Residual Force Enhancement Is Present in Consecutive Post-Stretch Isometric Contractions of the Hamstrings during a Training Simulation

Residual force enhancement (rFE) is observed when isometric force following an active stretch is elevated compared to an isometric contraction at corresponding muscle lengths. Acute rFE has been confirmed in vivo in upper and lower limb muscles. However, it is uncertain whether rFE persists using multiple, consecutive contractions as per a training simulation. Using the knee flexors, 10 recreationally active participants (seven males, three females; age 31.00 years ± 8.43 years) performed baseline isometric contractions at 150° knee flexion (180° representing terminal knee extension) of 50% maximal voluntary activation of semitendinosus. Participants performed post-stretch isometric (PS-ISO) contractions (three sets of 10 repetitions) starting at 90° knee extension with a joint rotation of 60° at 60°·s−1 at 50% maximal voluntary activation of semitendinosus. Baseline isometric torque and muscle activation were compared to PS-ISO torque and muscle activation across all 30 repetitions. Significant rFE was noted in all repetitions (37.8–77.74%), with no difference in torque between repetitions or sets. There was no difference in activation of semitendinosus or biceps femoris long-head between baseline and PS-ISO contractions in all repetitions (ST; baseline ISO = 0.095–1.000 ± 0.036–0.039 Mv, PS-ISO = 0.094–0.098 ± 0.033–0.038 and BFlh; baseline ISO = 0.068–0.075 ± 0.031–0.038 Mv). This is the first investigation to observe rFE during multiple, consecutive submaximal PS-ISO contractions. PS-ISO contractions have the potential to be used as a training stimulus

Surfboard Paddling Technique and Neuromechanical Control: A Narrative Review

Surfboard paddling is an essential activity when surfing. Research investigating surfboard paddling, especially as it pertains to neuromechanical control and techniques used, is limited. Previous research made use of swim ergometers to examine surfboard paddling demands. The validity of using swim ergometers in surfboard paddling research and training deserves further analysis. To establish ecologically valid findings, researchers have begun to use swim flumes and still-water paddling environments to investigate paddling efficiency and technique. This emerging body of research has reported that muscle activation patterns, intensities, and timings differ as surfers move through different paddle stroke phases. A deeper understanding of paddling\u27s neuromechanical control may help enhance the understanding of how to improve paddle performance and perhaps reduce injury risk. Therefore, the purpose of this review was to identify the gaps in the existing literature to help identify future research directions in relation to surfboard paddling techniques and neuromechanical control

Are ankle mechanics during drop landings affected by two different measures of passive plantar-flexor flexibility and what are the effects of training: implications for injury?

BACKGROUND Ankle flexibility defined by passive dorsiflexion ROM and plantar-flexor stiffness, is associated with injury risk, particularly during landing tasks involving rapid dorsiflexion and elongation of the plantar-flexor MTU. However, the biomechanical mechanisms associated with poor ankle flexibility that may be inferred as potentially injurious during landing movements are not thoroughly understood. Furthermore, although dorsiflexion ROM and plantar-flexor stiffness may be affected by training, adaptations reported in the literature have been conflicting and the possible effects of training on landing biomechanics have not been investigated. THESIS AIM The primary purpose of this thesis was to determine whether variations in ankle dorsiflexion ROM affect ankle biomechanics during a drop landing task and whether these effects were moderated by training that was designed to alter dorsiflexion ROM. METHODS Using a randomised controlled trial (RCT) study design, ankle flexibility and landing biomechanics were assessed in 48 male volunteers, each assigned to one of three experimental training interventions (stretch, eccentric strength or landing training) or a control group. Results from this RCT were analysed and presented in three main parts, with each part systematically contributing to the primary thesis aim. Part I explored the baseline RCT data to investigate the relationship between dorsiflexion ROM (in weight-bearing and non-weight-bearing) and plantar-flexor stiffness in order to establish whether these measures of ankle flexibility assessed different characteristics (Chapter 2). Part II again explored the baseline RCT data to determine the effect of dorsiflexion ROM and plantar-flexor stiffness on ankle biomechanics and plantar-flexor loading during drop landings (Chapters 3 and 4). Part III then used the whole RCT data set (baseline and post-intervention) to investigate the effects of different training interventions, designed to increase dorsiflexion ROM or take advantage of the concept of training specificity, on flexibility characteristics and ankle biomechanics during drop landings (Chapters 5 and 6). Baseline and post-intervention assessments included measurements of passive DROM, passive plantar-flexor stiffness and ankle biomechanics during a single-limb drop landing task. Data collection for the outcome variables characterising landing biomechanics included EMG from four shank muscles and three-dimensional kinematics of the foot and shank as participants landed on a force platform. These biomechanical data provided input for inverse dynamic calculations of ankle kinetics and an estimation of Achilles tendon force generated during landing. MAJOR CONCLUSIONS Passive DROM or passive plantar-flexor stiffness do not affect ankle biomechanics during drop landings. However, relative to the demands of the task, athletes with a low DROM may be absorbing landing loads with their plantar-flexor MTU in a more extended length, thereby exposing them to an increased risk of both acute and repetitive overuse plantar-flexor MTU strain injuries. Long-term static stretch training is recommended, as more effective than eccentric strength or task-specific landing training, in order to increase DROM. Static stretch training may also provide some biomechanical advantages during drop landings, with respect to injury mechanisms, by potentially reducing plantar-flexor MTU strain. Specificity provided by landing training may also offer protection from injury during drop landings by developing more synchronous plantar-flexor muscle activation to control the landing movement and increase the time over which to absorb the potentially injurious loads generated during high impact landing tasks

Monopolar electromyographic signals recorded by a current amplifier in air and under water without insulation

It was recently proposed that one could use signal current instead of voltage to collect surface electromyography (EMG). With EMG-current, the electrodes remain at the ground potential, thereby eliminating lateral currents. The purpose of this study was to determine whether EMG-currents can be recorded in Tap and Salt water, as well as in air, without electrically shielding the electrodes. It was hypothesized that signals would display consistent information between experimental conditions regarding muscle responses to changes in contraction effort. EMG-currents were recorded from the flexor digitorum muscles as participant’s squeezed a pre-inflated blood pressure cuff bladder in each experimental condition at standardized efforts. EMG-current measurements performed underwater showed no loss of signal amplitude when compared to measurements made in air, although some differences in amplitude and spectral components were observed between conditions. However, signal amplitudes and frequencies displayed consistent behavior across contraction effort levels, irrespective of the experimental condition. This new method demonstrates that information regarding muscle activity is comparable between wet and dry conditions when using EMG-current. Considering the difficulties imposed by the need to waterproof traditional bipolar EMG electrodes when underwater, this new methodology is tremendously promising for assessments of muscular function in aquatic environments

Does foot pitch at ground contact affect parachute landing technique?

The Australian Defence Force Parachute Training School instructs trainees to make initial ground contact using a fl at foot whereas United States paratroopers are taught to contact the ground with the ball of the foot fi rst. This study aimed to determine whether differences in foot pitch affected parachute landing technique. Kinematic, ground reaction force and electromyographic data were analyzed for 28 parachutists who performed parachute landings (vertical descent velocity = 3.4 m·s −1 ) from a monorail apparatus. Independent t -tests were used to determine signifi cant ( p \u3c 0.05) differences between variables characterizing foot pitch. Subjects who landed fl at-footed displayed less knee and ankle fl exion, sustained higher peak ground reaction forces, and took less time to reach peak force than those who landed on the balls of their feet. Although forefoot landings lowered ground reaction forces compared to landing fl at-footed, further research is required to confi rm whether this is a safer parachute landing strategy

Dorsiflexion capacity affects achilles tendon loading during drop landings

Purpose: Evidence suggests a link between decreased dorsiflexion range of motion (DROM) and injury risk during landings. The purpose of this study was to determine the effect of weight-bearing DROM on ankle mechanics during drop landings. Methods: Forty-eight men (mean ± SD = 22.5 ± 4.7 yr) were measured for DROM. Participants performed drop landings onto a force platform at two vertical descent velocities (2.25 ± 0.15 and 3.21 ± 0.17 m·s−1), while EMG activity of four shank muscles and three-dimensional ankle joint kinematics were recorded. Participants were classified into low (37.7° ± 2.5°) and high (48.4° ± 2.5°) DROM groups. Results: Ground reaction force, EMG, dorsiflexion angle, plantarflexion moment, and Achilles tendon force outcome variables were all equivalent for the two DROM groups during each landing condition. However, the low DROM group performed each landing condition at a significantly greater percentage of their DROM and displayed significantly more ankle eversion throughout most of the movement. The low and high DROM groups displayed DROM percentages of 27 ± 11 and 10 ± 11 (P = 0.013), 32 ± 9 and 23 ± 9 (P = 0.056), 60 ± 13 and 46 ± 13 (P = 0.004), and 66 ± 16 and 54 ± 9 (P = 0.003) when they encountered the peak plantarflexion moments, Achilles tendon force, eversion angles, and dorsiflexionangles, respectively. Conclusion: Participants with a low DROM absorbed the landing impact forces with their plantarflexor muscle-tendon units in a more lengthened and everted position. Athletes with a low DROM may be more likely to regularly overload their plantarflexor muscle-tendon units, thereby potentially exposing themselves to a higher likelihood of incurring injuries such as Achilles tendinopathy

Parachute landing fall characteristics at three realistic vertical descent velocities

Introduction: Although parachute landing injuries are thought to be due in part to a lack of exposure of trainees to realistic descent velocities during parachute landing fall (PLF) training, no research has systematically investigated whether PLF technique is affected by different vertical descent conditions, with standardized and realistic conditions of horizontal drift. This study was designed to determine the effects of variations in vertical descent velocity on PLF technique. Methods: Kinematic, ground reaction force, and electromyographic data were collected and analyzed for 20 paratroopers while they performed parachute landings, using a custom-designed monorail apparatus, with a constant horizontal drift velocity (2.3 m · s−1) and at three realistic vertical descent velocities: slow (2.1 m · s−1), medium (3.3 m · s−1), and fast (4.6 m · s−1). Results: Most biomechanical variables characterizing PLF technique were significantly affected by descent velocity. For example, at the fast velocity, the subjects impacted the ground with 123° of plantar flexion and generated ground reaction forces averaging 13.7 times body weight, compared to 106° and 6.1 body weight, respectively, at the slow velocity. Furthermore, the subjects activated their antigravity extensor muscles earlier during the fast velocity condition to eccentrically control the impact absorption. Discussion: As vertical descent rates increased, the paratroopers displayed a significantly different strategy when performing the PLF. It is therefore recommended that PLF training programs include ground training activities with realistic vertical descent velocities to better prepare trainees to withstand the impact forces associated with initial aerial descents onto the Drop Zone and, ultimately, minimize the potential for injury