Body mass maximizes power output in human jumping: a strength-independent optimum loading behavior

It is well known that in vitro muscles maximize their power output when acting against a moderate resistance regarding their maximum strength. Similar behavior has been observed from in vivo muscular systems in both single-joint and most of the multi-joint maximum performance tasks. We refer to that phenomenon as a strength-dependent behavior, since the optimum external load that maximizes the mechanical power output of particular muscle(s) or neuro-musculoskeletal system corresponds to a certain percent of maximum strength. In this review paper, we present evidence that the optimum load in maximum vertical jumps is one's own body mass, regardless of the strength of the lower limb muscles (i.e., the strength-independent behavior). Although the discussed phenomenon is still underexplored, we believe that several neuro-mechanical mechanisms are involved. Among these are a long-term adaptation of the muscular force-velocity relationship to the body weight and inertia, alteration of the jumping technique, load-specific muscle activation and jumping skills. Further exploration of the discussed strength-independent behavior of the lower limb muscles is of importance for refining various training and rehabilitation procedures, as well as for understanding the design and function of lower limb muscles.

Evaluation of the effectiveness of anterior cruciate ligament injury prevention programme training components: a systematic review and meta-analysis

BACKGROUND

Anterior cruciate ligament (ACL) injury prevention programmes have shown mixed results, which may be due to differing emphasis on training components. The purpose of this study was to (1) quantify the overall and relative duration of each training component encompassed within these programmes and (2) examine the effect of these durations on ACL injury rates.

METHODS

A systematic review was completed and meta-analyses performed on eligible studies to produce a pooled OR estimate of the effectiveness of these programmes. Meta-regression was used to detect any relationship that programme duration and the duration of individual training components had on ACL injury rates.

RESULTS

13 studies were included for review. Results of the meta-analyses revealed a significant reduction of injuries after preventative training programmes for all ACL injuries (pooled OR estimate of 0.612, 95% CI 0.44 to 0.85; p=0.004) and for non-contact ACL injuries (OR 0.351, 95% CI 0.23 to 0.54; p<0.001). Results of meta-regression analysis revealed that a greater duration of balance training was associated with a higher injury risk for ACL injury (p=0.04), while greater durations of static stretching was associated with a lower injury risk for non-contact ACL injuries (p=0.04).

CONCLUSIONS

While ACL prevention programmes are successful in reducing the risk of ACL injury, the ideal combination and emphasis of training components within these programmes remains unclear. Evidence indicates that greater emphases on balance training and static stretching may be associated with an increase and decrease in injury risk, respectively.

Comparison of gluteal and hamstring activation during five commonly used plyometric exercises

BACKGROUND

Anterior cruciate ligament injuries occur frequently in athletics, and anterior cruciate ligament injury prevention programs may decrease injury risk. However, previous prevention programs that include plyometrics use a variety of exercises with little justification of exercise inclusion. Because gluteal and hamstring activation is thought to be important for preventing knee injuries, the purpose of this study was to determine which commonly used plyometric exercises produce the greatest activation of the gluteals and hamstrings.

METHODS

EMG (Electromyography) amplitudes of the hamstring and gluteal muscles during preparatory and loading phases of landing were recorded in 41 subjects during 5 commonly used plyometric exercises. Repeated measures ANOVAs (Analysis of Variance) were used on 36 subjects to examine differences in muscle activation.

FINDINGS

Differences in hamstring (P<.01) and gluteal (P<.01) activities were identified across exercises during the preparatory and landing phases. The single-leg sagittal plane hurdle hops produced the greatest gluteal and hamstring activity in both phases. The 180° jumps did not produce significantly greater gluteal or hamstring activity than any other exercise.

INTERPRETATION

Single-leg sagittal plane hurdle hops may be the most effective exercise to activate the gluteals and hamstrings and may be important to include in anterior cruciate ligament injury prevention programs, given the importance of these muscles for limiting valgus loading of the knee. Because 180° jumps do not produce greater gluteal and hamstring activation than other plyometric exercises, their removal from injury prevention programs may be warranted without affecting program efficacy.

Changes in selected physical, motor performance and anthropometric components of university-level rugby players after one microcycle of a combined rugby conditioning and plyometric training program

The purpose of this study was to determine the effects of a microcycle (4 weeks) combined rugby conditioning plyometric compared with a nonplyometric rugby conditioning program on selected physical and motor performance components and anthropometric measurements of university-level rugby players. Players (18.94 ± 0.40 years) were assigned to either a control (n = 16) or experimental group (n = 19) from the U/19 rugby teams of the North-West University (South Africa). Twenty-six direct and indirect anthropometric measurements were taken, and the players performed a battery of 5 physical and motor performance tests before and after a microcycle (4 week) combined rugby conditioning plyometric (experimental group) and a nonplyometric rugby conditioning program (control group). The dependent t-test results showed that the control group's upper-body explosive power decreased significantly, whereas the stature, skeletal mass, and femur breadth increased significantly from pre- to posttesting. The experimental group showed significant increases in wrist breadth, speed over 20 m, agility, and power and work measurements of the Wingate anaerobic test (WAnT). Despite these results, the independent t-test revealed that speed over 20 m, average power output at 20 seconds, relative work of the WAnT, and agility were the only components of the experimental group that improved significantly more than the control group. A microcycle combined rugby conditioning plyometric program therefore leads to significantly bigger changes in selected physical and motor performance components of university-level rugby players than a nonplyometric rugby conditioning program alone. Based on these findings, coaches and sport scientists should implement 3 weekly combined rugby conditioning plyometric programs in rugby players' training regimens to improve the players' speed, agility, and power.

Short-term high intensity plyometric training program improves strength, power and agility in male soccer players

The aim of the present study was to investigate the effects of a short-term in-season plyometric training program on power, agility and knee extensor strength. Male soccer players from a third league team were assigned into an experimental and a control group. The experimental group, beside its regular soccer training sessions, performed a periodized plyometric training program for six weeks. The program included two training sessions per week, and maximal intensity unilateral and bilateral plyometric exercises (total of 40 - 100 foot contacts/session) were executed. Controls participated only in the same soccer training routine, and did not perform plyometrics. Depth vertical jump height, agility (Illinois Agility Test, T Agility Test) and maximal voluntary isometric torque in knee extensors using Multicont II dynamometer were evaluated before and after the experiment. In the experimental group small but significant improvements were found in both agility tests, while depth jump height and isometric torque increments were greater. The control group did not improve in any of the measures. Results of the study indicate that plyometric training consisting of high impact unilateral and bilateral exercises induced remarkable improvements in lower extremity power and maximal knee extensor strength, and smaller improvements in soccer-specific agility. Therefore, it is concluded that short-term plyometric training should be incorporated in the in-season preparation of lower level players to improve specific performance in soccer.

Preferential type II muscle fiber damage from plyometric exercise

CONTEXT

Plyometric training has been successfully used in different sporting contexts. Studies that investigated the effect of plyometric training on muscle morphology are limited, and results are controversial with regard to which muscle fiber type is mainly affected.

OBJECTIVE

To analyze the skeletal muscle structural and ultrastructural change induced by an acute bout of plyometric exercise to determine which type of muscle fibers is predominantly damaged.

DESIGN

Descriptive laboratory study.

SETTING

Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Eight healthy, untrained individuals (age = 22 ± 1 years, height = 179.2 ± 6.4 cm, weight = 78.9 ± 5.9 kg).

INTERVENTION(S)

Participants completed an acute bout of plyometric exercise (10 sets of 10 squat-jumps with a 1-minute rest between sets).

MAIN OUTCOME MEASURE(S)

Blood samples were collected 9 days and immediately before and 6 hours and 1, 2, and 3 days after the acute intervention. Muscle samples were collected 9 days before and 3 days after the exercise intervention. Blood samples were analyzed for creatine kinase activity. Muscle biopsies were analyzed for damage using fluorescent and electron transmission microscopy.

RESULTS

Creatine kinase activity peaked 1 day after the exercise bout (529.0 ± 317.8 U/L). Immunofluorescence revealed sarcolemmal damage in 155 of 1616 fibers analyzed. Mainly fast-twitch fibers were damaged. Within subgroups, 7.6% of type I fibers, 10.3% of type IIa fibers, and 14.3% of type IIx fibers were damaged as assessed by losses in dystrophin staining. Similar damage was prevalent in IIx and IIa fibers. Electron microscopy revealed clearly distinguishable moderate and severe sarcomere damage, with damage quantifiably predominant in type II muscle fibers of both the glycolytic and oxidative subtypes (86% and 84%, respectively, versus only 27% of slow-twitch fibers).

CONCLUSIONS

We provide direct evidence that a single bout of plyometric exercise affected mainly type II muscle fibers.

Intensity-level assessment of lower body plyometric exercises based on mechanical output of lower limb joints

The present study aimed to quantify the intensity of lower extremity plyometric exercises by determining joint mechanical output. Ten men (age, 27.3 ± 4.1 years; height, 173.6 ± 5.4 cm; weight, 69.4 ± 6.0 kg; 1-repetition maximum [1RM] load in back squat 118.5 ± 12.0 kg) performed the following seven plyometric exercises: two-foot ankle hop, repeated squat jump, double-leg hop, depth jumps from 30 and 60 cm, and single-leg and double-leg tuck jumps. Mechanical output variables (torque, angular impulse, power, and work) at the lower limb joints were determined using inverse-dynamics analysis. For all measured variables, ANOVA revealed significant main effects of exercise type for all joints (P < 0.05) along with significant interactions between joint and exercise (P < 0.01), indicating that the influence of exercise type on mechanical output varied among joints. Paired comparisons revealed that there were marked differences in mechanical output at the ankle and hip joints; most of the variables at the ankle joint were greatest for two-foot ankle hop and tuck jumps, while most hip joint variables were greatest for repeated squat jump or double-leg hop. The present results indicate the necessity for determining mechanical output for each joint when evaluating the intensity of plyometric exercises.

Promoting lower extremity strength in elite volleyball players: effects of two combined training methods

OBJECTIVES

To compare the impact of short term training with resistance plus plyometric training (RT+P) or electromyostimulation plus plyometric training (EMS+P) on explosive force production in elite volleyball players.

DESIGN

Sixteen elite volleyball players of the first German division participated in a training study.

METHODS

The participants were randomly assigned to either the RT+P training group (n=8) or the EMS+P training group (n=8). Both groups participated in a 5-week lower extremity exercise program. Pre and post tests included squat jumps (SJ), countermovement jumps (CMJ), and drop jumps (DJ) on a force plate. The three-step reach height (RH) was assessed using a custom-made vertec apparatus. Fifteen m straight and lateral sprint (S15s and S15l) were assessed using photoelectric cells with interims at 5m and 10 m.

RESULTS

RT+P training resulted in significant improvements in SJ (+2.3%) and RH (+0.4%) performance. The EMS+P training group showed significant increases in performance of CMJ (+3.8%), DJ (+6.4%), RH (+1.6%), S15l (-3.8%) and after 5m and 10 m of the S15s (-2.6%; -0.5%). The comparison of training-induced changes between the two intervention groups revealed significant differences for the SJ (p=0.023) in favor of RT+P and for the S15s after 5m (p=0.006) in favor of EMS+P.

CONCLUSIONS

The results indicate that RT+P training is effective in promoting jump performances and EMS+P training increases jump, speed and agility performances of elite volleyball players.

Current concepts for rehabilitation and return to sport after knee articular cartilage repair in the athlete

Articular cartilage injury is observed with increasing frequency in both elite and amateur athletes and results from the significant acute and chronic joint stress associated with impact sports. Left untreated, articular cartilage defects can lead to chronic joint degeneration and athletic and functional disability. Treatment of articular cartilage defects in the athletic population presents a therapeutic challenge due to the high mechanical demands of athletic activity. Several articular cartilage repair techniques have been shown to successfully restore articular cartilage surfaces and allow athletes to return to high-impact sports. Postoperative rehabilitation is a critical component of the treatment process for athletic articular cartilage injury and should take into consideration the biology of the cartilage repair technique, cartilage defect characteristics, and each athlete's sport-specific demands to optimize functional outcome. Systematic, stepwise rehabilitation with criteria-based progression is recommended for an individualized rehabilitation of each athlete not only to achieve initial return to sport at the preinjury level but also to continue sports participation and reduce risk for reinjury or joint degeneration under the high mechanical demands of athletic activity.