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Cross-sectional studies revealed that inclusion of unstable elements in core-strengthening exercises produced increases in trunk muscle activity and thus potential extra stimuli to induce more pronounced performance enhancements in youth athletes. Thus, the purpose of the study was to investigate changes in neuromuscular and athletic performance following core strength training performed on unstable (CSTU) compared with stable surfaces (CSTS) in youth soccer players. Thirty-nine male elite soccer players (age: 17 +/- 1 years) were assigned to two groups performing a progressive core strength-training program for 9 weeks (2-3 times/week) in addition to regular in-season soccer training. CSTS group conducted core exercises on stable (i.e., floor, bench) and CSTU group on unstable (e.g., Thera-Band (R) Stability Trainer, Togu (c) Swiss ball) surfaces. Measurements included tests for assessing trunk muscle strength/activation, countermovement jump height, sprint time, agility time, and kicking performance. Statistical analysis revealed significant main effects of test (pre vs post) for trunk extensor strength (5%, P<0.05, d=0.86), 10-20-m sprint time (3%, P<0.05, d=2.56), and kicking performance (1%, P<0.01, d=1.28). No significant Groupxtest interactions were observed for any variable. In conclusion, trunk muscle strength, sprint, and kicking performance improved following CSTU and CSTS when conducted in combination with regular soccer training.
During the last two decades, instability training devices have become a popular means in athletic training and rehabilitation of mimicking unstable surfaces during movements like vertical jumps. Of note, under unstable conditions, trunk muscles seem to have a stabilizing function during exercise to facilitate the transfer of torques and angular momentum between the lower and upper extremities. The present thesis addresses the acute effects of surface instability on performance during jump-landing tasks. Additionally, the long-term effects (i.e., training) of surface instability were examined with a focus on the role of the trunk in athletic performance/physical fitness.
Healthy adolescent, and young adult subjects participated in three cross-sectional and one longitudinal study, respectively. Performance in jump-landing tasks on stable and unstable surfaces was assessed by means of a ground reaction force plate. Trunk muscle strength (TMS) was determined using an isokinetic device or the Bourban TMS test. Physical fitness was quantified by standing long jump, sprint, stand-and-reach, jumping sideways, Emery balance, and Y balance test on stable surfaces. In addition, activity of selected trunk and leg muscles and lower limb kinematics were recorded during jump-landing tasks.
When performing jump-landing tasks on unstable compared to stable surfaces, jump performance and leg muscle activity were significantly lower. Moreover, significantly smaller knee flexion angles and higher knee valgus angles were observed when jumping and landing on unstable compared to stable conditions and in women compared to men. Significant but small associations were found between behavioral and neuromuscular data, irrespective of surface condition. Core strength training on stable as well as on unstable surfaces significantly improved TMS, balance and coordination.
The findings of the present thesis imply that stable rather than unstable surfaces provide sufficient training stimuli during jump exercises (i.e., plyometrics). Additionally, knee motion strategy during plyometrics appears to be modified by surface instability and sex. Of note, irrespective of surface condition, trunk muscles only play a minor role for leg muscle performance/activity during jump exercises. Moreover, when implemented in strength training programs (i.e., core strength training), there is no advantage in using instability training devices compared to stable surfaces in terms of enhancement of athletic performance.
The present study investigated associations between trunk muscle strength, jump performance, and lower limb kinematics during drop jumps on stable and unstable surfaces. Next to this behavioral approach, correlations were also computed on a neuromuscular level between trunk and leg muscle activity during the same test conditions.
Twenty-nine healthy and physically active subjects (age 23 +/- A 3 years) were enrolled in this study. Peak isokinetic torque (PIT) of the trunk flexors and extensors was assessed separately on an isokinetic device. In addition, tests included drop jumps (DJ) on a force plate under stable and unstable (i.e., balance pad on top of the force plate) surfaces. Lower limb kinematics as well as electromyographic activity of selected trunk and leg muscles were analyzed.
Significant positive but small correlations (0.50 a parts per thousand currency sign r a parts per thousand currency sign 0.66, p < 0.05) were detected between trunk extensor PIT and athletic performance measures (i.e., DJ height, DJ performance index), irrespective of surface condition. Further, significant negative but small correlation coefficients were examined between trunk extensor PIT and knee valgus motion under stable and unstable surface conditions (-0.48 a parts per thousand currency sign r a parts per thousand currency sign -0.45, p < 0.05). In addition, significant positive but small correlations (0.45 a parts per thousand currency sign r a parts per thousand currency sign 0.68, p < 0.05) were found between trunk and leg muscle activity, irrespective of surface condition.
Behavioral and neuromuscular data from this study indicate that, irrespective of the surface condition (i.e., jumping on stable or unstable ground), the trunk plays a minor role for leg muscle performance/activity during DJ. This implies only limited effects of trunk muscle strengthening on jump performance in the stretch-shortening cycle.
The purpose of this study was to investigate the effects of plyometric training on stable (SPT) vs. highly unstable surfaces (IPT) on athletic performance in adolescent soccer players. 24 male sub-elite soccer players (age: 15 +/- 1 years) were assigned to 2 groups performing plyometric training for 8 weeks (2 sessions/week, 90min each). The SPT group conducted plyometrics on stable and the IPT group on unstable surfaces. Tests included jump performance (countermovement jump [CMJ] height, drop jump [DJ] height, DJ performance index), sprint time, agility and balance. Statistical analysis revealed significant main effects of time for CMJ height (p<0.01, f=1.44), DJ height (p<0.01, f=0.62), DJ performance index (p<0.05, f=0.60), 0-10-m sprint time (p<0.05, f=0.58), agility (p<0.01, f=1.15) and balance (p<0.05, 0.46f1.36). Additionally, a Training groupxTime interaction was found for CMJ height (p<0.01, f=0.66) in favor of the SPT group. Following 8 weeks of training, similar improvements in speed, agility and balance were observed in the IPT and SPT groups. However, the performance of IPT appears to be less effective for increasing CMJ height compared to SPT. It is thus recommended that coaches use SPT if the goal is to improve jump performance.
This study investigated sex-specific effects of surface instability on kinetics and lower extremity kinematics during drop jumping and landing. Ground reaction forces as well as knee valgus and flexion angles were tested in 14 males (age: 23 +/- 2 years) and 14 females (age: 24 +/- 3 years) when jumping and landing on stable and unstable surfaces. Jump height was found to be significantly lower (9 %, p < 0.001) when drop jumps were performed on unstable vs. stable surface. Significantly higher peak ground reaction forces were observed when jumping was performed on unstable vs. stable surfaces (5 %, p = 0.022). Regarding frontal plane kinematics during jumping and landing, knee valgus angles were higher on unstable compared to stable surfaces (1932 %, p < 0.05). Additionally, at the onset of ground contact during landings, females showed higher knee valgus angles than males (222 %, p = 0.027). Sagittal plane kinematics indicated significantly smaller knee flexion angles (6-35 %, p < 0.05) when jumping and landing on unstable vs. stable surfaces. During drop jumps and landings, women showed smaller knee flexion angles at ground contact compared to men (27-33 %, p < 0.05). These findings imply that knee motion strategies were modified by surface instability and sex during drop jumps and landings.
Background
It has been demonstrated that core strength training is an effective means to enhance trunk muscle strength (TMS) and proxies of physical fitness in youth. Of note, cross-sectional studies revealed that the inclusion of unstable elements in core strengthening exercises produced increases in trunk muscle activity and thus provide potential extra training stimuli for performance enhancement. Thus, utilizing unstable surfaces during core strength training may even produce larger performance gains. However, the effects of core strength training using unstable surfaces are unresolved in youth. This randomized controlled study specifically investigated the effects of core strength training performed on stable surfaces (CSTS) compared to unstable surfaces (CSTU) on physical fitness in school-aged children.
Methods
Twenty-seven (14 girls, 13 boys) healthy subjects (mean age: 14 ± 1 years, age range: 13–15 years) were randomly assigned to a CSTS (n = 13) or a CSTU (n = 14) group. Both training programs lasted 6 weeks (2 sessions/week) and included frontal, dorsal, and lateral core exercises. During CSTU, these exercises were conducted on unstable surfaces (e.g., TOGU© DYNAIR CUSSIONS, THERA-BAND© STABILITY TRAINER).
Results
Significant main effects of Time (pre vs. post) were observed for the TMS tests (8-22%, f = 0.47-0.76), the jumping sideways test (4-5%, f = 1.07), and the Y balance test (2-3%, f = 0.46-0.49). Trends towards significance were found for the standing long jump test (1-3%, f = 0.39) and the stand-and-reach test (0-2%, f = 0.39). We could not detect any significant main effects of Group. Significant Time x Group interactions were detected for the stand-and-reach test in favour of the CSTU group (2%, f = 0.54).
Conclusions
Core strength training resulted in significant increases in proxies of physical fitness in adolescents. However, CSTU as compared to CSTS had only limited additional effects (i.e., stand-and-reach test). Consequently, if the goal of training is to enhance physical fitness, then CSTU has limited advantages over CSTS.
The purpose of this study was to assess intrasession and intersession reliability of maximal and explosive isometric torque production of the elbow flexors and its respective neuromuscular activation pattern. Subjects (13 men, age: 24.8 +/- 3.1 years, height: 1.9 +/- 0.1 m, body mass: 83.7 +/- 12.7 kg; and 6 women, age: 26.5 +/- 1.4 years, height: 1.7 +/- 0.1 m, body mass: 62.7 +/- 7.0 kg) were tested and retested 2-7 days later performing unilateral maximal isometric elbow flexions. Absolute (coefficient of variation[CV], test-retest variability[TRV], Bland-Altman plots with 95% limits of agreement) and relative reliability statistics (intraclass correlation coefficient) were calculated for various mechanical (i.e., maximal isometric torque, rate of torque development, impulse) and electromyographical measures (i.e., mean average voltage) at different time intervals relative to onset of torque (i. e., 30, 50, 100, 200, 300, 400, 100-200 ms). Intraclass correlation coefficient values were >= 0.61 for all mechanical and electromyographical measures and time intervals indicating good to excellent intrasession and intersession reliability. BlandAltman plots confirmed these findings by showing that only 0-2 (<= 3.3%) data points were beyond the limits of agreement. Regarding torque and electromyographic measures, CV (11.9-32.3%) and TRV (18.4-53.8%) values were high during the early intervals of torque development (<= 100 ms) indicating high variability. During the later intervals (>100 ms), lower CV (i. e., 5.0-29.9%) and TRV values (i.e., 5.4-34.6%) were observed indicating lower variability. The present study revealed that neuromuscular performance during explosive torque production of the elbow flexors is reproducible in time intervals >100 ms after onset of isometric actions, whereas during earlier time intervals variability is high.
The purpose of this study was to investigate the effects of surface instability on measures of performance and activity of leg and trunk muscles during drop jumps and landings.
Drop jumps and landings were assessed on a force plate under stable and unstable (balance pad on top of the force plate) conditions. Performance measures (contact time, jump height, peak ground reaction force) and electromyographic (EMG) activity of leg and trunk muscles were tested in 27 subjects (age 23 +/- A 3 years) during different time intervals (preactivation phase, braking phase, push-off phase).
The performance of drop jumps under unstable compared to stable conditions produced a decrease in jump height (9 %, p < 0.001, f = 0.92) and an increase in peak ground reaction force (5 %, p = 0.022, f = 0.72), and time for braking phase (12 %, p < 0.001, f = 1.25). When performing drop jumps on unstable compared to stable surfaces, muscle activity was reduced in the lower extremities during the preactivation, braking and push-off phases (11-25 %, p < 0.05, 0.48 a parts per thousand currency sign f a parts per thousand currency sign 1.23). Additionally, when landing on unstable compared to stable conditions, reduced lower limb muscle activities were observed during the preactivation phase (7-60 %, p < 0.05, 0.50 a parts per thousand currency sign f a parts per thousand currency sign 3.62). Trunk muscle activity did not significantly differ between the test conditions for both jumping and landing tasks.
The present findings indicate that modified feedforward mechanisms in terms of lower leg muscle activities during the preactivation phase and/or possible alterations in leg muscle activity shortly after ground contact (i.e., braking phase) are responsible for performance decrements during jumping on unstable surfaces.