISSN:0959-3020ISSN:1878-591