Plyometric Training Favors Optimizing Muscle-Tendon Behavior during Depth Jumping

Achilles tendon and triceps surae muscle properties in athletes

PURPOSE

The aim of this study was to investigate internal Achilles tendon (AT) displacement, AT shear wave velocity (SWV), and triceps surae (TS) muscle shear modulus in athletes.

METHODS

Internal AT displacement was assessed using ultrasound during isometric contraction. Shear wave elastography was used to assess AT SWV (m × s-1) at rest and TS muscle shear modulus (kPa) during passive ankle dorsiflexion.

RESULTS

A total of 131 athletes participated in this study. Athletes who had not exercised within two days had greater AT non-uniformity and mean anterior tendon displacement, and lower SWV at the proximal AT measurement site (mean difference [95% CI]: 1.8 mm [0.6-2.9], p = 0.003; 1.6 mm [0.2-2.9], p = 0.021; - 0.9 m × s-1 [- 1.6 to - 0.2], p = 0.014, respectively). Male basketball players had a lower mean AT displacement compared to gymnasts (- 3.7 mm [- 6.9 to - 0.5], p = 0.042), with the difference localised in the anterior half of the tendon (- 5.1 mm [- 9.0 to - 1.1], p = 0.022). Male gymnasts had a smaller absolute difference in medial gastrocnemius-minus-soleus shear modulus than basketball players (59.6 kPa [29.0-90.2], p < 0.001) and track and field athletes (52.7 kPa [19.2-86.3], p = 0.004). Intraclass correlation coefficients of measurements ranged from 0.720 to 0.937 for internal AT displacement, from 0.696 to 0.936 for AT SWE, and from 0.570 to 0.890 for TS muscles.

CONCLUSION

This study provides a reliability assessment of muscle and tendon SWV. The relative differences in passive TS muscle shear modulus suggest sport-specific adaptation. Importantly, in healthy individuals, lower AT displacement after exercise may reflect the time required for tendon recovery.

Energy transfer in reactive movements as a function of individual stretch load

Background: By directly recording electromyographic activity profiles and muscle-tendon interaction, this study aimed to elucidate the mechanisms why well-trained track and field athletes (experts) are able to outperform untrained individuals without former systematic experience in reactive jump training (novices). In particular, reactive power output and the elastic recoil properties of the muscle-tendon unit (MTU) were of special interest. For this purpose, stiffness regulation on muscle and joint level, energy management in terms of storing or dissipating elastic energy were compared between experts and novices during various stretch loads. Methods: Experts were compared with novices during reactive drop jumps (DJs) from drop heights ranging between 25 and 61 cm. Delta kinetic energy (Ekin) was calculated as the difference between the Ekin at take-off and ground contact (GC) to determine energy management. By recording electromyography of the lower limb muscles, in vivo fascicle dynamics (gastrocnemius medialis) and by combining kinematics and kinetics in a 3D inverse dynamics approach to compute ankle and knee joint kinetics, this study aimed to compare reactive jump performance, the neuromuscular activity and muscle-tendon interaction between experts and novices among the tested stretch loads. Results: Experts demonstrated significantly higher power output during DJs. Among all drop heights experts realized higher delta Ekin compared to novices. Consequently, higher reactive jump performance shown for experts was characterized by shorter GC time (GCT), higher jump heights and higher neuromuscular activity before and during the GC phase compared to novices. Concomitantly, experts were able to realize highest leg stiffness and delta Ekin in the lowest stretch load; however, both groups compensated the highest stretch load by prolonged GCT and greater joint flexion. On muscle level, experts work quasi-isometrically in the highest stretch load, while in novices GM fascicles were forcefully stretched. Conclusion: Group-specific stiffness regulation and elastic recoil properties are primarily influenced by the neuromuscular system. Due to their higher neuromuscular activity prior and during the GC phase, experts demonstrate higher force generating capacity. A functionally stiffer myotendinous system through enhanced neuromuscular input enables the experts loading their elastic recoil system more efficiently, thus realizing higher reactive power output and allowing a higher amount of energy storage and return. This mechanism is regulated in a stretch load dependent manner.

Effects of Upper-Body Plyometric Training on Physical Fitness in Healthy Youth and Young Adult Participants: A Systematic Review with Meta-Analysis

BACKGROUND

Upper-body plyometric training (UBPT) is a commonly used training method, yet its effects on physical fitness are inconsistent and there is a lack of comprehensive reviews on the topic.

OBJECTIVE

To examine the effects of UBPT on physical fitness in healthy youth and young adult participants compared to active, specific-active, and passive controls.

METHODS

This systematic review followed PRISMA 2020 guidelines and utilized the PICOS framework. PubMed, WOS, and SCOPUS were searched. Studies were assessed for eligibility using the PICOS framework. The effects of UBPT on upper-body physical fitness were assessed, including maximal strength, medicine ball throw performance, sport-specific throwing performance, and upper limb muscle volume. The risk of bias was evaluated using the PEDro scale. Means and standard deviations were used to calculate effect sizes, and the I2 statistic was used to assess heterogeneity. Publication bias was assessed using the extended Egger's test. Certainty of evidence was rated using the GRADE scale. Additional analyses included sensitivity analyses and adverse effects.

RESULTS

Thirty-five studies were included in the systematic review and 30 studies in meta-analyses, involving 1412 male and female participants from various sport-fitness backgrounds. Training duration ranged from 4 to 16 weeks. Compared to controls, UBPT improved maximal strength (small ES = 0.39 95% CI = 0.15-0.63, p = 0.002, I2 = 29.7%), medicine ball throw performance (moderate ES = 0.64, 95% CI = 0.43-0.85, p < 0.001, I2 = 46.3%), sport-specific throwing performance (small ES = 0.55, 95% CI = 0.25-0.86, p < 0.001, I2 = 36.8%), and upper limbs muscle volume (moderate ES = 0.64, 95% CI = 0.20-1.08, p = 0.005, I2 = 0.0%). The GRADE analyses provided low or very low certainty for the recommendation of UBPT for improving physical fitness in healthy participants. One study reported one participant with an injury due to UBPT. The other 34 included studies provided no report measure for adverse effects linked to UBPT.

CONCLUSIONS

UBPT interventions may enhance physical fitness in healthy youth and young adult individuals compared to control conditions. However, the certainty of evidence for these recommendations is low or very low. Further research is needed to establish the optimal dose of UBPT and to determine its effect on female participants and its transfer to other upper-body dominated sports.

Effects of tendon elastic energy and electromyographic activity pattern on jumping height and pre-stretch augmentation during jumps with different pre-stretch intensity

The present study aimed to investigate the effects of tendon elastic energy and electromyographic activity patterns (ratio of pre-landing to concentric: mEMG PLA/CON; ratio of eccentric to concentric; mEMG ECC/CON) on jump performance. Twenty-nine males performed five kinds of unilateral jumps using only ankle joint (no-countermovement jump: noCMJ; countermovement jump: CMJ; drop jumps at 10, 20 and 30 cm drop height: DJ10, DJ20 and DJ30). Jumping height, pre-stretch augmentation and electromyographic activity of the plantar flexor muscles were measured. The elastic energy of the Achilles tendon was measured during isometric contractions. Relative tendon elastic energy (to body mass) was highly correlated with jumping heights of CMJ, DJ10 and DJ20 but not with noCMJ and DJ30, whereas that was significantly correlated with pre-stretch augmentation in CMJ, but not with three DJs. The mEMG PLA/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with DJ10, whereas the mEMG ECC/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with CMJ and DJ10. These results suggested that jumping exercises with low pre-stretch intensity benefited from tendon elastic energy, but those with high pre-stretch intensity benefited from electromyographic activity patterns.

Comparative Effects of Contrast Training and Progressive Resistance Training on Strength and Power-Related Measures in Subelite Australian Rules Football Players

ABSTRACT

Schneiker, KT, Fyfe, JJ, Teo, SYM, and Bishop, DJ. Comparative effects of contrast training and progressive resistance training on strength and power-related measures in subelite Australian Rules Football players. J Strength Cond Res 37(7): 1440-1448, 2023-The aim of this study was to determine whether contrast training (CST) elicits superior strength-related and power-related outcomes compared with progressive resistance training (PRT). Sixteen male amateur Australian Rules Football players (age, 19 ± 2 years; height, 183 ± 8 cm; body mass, 78.5 ± 8.8 kg; mean ± SD ) completed 2 weeks of standardized resistance training followed by 6 weeks of either CST or PRT. Both CST and PRT improved absolute (20 and 19%) and relative (19 and 16%) 1 repetition maximum (1RM) half squat strength, absolute (8.7 and 8.7%, respectively) and relative (8.2 and 6.1%, respectively) squat jump peak power, and 20-m sprint performance (5.4 and 4.7%, respectively), including both 0 to 5 m (15 and 14%, respectively) and 0 to 10 m (8.6 and 7.5%, respectively) splits. Vertical jump height only improved with CST (6.5%), whereas body mass only increased after PRT (1.9%). There were negative associations between the baseline power-to-strength ratio (PSR) and improvements in both absolute ( r2 = 0.51 and 0.72 for CST and PRT, respectively) and relative ( r2 = 0.65 and 0.60 for CST and PRT, respectively) squat jump peak power. There were no statistically significant (i.e., p ≥ 0.05) between-group differences for all training outcomes. Both interventions improved various strength-related and power-related measures, although a lower baseline PSR was associated with greater improvements in power-related outcomes after both interventions. Contrast training is therefore an effective alternative to progressive resistance training during relatively short-term (6-8 weeks) training periods in young, male, team-sport athletes, particularly in those with a lower power-to-strength ratio.

Ankle-Specific Training Does Not Alter Drop Jumping Biomechanics Despite Increased Plantar Flexor Strength and Jumping Performance

INTRODUCTION

Power plays a crucial role in determining an athlete's final performance, as it signifies the ability to rapidly generate force. The plantar flexor muscles have a crucial role in producing the necessary power. The plantar flexor muscles are important in explosive sports movements due to their ability to generate substantial force quickly during the propulsion phase and facilitate efficient energy transfer through the joints. This study aimed to investigate the effects of specific plantar flexor training on drop jumping (DJ) biomechanics, muscle activation, and muscle strength.

MATERIAL AND METHODS

A total of 30 male participants were divided into three groups: the incline hopping (IH) group, which performed continuous jumps on a 15° inclined surface; the plane hopping (PH) group, which performed jumps on a plane surface; and the electrostimulation (EMS) group (n = 10 for each group). All groups trained four times weekly, performing 10 sets of 10 jumps per session. The intervention period lasted four weeks. Participants' drop jumping ability was assessed before and immediately after the training period using hip, knee, and ankle kinematics and electromyographic (EMG) activity of the medial gastrocnemius (MGas), tibialis anterior (TA), rectus femoris (RF), and semitendinosus (ST) muscles. In addition, maximal isokinetic plantar flexor force measurements were evaluated in eccentric and concentric conditions.

RESULTS

Analysis of variance (ANOVA) revealed that only the inclined hopping showed significant improvements in the take-off velocity (Vto) of the fast drop jump (bounce drop jump (BDJ)) (p < 0.05). These improvements were accompanied by significantly higher MGas activity during the propulsion phase of the jump (p < 0.05). In addition, all groups demonstrated greater eccentric torque (p < 0.05), while IH also improved concentric torque (p < 0.05).

CONCLUSIONS

The results support the efficacy of inclined hopping in improving the Vto of BDJs. The increased MGas activity and stable co-activation index (CI) during the propulsion phase are likely to contribute to these improvements. Coaches should consider incorporating incline hopping into the periodization of athletes, while level hopping and electrostimulation could be used to increase overall strength.

The effects of plyometric jump training on lower-limb stiffness in healthy individuals: A meta-analytical comparison

PURPOSE

This study aimed to examine the effects of plyometric jump training (PJT) on lower-limb stiffness.

METHODS

Systematic searches were conducted in PubMed, Web of Science, and Scopus. Study participants included healthy males and females who undertook a PJT programme isolated from any other training type.

RESULTS

There was a small effect size (ES) of PJT on lower-limb stiffness (ES = 0.33, 95% confidence interval (95%CI): 0.07-0.60, z = 2.47, p = 0.01). Untrained individuals exhibited a larger ES (ES = 0.46, 95%CI: 0.08-0.84, p = 0.02) than trained individuals (ES = 0.15, 95%CI: ‒0.23 to 0.53, p = 0.45). Interventions lasting a greater number of weeks (>7 weeks) had a larger ES (ES = 0.47, 95%CI: 0.06-0.88, p = 0.03) than those lasting fewer weeks (ES = 0.22, 95%CI: ‒0.12 to 0.55, p = 0.20). Programmes with ≤2 sessions per week exhibited a larger ES (ES = 0.39, 95%CI: 0.01-0.77, p = 0.04) than programmes that incorporated >2 sessions per week (ES = 0.20, 95%CI: -0.10 to 0.50, p = 0.18). Programmes with <250 jumps per week (ES = 0.50, 95%CI: 0.02-0.97, p = 0.04) showed a larger effect than programmes with 250-500 jumps per week (ES = 0.36, 95%CI: 0.00-0.72, p = 0.05). Programmes with >500 jumps per week had negative effects (ES = -0.22, 95%CI: -1.10 to 0.67, p = 0.63). Programmes with >7.5 jumps per set showed larger effect sizes (ES = 0.55, 95%CI: 0.02-1.08, p = 0.04) than those with <7.5 jumps per set (ES = 0.32, 95%CI: 0.01-0.62, p = 0.04).

CONCLUSION

PJT enhances lower-body stiffness, which can be optimised with lower volumes (<250 jumps per week) over a relatively long period of time (>7 weeks).

Mechanical Linkage between Achilles Tendon and Plantar Fascia Accounts for Range of Motion of Human Ankle-Foot Complex

PURPOSE

The human ankle-foot complex possesses a passive range of motion (ROM) through changes in tibiocalcaneal ( θcal ) and foot arch ( θarch ) angles. Based on the anatomical linkage between the Achilles tendon (AT) and plantar fascia (PF), we hypothesized that AT and PF with different mechanical properties conjointly modulate the passive ROM of the human ankle-foot complex. We examined the association of AT and PF stiffness with passive ankle-foot ROM and further addressed differences between sexes.

METHODS

A series of sagittal magnetic resonance images of the foot and passive ankle plantar flexion torque were obtained for 20 men and 20 women with their ankle-foot passively rotated from 30° of plantar flexion to 20° of dorsiflexion. Based on the measured changes in AT and PF lengths, θcal , θarch , and passive torque, AT and PF stiffness were determined.

RESULTS

Upon passive ankle dorsiflexion, AT and PF were lengthened; their length changes were inversely correlated. Men showed a stiffer AT, more compliant PF, less calcaneal rotation, and greater foot arch deformation compared with women. Furthermore, we found inverse correlations between AT stiffness and ROM of θcal , and between PF stiffness and ROM of θarch in men and women.

CONCLUSIONS

Passive AT and PF extensibility counter each other. AT and PF stiffness and passive ROM of ankle-foot components were countered between sexes; however, associations between stiffness and passive ROM of the ankle-foot complex were consistent between sexes. Our findings support the notion that the balanced mechanical interaction between the AT and PF can account for the passive ROM of the human ankle-foot complex in vivo , and the differences between sexes.

Mechanical, Material and Morphological Adaptations of Healthy Lower Limb Tendons to Mechanical Loading: A Systematic Review and Meta-Analysis

BACKGROUND

Exposure to increased mechanical loading during physical training can lead to increased tendon stiffness. However, the loading regimen that maximises tendon adaptation and the extent to which adaptation is driven by changes in tendon material properties or tendon geometry is not fully understood.

OBJECTIVE

To determine (1) the effect of mechanical loading on tendon stiffness, modulus and cross-sectional area (CSA); (2) whether adaptations in stiffness are driven primarily by changes in CSA or modulus; (3) the effect of training type and associated loading parameters (relative intensity; localised strain, load duration, load volume and contraction mode) on stiffness, modulus or CSA; and (4) whether the magnitude of adaptation in tendon properties differs between age groups.

METHODS

Five databases (PubMed, Scopus, CINAHL, SPORTDiscus, EMBASE) were searched for studies detailing load-induced adaptations in tendon morphological, material or mechanical properties. Standardised mean differences (SMDs) with 95% confidence intervals (CIs) were calculated and data were pooled using a random effects model to estimate variance. Meta regression was used to examine the moderating effects of changes in tendon CSA and modulus on tendon stiffness.

RESULTS

Sixty-one articles met the inclusion criteria. The total number of participants in the included studies was 763. The Achilles tendon (33 studies) and the patella tendon (24 studies) were the most commonly studied regions. Resistance training was the main type of intervention (49 studies). Mechanical loading produced moderate increases in stiffness (standardised mean difference (SMD) 0.74; 95% confidence interval (CI) 0.62-0.86), large increases in modulus (SMD 0.82; 95% CI 0.58-1.07), and small increases in CSA (SMD 0.22; 95% CI 0.12-0.33). Meta-regression revealed that the main moderator of increased stiffness was modulus. Resistance training interventions induced greater increases in modulus than other training types (SMD 0.90; 95% CI 0.65-1.15) and higher strain resistance training protocols induced greater increases in modulus (SMD 0.82; 95% CI 0.44-1.20; p = 0.009) and stiffness (SMD 1.04; 95% CI 0.65-1.43; p = 0.007) than low-strain protocols. The magnitude of stiffness and modulus differences were greater in adult participants.

CONCLUSIONS

Mechanical loading leads to positive adaptation in lower limb tendon stiffness, modulus and CSA. Studies to date indicate that the main mechanism of increased tendon stiffness due to physical training is increased tendon modulus, and that resistance training performed at high compared to low localised tendon strains is associated with the greatest positive tendon adaptation. PROSPERO registration no.: CRD42019141299.

Effect of Plyometric Jump Training on Skeletal Muscle Hypertrophy in Healthy Individuals: A Systematic Review With Multilevel Meta-Analysis

Objective: To examine the effect of plyometric jump training on skeletal muscle hypertrophy in healthy individuals.

Methods: A systematic literature search was conducted in the databases PubMed, SPORTDiscus, Web of Science, and Cochrane Library up to September 2021.

Results: Fifteen studies met the inclusion criteria. The main overall finding (44 effect sizes across 15 clusters median = 2, range = 1–15 effects per cluster) indicated that plyometric jump training had small to moderate effects [standardised mean difference (SMD) = 0.47 (95% CIs = 0.23–0.71); p < 0.001] on skeletal muscle hypertrophy. Subgroup analyses for training experience revealed trivial to large effects in non-athletes [SMD = 0.55 (95% CIs = 0.18–0.93); p = 0.007] and trivial to moderate effects in athletes [SMD = 0.33 (95% CIs = 0.16–0.51); p = 0.001]. Regarding muscle groups, results showed moderate effects for the knee extensors [SMD = 0.72 (95% CIs = 0.66–0.78), p < 0.001] and equivocal effects for the plantar flexors [SMD = 0.65 (95% CIs = −0.25–1.55); p = 0.143]. As to the assessment methods of skeletal muscle hypertrophy, findings indicated trivial to small effects for prediction equations [SMD = 0.29 (95% CIs = 0.16–0.42); p < 0.001] and moderate-to-large effects for ultrasound imaging [SMD = 0.74 (95% CIs = 0.59–0.89); p < 0.001]. Meta-regression analysis indicated that the weekly session frequency moderates the effect of plyometric jump training on skeletal muscle hypertrophy, with a higher weekly session frequency inducing larger hypertrophic gains [β = 0.3233 (95% CIs = 0.2041–0.4425); p < 0.001]. We found no clear evidence that age, sex, total training period, single session duration, or the number of jumps per week moderate the effect of plyometric jump training on skeletal muscle hypertrophy [β = −0.0133 to 0.0433 (95% CIs = −0.0387 to 0.1215); p = 0.101–0.751].

Conclusion: Plyometric jump training can induce skeletal muscle hypertrophy, regardless of age and sex. There is evidence for relatively larger effects in non-athletes compared with athletes. Further, the weekly session frequency seems to moderate the effect of plyometric jump training on skeletal muscle hypertrophy, whereby more frequent weekly plyometric jump training sessions elicit larger hypertrophic adaptations.

Does the Level of Training Interfere with the Sustainability of Static and Dynamic Strength in Paralympic Powerlifting Athletes?

Background: Paralympic powerlifting (PP) presents adaptations that the training tends to provide, mainly concerning the mechanical variables. Objective: Our aim was to analyze mechanical, dynamic and static indicators, at different intensities, on the performance of paralympic powerlifting athletes. Methods: 23 athletes of PP, 11 national level (NL) and 12 regional level (RL) performed dynamic and static tests over a comprehensive range of loads. The study evaluated regional and national level athletes and the influence on the training level on the performance of strength. The study was carried out in four weeks, with the first week to familiarize with the one repetition maximum (1RM), day 1, and there was a 72-h rest and familiarization with dynamic and static tests carried out day 2. In week 2, the 1RM tests were performed (day 1 and 72 h later), and the static tests were performed with a distance of 15 cm from the bar to the chest, with the tests of maximum isometric strength, time to maximum isometric strength, rate of force development (RFD), impulse, variability and fatigue index (IF) taking place on day 2. In weeks three and four dynamic tests were performed, including means propulsive velocity, maximum velocity, power and prediction of one maximum repeat. Results: Differences were found, with better results than for RL in relation to NL in MVP (45%, 55%, and 75% 1RM), in VMax (50%, 55%, 75% and 95% 1RM). In power, the NL had better results (40%, 45%, 50%, 60% and 95% 1RM). Conclusion: RL athletes tend to present better results with regard to velocity, however in power, NL athletes tend to present better performances.

Plyometric training effects on physical fitness and muscle damage in high school baseball players

[Purpose]

The objective of this study was to investigate the effects of plyometric training on physical fitness and muscle damage in high school baseball players.

[Methods]

The participants in the study included 21 high school baseball players who are healthy and well-training. The participants were randomly allocated to the plyometric training (n=11) and control (n=10) groups. The plyometric training was applied 3 times a week for a total of 8 weeks and the control group took part in only regular baseball skills training without plyometric training. For physical fitness, measures included maximal strength (left and right hand-grip strength), muscle endurance (sit-up), agility (side-step), power (standing long jump), and balance (left and right Rhomberg test). For muscle damage, creatine kinase (CK) and lactate dehydrogenase (LDH) levels were measured.

[Results]

The results showed a significant interaction effect between time and groups in left hand-grip strength (P = 0.022), side-step (P = 0.004), and standing long jump (P < 0.001) after the 8-week plyometric training, with greater improvement in the plyometric training group than the control group. On the other hand, there was no significant interaction effect between time and groups in right hand-grip strength, situp, left and right Rhomberg test, CK level, and LDH level (P > 0.05).

[Conclusion]

In conclusion, 8-week plyometric training had a positive effect on improving physical fitness, such as maximal strength, agility, and power, in high school baseball players without causing additional muscle damage.

Effects of Plyometric Training on Lower Body Muscle Architecture, Tendon Structure, Stiffness and Physical Performance: A Systematic Review and Meta-analysis

BACKGROUND

Plyometric training (PT) has been widely studied in sport science. However, there is no review that determines the impact of PT on the structural variables and mechanical properties of the lower limbs and physical performance.

OBJECTIVE

The aim of this systematic review and meta-analysis was to determine the effects of PT on lower body muscle architecture, tendon structure, stiffness and physical performance.

METHODS

Five electronic databases were analysed. The inclusion criteria were: (1) Availability in English; (2) Experimental studies that included a PT of at least eight sessions; and (3) Healthy adults subjects. Four meta-analyses were performed using Review Manager software: (1) muscle architecture; (2) tendon structure; (3) muscle and tendon stiffness; (4) physical performance.

RESULTS

From 1008 search records, 32 studies were eligible for meta-analysis. Muscle architecture meta-analysis found a moderate effect of PT on muscle thickness (Standard Mean Difference (SMD): 0.59; [95% Confidence Interval (CI) 0.47, 0.71]) and fascicle length (SMD: 0.51; [95% CI 0.26, 0.76]), and a small effect of PT on pennation angle (SMD: 0.29; [95% CI 0.02, 0.57]). The meta-analysis found a moderate effect of PT on tendon stiffness (SMD: 0.55; [95% CI 0.28, 0.82]). The lower body physical performance meta-analysis found a moderate effect of PT on jumping (SMD: 0.61; [95% CI 0.47, 0.74]) and strength (SMD: 0.57; [95% CI 0.42, 0.73]).

CONCLUSION

PT increased the thickness, pennation angle and fascicle length of the evaluated muscles. In addition, plyometrics is an effective tool for increasing tendon stiffness and improving jump and strength performance of the lower body.

Mechanical properties of muscles and tendon structures in middle-aged and young men

The purpose of this study was to compare the mechanical properties of muscles and tendon structures for plantar flexor muscles at various strain rates and jump performances using single joint between middle-aged and young men in order to clarify the mechanisms of age-related decline in power output during vertical jump of middle-aged people previously reported. Passive muscle stiffness of the medial gastrocnemius muscle was determined based on passive muscle force and fascicle length during passive stretching at four angular velocities. Active muscle stiffness was calculated based on changes in muscle force and fascicle length during stretching at five angular velocities after submaximal isometric contractions. Maximal elongation and hysteresis of tendon structures were assessed from estimated muscle force—tendon elongation during ramp and ballistic contractions. Two kinds of unilateral jump heights using only ankle joint (no-countermovement and countermovement jumps) were measured. No significant differences in passive and active muscle stiffness, tendon structure properties (except for maximal elongation during ramp contraction), or jump heights were found between middle-aged and young men. The results suggest that the mechanical properties of muscles and tendon structures for plantar flexor muscles and jump performances using only ankle joint do not show age-related changes in middle-aged men.

Relationship Between Isokinetic Lower-Limb Joint Strength, Isometric Time Force Characteristics, and Leg-Spring Stiffness in Recreational Runners

Neuromuscular characteristics, such as lower-limb joint strength and the ability to rapidly generate force, may play an important role in leg-spring stiffness regulation. This study aimed to investigate the relationship between isokinetic knee and ankle joint peak torque (PT), the force-time characteristics of isometric mid-thigh pull (IMTP), and leg stiffness (K_leg)/vertical stiffness (K_vert) in recreationally trained runners. Thirty-one male runners were recruited and underwent three separate tests. In the first session, the body composition, K_leg, and K_vert at running speeds of 12 and 14 km⋅h^–1 were measured. In the second session, isokinetic knee and ankle joint PT at 60°⋅s^–1 were tested. The force-time characteristics of the IMTP were evaluated in the final session. Pearson’s product-moment correlations, with the Benjamini–Hochberg correction procedure, showed that the knee flexor concentric and eccentric and extensor concentric PT (r = 0.473–0.654, p < 0.05) were moderate to largely correlated with K_leg and K_vert at 12 and 14 km⋅h^–1. The knee extensor eccentric PT (r = 0.440, p = 0.050) was moderately correlated with the 14 km⋅h^–1K_vert. The ankle plantar flexor concentric and dorsiflexor eccentric PT (r = 0.506–0.571, p < 0.05) were largely correlated with K_leg at 12 km⋅h^–1. The ankle plantar flexor concentric and eccentric and dorsiflexor eccentric PT (r = 0.436–0.561, p < 0.05) were moderate to largely correlated with K_vert at 12 and 14 km⋅h^–1. For IMTP testing, high correlation was only found between the IMPT peak force (PF) and K_vert at 14 km⋅h^–1 (r = 0.510, p = 0.014). Thus, superior leg-spring stiffness in recreational runners may be related to increased knee and ankle joint strength, eccentric muscular capacity, and maximal force production.

Mechanisms of Achilles Tendon Rupture in National Basketball Association Players

A systematic search was performed of online databases for any Achilles tendon (AT) injuries occurring within the National Basketball Association (NBA). Video was obtained of injuries occurring during competition and downloaded for analysis in Dartfish. NBA athletes (n = 27) were identified with AT rupture over a 30-year period (1991–2021). Of the 27 NBA athletes found to have AT ruptures (mean age: 29.3 [3.3] y; average time in the NBA: 8.5 [3.8] y), 15 in-game videos were obtained for analysis. Noncontact rupture was presumed to have occurred in 12/13 cases. Eight of the 13 athletes had possession of the ball during time of injury. The ankle joint of the injured limb for all 13 athletes was in a dorsiflexed position during the time of injury (47.9° [6.5°]). All 13 athletes performed a false-step mechanism at time of injury where they initiated the movement by taking a rearward step posterior to their center of mass with the injured limb before translating forward. NBA basketball players that suffered AT ruptures appeared to present with a distinct sequence of events, including initiating a false step with ankle dorsiflexion of the injured limb at the time of injury.

Isolated Joint Block Progression Training Improves Leaping Performance in Dancers

The purpose of this study was to investigate the effect of a 12-week ankle-specific block progression training program on saut de chat leaping performance [leap height, peak power (PP), joint kinetics and kinematics], maximal voluntary isometric plantar flexion (MVIP) strength, and Achilles tendon (AT) stiffness. Dancers (training group n = 7, control group n = 7) performed MVIP at plantarflexed (10◦) and neutral ankle positions (0◦) followed by ramping isometric contractions equipped with ultrasound to assess strength and AT stiffness, respectively. Dancers also performed saut de chat leaps surrounded by 3-D motion capture atop force platforms to determine center of mass and joint kinematics and kinetics. The training group then followed a 12-week ankle-focused program including isometric, dynamic constant external resistance, accentuated eccentric loading, and plyometric training modalities, while the control group continued dancing normally. We found that the training group's saut de chat ankle PP (59.8%), braking ankle stiffness (69.6%), center of mass PP (11.4%), and leap height (12.1%) significantly increased following training. We further found that the training group's MVIP significantly increased at 10◦ (17.0%) and 0◦ (12.2%) along with AT stiffness (29.6%), while aesthetic leaping measures were unchanged (peak split angle, mean trunk angle, trunk angle range). Ankle-specific block progression training appears to benefit saut de chat leaping performance, PP output, ankle-joint kinetics, maximal strength, and AT stiffness, while not affecting kinematic aesthetic measures. We speculate that the combined training blocks elicited physiological changes and enhanced neuromuscular synchronization for increased saut de chat leaping performance in this cohort of dancers.

Effects of plyometric training on muscle-tendon mechanical properties and behavior of fascicles during jumping

The purpose of this study was to investigate the effects of plyometric training on the muscle-tendon mechanical properties and behavior of fascicles during jumping in order to elucidate the mechanisms of improved jump performance due to plyometric training. Eleven subjects completed a 12-week unilateral training program for plantar flexors. Active muscle stiffness with (100°·s-1 ) and without (250°·s-1 ) stretch reflex were calculated according to changes in muscle force and fascicle length during fast stretching after submaximal isometric contractions. Stiffness and hysteresis of tendon were measured using ultrasonography during ramp and ballistic contractions. Three kinds of unilateral jump heights using only ankle joint (no counter-movement jump: no-CMJ; counter-movement jump: CMJ; drop jump: DJ) on sledge apparatus were measured. During jumping, electromyographic activities (mEMG) of plantar flexors and fascicle length of the medial gastrocnemius muscle were measured. Active muscle stiffness at 250 and 100°·s-1 and maximal tendon elongation during ballistic contraction significantly increased after training. Tendon hysteresis during ballistic contraction significantly decreased after training, whereas that during ramp contraction did not. The heights of three jump tests, the ratio of mEMG during eccentric to that during concentric phases for CMJ, and the amount of fascicle shortening and shortening velocity during eccentric phase of DJ significantly increased after training. These results suggest that an increase in CMJ height was associated with changes in the muscle-tendon mechanical properties and muscle activation strategy, whereas an increase in DJ height could be explained by changes in the muscle-tendon mechanical properties, but not muscle activation strategy.

High-speed stretch-shortening cycle exercises as a strategy to provide eccentric overload during resistance training

Resistance exercises eliciting eccentric overload (EO) are considered to strongly promote muscular hypertrophy and broad neuromuscular adaptations but typically require specialized equipment. The aims of these experiments were to assess whether EO is achieved during common high-speed stretch-shortening cycle (SSC) exercises (rebound bench press throw [RBPT] and squat jump [SJ]), and to test the effect of the external load on the EO achieved. Twenty-nine under 18 handball players and fifteen physically active males (24.9 ± 3.2 years) took part in the experiments. Testing consisted of a single set of 6 repetitions with light (25%-30% 1-RM), moderate (50% 1-RM), and heavy (70%-75% 1-RM) loads. Eccentric and concentric force near the zero-velocity point (50-200 ms) as well as eccentric-concentric force ratio (EO; %) were calculated. In RBPT, higher EO values were found at 50% 1-RM than 70% 1-RM in the time interval 50 ms before and after the zero-velocity point. Higher EO values were also found at 50% 1-RM than both 30% 1-RM and 70% 1-RM 100 ms before and after the zero-velocity point. For the SJ, higher EO values were found at 50% 1-RM and 75% 1-RM than 25% 1-RM 100 ms before and after the zero-velocity point. Higher EO values were found at 50% 1-RM than 25% 1-RM 200 ms before and after the zero-velocity point. However, the higher EO values in the SJ were found far from the zero-velocity point. High-speed SSC resistance training provides similar EO to other methods and thus should promote muscle hypertrophy and other neuromuscular adaptations.

Substantial Achilles adaptation following strength training has no impact on tendon function during walking

Tendons are responsive to mechanical loading and their properties are often the target of intervention programs. The tendon's mechanical properties, particularly stiffness, also govern its function, therefore changes to these properties could have substantial influence on energy-saving mechanisms during activities utilizing the stretch-shortening cycle. We investigated Achilles tendon (AT) function in vivo during walking with respect to a training intervention that elicited significant increases in AT stiffness. 14 men and women completed 12-weeks of isometric plantarflexor strength training that increased AT stiffness, measured during isometric MVC, by ~31%. Before and after the intervention, participants walked shod at their preferred velocity on a fully-instrumented treadmill. Movement kinematics, kinetics and displacement of the gastrocnemius medialis muscle-tendon junction were captured synchronously using 3D motion capture and ultrasound imaging, respectively. A MANOVA test was used to examine changes in AT force, stress, strain, stiffness, Young's modulus, hysteresis and strain energy, measured during walking, before and following strength training. All were non-significant for a main effect of time, therefore no follow-up statistical tests were conducted. Changes in joint kinematics, tendon strain, velocity, work and power and muscle activity during the stance phase were assessed with 1D statistical parametric mapping, all of which also demonstrated a lack of change in response to the intervention. This in vivo examination of tendon function in walking provides an important foundation for investigating the functional consequences of training adaptations. We found substantial increases in AT stiffness did not impact on tendon function during walking. AT stiffness measured during walking, however, was unchanged with training, which suggests that increases in stiffness may not be evident across the whole force-elongation relation, a finding which may help explain previously mixed intervention results and guide future investigations in the functional implications of tendon adaptation.

Effects of Plyometric Jump Training on Electromyographic Activity and Its Relationship to Strength and Jump Performance in Healthy Trained and Untrained Populations: A Systematic Review of Randomized Controlled Trials

ABSTRACT

Ramirez-Campillo, R, Garcia-Pinillos, F, Chaabene, H, Moran, J, Behm, DG, and Granacher, U. Effects of plyometric jump training on electromyographic activity and its relationship to strength and jump performance in healthy trained and untrained populations: a systematic review of randomized controlled trials. J Strength Cond Res 35(7): 2053-2065, 2021-This systematic review analyzed the effects of plyometric jump training (PJT) on muscle activation assessed with surface electromyography during the performance of strength and jumping tasks in healthy populations across the lifespan. A systematic literature search was conducted in the electronic databases PubMed/MEDLINE, Web of Science, and SCOPUS. Only randomized controlled studies were eligible to be included in this study. Our search identified 17 studies comprising 23 experimental groups and 266 subjects aged 13-73 years, which were eligible for inclusion. The included studies achieved a median Physiotherapy Evidence Database score of 6. No injuries were reported among the included studies. Significant PJT-related improvements were reported in 7 of 10 studies and in 6 of 10 studies for measures of muscle activation during the performance of strength and jumping tasks, respectively. Moreover, a secondary correlational analysis showed significant positive relationships (r = 0.86; p = 0.012; r2 = 0.74) between changes in muscle activation and changes in jump performance. However, from the total number (n = 287) of muscle activation response variables analyzed for strength and jumping tasks, ∼80% (n = 226) were reported as nonsignificant when compared with a control condition. In conclusion, PJT may improve muscle activation during the performance of strength and jumping tasks. However, conflicting results were observed probably arising from (a) studies that incorporated a large number of outcomes with reduced sensitivity to PJT, (b) methodological limitations associated to muscle activation measurement during strength and jumping tasks, and (c) limitations associated with PJT prescription. Future studies in this field should strive to solve these methodological shortcomings.

Interval Return to Play Programs for the Tennis Athlete

PURPOSE OF REVIEW

General guidelines exist for return to sport after injury. The goal of these guidelines is to outline phases of recovery that will minimize the risk of reinjury and promote an optimal return to function for the athlete. This paper analyzes the current research pertaining to interval return to play programs with a special focus on the tennis athlete. The authors examine the different components of an interval tennis program and work to develop what they feel are the necessary elements of the optimal return to sport guidelines for tennis athletes. These criteria are based on the available literature, research, and preliminary data collection as well as the personal experiences and clinical observations of the authors.

RECENT FINDINGS

Interval sports programs are typically designed to guide the athlete back to competition after an injury. The current research reveals the obstacles present in implementing an appropriate interval sports program including a lack of consensus on which criteria is actually necessary and relevant. Limited research is present for interval tennis programs. Return to sport competition and sports science is continuously evolving. The authors highlight the necessary components in rehabilitation and performance principles for establishing a comprehensive interval tennis program. In addition, the role of technology in sports rehabilitation is assessed as it pertains to return to play. The authors proposed that interval sports program can help guide and direct future clinicians in their rehabilitation of the tennis athlete.

The Anticipation of Gravity in Human Ballistic Movement

Stretch-shortening type actions are characterized by lengthening of the pre-activated muscle-tendon unit (MTU) in the eccentric phase immediately followed by muscle shortening. Under 1 g, pre-activity before and muscle activity after ground contact, scale muscle stiffness, which is crucial for the recoil properties of the MTU in the subsequent push-off. This study aimed to examine the neuro-mechanical coupling of the stretch-shortening cycle in response to gravity levels ranging from 0.1 to 2 g. During parabolic flights, 17 subjects performed drop jumps while electromyography (EMG) of the lower limb muscles was combined with ultrasound images of the gastrocnemius medialis, 2D kinematics and kinetics to depict changes in energy management and performance. Neuro-mechanical coupling in 1 g was characterized by high magnitudes of pre-activity and eccentric muscle activity allowing an isometric muscle behavior during ground contact. EMG during pre-activity and the concentric phase systematically increased from 0.1 to 1 g. Below 1 g the EMG in the eccentric phase was diminished, leading to muscle lengthening and reduced MTU stretches. Kinetic energy at take-off and performance were decreased compared to 1 g. Above 1 g, reduced EMG in the eccentric phase was accompanied by large MTU and muscle stretch, increased joint flexion amplitudes, energy loss and reduced performance. The energy outcome function established by linear mixed model reveals that the central nervous system regulates the extensor muscles phase- and load-specifically. In conclusion, neuro-mechanical coupling appears to be optimized in 1 g. Below 1 g, the energy outcome is compromised by reduced muscle stiffness. Above 1 g, loading progressively induces muscle lengthening, thus facilitating energy dissipation.

The influence of lower limb plyometric and resistance training on the stiffness of Achilles and patellar tendons in recreational athletes

Study aim: This study aimed to investigate the influence of combined plyometric and resistance training of lower limbs when administered for a shorter duration of six weeks on the stiffness of Achilles and patellar tendons as well as the jump height.

Materials and methods: Twenty recreational athletes were administered six weeks of a single session of lower limb resistance training and one session of plyometric training every week for a total duration of six weeks. Tendon stiffness was measured using MyotonPro, and vertical jump height was derived from the force plate at baseline and six weeks after the intervention.

Results: There was a statistically significant difference (p < 0.01) between the baseline and post-training measures of patellar and Achilles tendons stiffness as well as the squat jump (SJ) and countermovement jump (CMJ) height.

Conclusion: Both resistance and plyometric training may be incorporated into the training session as combined training showed significant improvements in jump height and tendon stiffness after six weeks of combined RT and PT.

Monitoring Muscle-Tendon Adaptation Over Several Years of Athletic Training and Competition in Elite Track and Field Jumpers

Differences in muscle and tendon responsiveness to mechanical stimuli and time courses of adaptive changes may disrupt the interaction of the musculotendinous unit (MTU), increasing the risk for overuse injuries. We monitored training-induced alterations in muscle and tendon biomechanical properties in elite jumpers over 4 years of athletic training to detect potential non-synchronized adaptations within the triceps surae MTU. A combined cross-sectional and longitudinal investigation over 4 years was conducted by analyzing triceps surae MTU mechanical properties in both legs via dynamometry and ultrasonography in 67 elite track and field jumpers and 24 age-matched controls. Fluctuations in muscle and tendon adaptive changes over time were quantified by calculating individual residuals. The cosine similarity of the relative changes of muscle strength and tendon stiffness between sessions served as a measure of uniformity of adaptive changes. Our cross-sectional study was unable to detect clear non-concurrent differences in muscle strength and tendon stiffness in elite jumpers. However, when considering the longitudinal data over several years of training most of the jumpers demonstrated greater fluctuations in muscle strength and tendon stiffness and hence tendon strain compared to controls, irrespective of training period (preparation vs. competition). Moreover, two monitored athletes with chronic Achilles tendinopathy showed in their affected limb lower uniformity in MTU adaptation as well as higher fluctuations in tendon strain over time. Habitual mechanical loading can affect the MTU uniformity in elite jumpers, leading to increased mechanical demand on the tendon over an athletic season and potentially increased risk for overuse injuries.

Mechanical properties of muscle and tendon at high strain rate in sprinters

The aim of the present study was to compare the mechanical properties of muscles and tendons at high strain rates between sprinters and untrained men. Fifteen sprinters and 18 untrained men participated in this study. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast stretching at five different angular velocities (100, 200, 300, 500, and 600 deg·s-1 ) after submaximal isometric contractions. Stiffness and hysteresis of tendon structures were measured during ramp and ballistic contractions. Active muscle stiffness at 500 deg·s-1 (p = .070) and 600 deg·s-1 (p = .041) was greater in sprinters than untrained men, whereas no differences in those at 100, 200, and 300 deg·s-1 were found between the two groups. There were no differences in stiffness or hysteresis of tendon structures measured during ramp and ballistic contractions between the two groups. These results suggest that, for sprinters, greater active muscle stiffness at a high angular velocity is caused by exercising with a high angular velocity that is typical of their training.

Changes in joint, muscle, and tendon stiffness following repeated hopping exercise

The purpose of this study was to elucidate the mechanisms of decline in joint stiffness after repeated stretch‐shortening cycle exercises according to changes in both muscle‐tendon properties and neuromuscular activities. Eleven males performed fatigue task (5 sets of 50 hopping). Ankle joint stiffness and electromyographic activities (mEMG) of plantar flexor and tibial anterior muscles during drop jump were measured before and after fatigue task. Active muscle stiffness with (100 deg·sec⁻¹) and without (250 deg·sec⁻¹) stretch reflex were calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions. Tendon stiffness was measured during ramp and ballistic contractions. After fatigue task, joint stiffness significantly decreased by 20.7 %, whereas mEMG of measured muscles during drop jump did not. After fatigue task, active muscle stiffness with and without stretch reflex significantly decreased by 15.7 % and 21.5 %, and tendon stiffness measured during ramp and ballistic contractions did not change. In addition, the relative change in joint stiffness was significantly correlated with that in active muscle stiffness with stretch reflex (r = 0.737, P = 0.009), but not with those in the other measured variables. These results suggested that the decline in joint stiffness after repeated hopping exercises would be caused by changes in active muscle stiffness, but not those in tendon properties or neuromuscular activities.

The Time-Course of Changes in Muscle Mass, Architecture and Power During 6 Weeks of Plyometric Training

Purpose

To investigate the time-course of changes in knee-extensors muscle mass, architecture and function in response to plyometric training (PLT) performed on a novel training device, the Tramp-Trainer. This machine consists in a trampoline connected to an inclined sledge which allows the performance of repeated jumps while the subject is sitting on a chair.

Methods

Eight healthy males (173.6 ± 4.7 cm, 69.7 ± 13.5 kg, 25.3 ± 4.6 years) underwent 6 weeks of bilateral PLT on the tramp-trainer machine. Training was performed three times per week (between 120 and 150 bounces per session). Knee-extensor maximum voluntary torque (MVT) and power, quadriceps femoris (QF) volume (VOL), cross-sectional area from the 20% to the 60% of femur length and CSA_mean, together with vastus lateralis (VL) architecture (fascicle length, Lf, and pennation angle, PA) were assessed after 2, 4, and 6 weeks of PLT.

Results

All results are presented as changes versus baseline values. MVT increased by 17.8% (week 2, p < 0.001) and 22.2% (week 4, p < 0.01), respectively, and declined to 13.3% (p < 0.05) at week 6 of PLT. Power increased by 18.2% (week 4, p < 0.05) and 19.7% (week 6, p < 0.05). QF VOL increased by 4.7% (week 4, p < 0.05) and 5.8% (week 6, p < 0.01); VL VOL increased by 5.2%, (p < 0.05), 8.2%, (p < 0.01), and 9.6% (p < 0.05) at weeks 2, 4, and 6, respectively. An increase in Lf was detected already at wk 2 (2.2%, p < 0.05), with further increase at 4 and 6 weeks of PLT (4 and 4.4%, respectively, p < 0.01). PA increased by 5.8% (p < 0.05) at week 6. Significant positive correlations were found between CSA_mean and Power (R² = 0.46, p < 0.001) and between QF VOL and Power (R² = 0.44, p < 0.024).

Conclusions

PLT induced rapid increases in muscle volume, fascicle length, pennation angle, torque and power in healthy younger adults. Notably, changes in VL VOL and Lf were detectable already after 2 weeks, followed by increases in knee extensors VOL and power from week 4 of PLT. Since the increase in CSA_mean and QF VOL cannot fully explain the increment in muscle power, it is likely that other factors (such as adaptations in neural drive or tendon mechanical properties) may have contributed to such fucntional changes.

Effects of in Season Multi-Directional Plyometric Training on Vertical Jump Performance, Change of Direction Speed and Dynamic Postural Control in U-21 Soccer Players

The aim of the study was to assess the effects of multi-directional plyometric training (MPT) on vertical jump height, change of direction speed (CODS), and dynamic postural control (DPC) of soccer players under 21 year (U-21). Twenty-seven male soccer players were randomly allocated to either an experimental group (EG; n = 14; age: 19.0 ± 0.9 years) or a control group (CG; n = 13; age: 19.0 ± 0.7 years). The EG introduced 6 weeks MPT, 2 days per week into their in-season training, while CG continued training without change. Measurements of vertical jump height, CODS and DPC were completed at the beginning and end of the 6 week MPT. ANOVA demonstrated a significant group × time interaction for SJ (F = 6.03, p = 0.021), CMJ (F = 9.10, p = 0.006), and T-Test (F = 10.46, p = 0.002). The Bonferroni Post Hoc test demonstrated significant increase for the three tests in both group (EG and CG). For SJ (EG: p < 0.001; CG: p < 0.001), CMJ (EG: p < 0.001; CG: p = 0.005) and T-Test (EG: p < 0.001; CG: p = 0.02). For DPC on the dominant leg, there was a significant group × time interaction for four axes [anterior (F = 5.48, p = 0.028), antero-lateral (F = 4.82, p = 0.038), postero-lateral (F = 4.82, p = 0.038), and medial (F = 6.77, p = 0.015)]. The Bonferroni Post Hoc test demonstrated significant increase in EG (p < 0.001), but no significant change in CG in four axes (anterior, antero-lateral, postero-lateral and medial). Furthermore DPC on the non-dominant leg, there was a significant group × time interaction for three axes [lateral (F = 8.09, p = 0.009), postero-lateral (F = 11.92, p = 0.002), and medial (F = 5.84, p = 0.023)]. The Bonferroni Post Hoc test demonstrated significant increase in EG (p < 0.001), but no significant change in CG in three axes (lateral, postero-lateral, and medial). In conclusion, incorporating MPT into the in-season regimen of under 21 soccer players improved performance of various indices related to soccer activity (i.e., CMJ, CODS, and DPC). MPT has the potential to be appealing to coaches, as it requires little time while yielding valuable results in the physical preparation of soccer players.

Hamstring Eccentric Strengthening Program: Does Training Volume Matter?

AIM

To compare the effect of low versus high volume of eccentric-biased hamstring training programs on knee-flexor strength and fascicle length changes in elite soccer players.

METHODS

A total of 19 elite youth soccer players took part in this study and were randomly assigned into 2 subgroups. For 6 weeks in-season, the groups performed either a low-volume (1 set per exercise; 10 repetitions in total) or a high-volume (4 sets; 40 repetitions) eccentric training of their knee flexors. After 6-weeks midtraining (MID), players performed the alternate training regimen. Each training set consisted of 4 repetitions of the Nordic hamstring exercise and 6 repetitions of the bilateral stiff-leg deadlift. Eccentric knee-flexor strength (NordBord) as well as biceps femoris long head and semimembranosus fascicle length (scanned with ultrasound scanner) were assessed during pretraining (PRE), MID, and posttraining (POST) tests.

RESULTS

Knee-flexor eccentric strength very likely increased from PRE to MID (low volume: +11.3% [7.8%] and high volume: 11.4% [5.3%]), with a possibly-to-likely increase in biceps femoris long head (+4.5% [5.0%] and 4.8% [2.5%]) and semimembranosus (+4.3% [4.7%] and 6.3% [6.3%]) fascicle length in both groups. There was no substantial changes between MID and POST. Overall, there was no clear between-group difference in the changes from PRE to MID and MID to POST for neither knee-flexor eccentric strength, biceps femoris long head, nor semimembranosus fascicle length.

CONCLUSIONS

Low-volume knee-flexor eccentric training is as effective as a greater training dose to substantially improve knee-flexor strength and fascicle length in-season in young elite soccer players. Low volume is, however, likely more appropriate to be used in an elite team facing congested schedules.

Multidirectional Plyometric Training: Very Efficient Way to Improve Vertical Jump Performance, Change of Direction Performance and Dynamic Postural Control in Young Soccer Players

The aim of the study was to assess the effects of multidirectional plyometric training (MPT) on vertical jump height, change of direction performance (CODP), and dynamic postural control (DPC) in young soccer players. Twenty-eight young male soccer players were randomly assigned to an experimental group (EG, n = 14; age: 11.8 ± 0.4 years) and a control group (CG, n = 14; age: 11.6 ± 0.5 years). The EG introduced 8-week MPT, two days per week into their in-season training, while CG continued training without change. Measurements of vertical jump height, CODP, and DPC were completed at the beginning and end of the 8-week MPT. A significant group × time interaction was observed for Squat-Jump (p < 0.05), for Counter-Movement Jump (p < 0.05), and for CODP test (p < 0.05). In addition, a significant group × time interaction was observed for DPC in seven axes for the dominant- (anterior, lateral, postero-lateral, posterior, postero-medial, medial, and antero-medial; p < 0.05 for all) and in seven axes for the non-dominant- (anterior, antero-lateral, lateral, posterior, postero-medial, medial and antero-medial; p < 0.05 for all) legs. The rest of the axes of both legs did not show any significant group × time interaction (p > 0.05). In conclusion, incorporating MPT into the in-season regimen of young male soccer players improved performance of various indices related to soccer activity (i.e., vertical jump height, CODP, and DPC). MPT has the potential to be appealing to coaches, as it requires little time while yielding valuable results in the physical preparation of young soccer players.

Anticipation of drop height affects neuromuscular control and muscle-tendon mechanics

This study examined the effect of drop height on neuromechanical control of the plantarflexors in drop jumps (DJs) before and during ground contact (GC). The effect of anticipation on muscle mechanical configurations was investigated in 22 subjects in three conditions (20, 30, and 40 cm): (i) known, (ii) unknown, or (iii) cheat falling heights (announced 40 cm, but actual drop height was 20 cm). Electromyographic (EMG) activity of the m. gastrocnemius medialis (GM) and other shank muscles was recorded and analyzed before GC and during GC separately for the short-, medium-, and long-latency responses (SLR, MLR, and LLR). Changes in GM fascicle length (L_M ) were determined via B-mode ultrasound, and muscle-tendon unit length (L_MTU ) was estimated. Peak force (P < .001), rate of force development (RFD) (P = .001) and GM EMG activity prior to (P = .003) and during GC (P = .007) was reduced in the unknown compared with the known conditions (P < .05). The amount of shortening in L_MTU during GC in unknown and cheat was less compared with the known conditions (P = .005; P = .049). Changes in L_MTU lengthening negatively correlated with changes in GM activity around SLR and MLR (P = .006; P = .02) in known and unknown conditions. Taken together, it seems that the central nervous system applies a protective strategy in the unknown condition by reducing muscle activity to result in a lower muscular stiffness and increased tendinous lengthening prior to and during GC. This might be a mechanism to absorb greater elastic energy in the tendon and reduce the magnitude and rate of muscle lengthening and subsequent stretch-induced muscle damage.

Mechanical properties, physiological behavior, and function of aponeurosis and tendon

During human movement, the muscle and tendinous structures interact as a mechanical system in which forces are generated and transmitted to the bone and energy is stored and released to optimize function and economy of movement and/or to reduce risk of injury. The present review addresses certain aspects of how the anatomical design and mechanical and material properties of the force-transmitting tissues contribute to the function of the muscle-tendon unit and thus overall human function. The force-bearing tissues are examined from a structural macroscopic point of view down to the nanoscale level of the collagen fibril. In recent years, the understanding of in vivo mechanical function of the force-bearing tissues has increased, and it has become clear that these tissues adapt to loading and unloading and furthermore that force transmission mechanics is more complex than previously thought. Future investigations of the force-transmitting tissues in three dimensions will enable a greater understanding of the complex functional interplay between muscle and tendon, with relevance for performance, injury mechanisms, and rehabilitation strategies.

Triceps Surae Muscle-Tendon Unit Properties in Preadolescent Children: A Comparison of Artistic Gymnastic Athletes and Non-athletes

Knowledge regarding the effects of athletic training on the properties of muscle and tendon in preadolescent children is scarce. The current study compared Achilles tendon stiffness, plantar flexor muscle strength and vertical jumping performance of preadolescent athletes and non-athletes to provide insight into the potential effects of systematic athletic training. Twenty-one preadolescent artistic gymnastic athletes (9.2 ± 1.6 years, 15 girls) and 11 similar-aged non-athlete controls (9.0 ± 1.7 years, 6 girls) participated in the study. The training intensity and volume of the athletes was documented for the last 6 months before the measurements. Subsequently, vertical ground reaction forces were measured with a force plate to assess jumping performance during squat (SJ) and countermovement jumps (CMJ) in both groups. Muscle strength of the plantar flexor muscles and Achilles tendon stiffness were examined using ultrasound, electromyography, and dynamometry. The athletes trained 6 days per week with a total of 20 h of training per week. Athletes generated significantly greater plantar flexion moments normalized to body mass compared to non-athletes (1.75 ± 0.32 Nm/kg vs. 1.31 ± 0.33 Nm/kg; p = 0.001) and achieved a significantly greater jump height in both types of jumps (21.2 ± 3.62 cm vs. 14.9 ± 2.32 cm; p < 0.001 in SJ and 23.4 ± 4.1 cm vs. 16.4 ± 4.1 cm; p < 0.001 in CMJ). Achilles tendon stiffness did not show any statistically significant differences (p = 0.413) between athletes (116.3 ± 32.5 N/mm) and non-athletes (106.4 ± 32.8 N/mm). Athletes were more likely to reach strain magnitudes close to or higher than 8.5% strain compared to non-athletes (frequency: 24% vs. 9%) indicating an increased mechanical demand for the tendon. Although normalized muscle strength and jumping performance were greater in athletes, gymnastic-specific training in preadolescence did not cause a significant adaptation of Achilles tendon stiffness. The potential contribution of the high mechanical demand for the tendon to the increasing risk of tendon overuse call for the implementation of specific exercises in the athletic training of preadolescent athletes that increase tendon stiffness and support a balanced adaptation within the muscle-tendon unit.

Evidence of a Uniform Muscle-Tendon Unit Adaptation in Healthy Elite Track and Field Jumpers: A Cross Sectional Investigation

Different adaptive responses to mechanical loading between muscle and tendon can lead to non-uniform biomechanical properties within the muscle-tendon unit. The current study aimed to analyze the mechanical properties of the triceps surae muscle-tendon unit in healthy male and female elite track and field jumpers in order to detect possible inter-limb differences and intra-limb non-uniformities in muscle and tendon adaptation. The triceps surae muscle strength and tendon stiffness were analyzed in both limbs during maximal voluntary isometric plantar flexion contractions using synchronous dynamometry and ultrasonography in sixty-seven healthy young male (n = 35) and female (n = 32) elite international level track and field jumpers (high jump, long jump, triple jump, pole vault). Triceps surae muscle-tendon unit intra-limb uniformity was assessed using between limb symmetry indexes in the muscle strength and tendon stiffness. Independent from sex and jumping discipline the take-off leg showed a significantly higher (p < 0.05) triceps surae muscle strength and tendon stiffness, suggesting different habitual mechanical loading between legs. However, despite these inter-limb discrepancies no differences were detected in the symmetry indexes of muscle strength (5.9 ± 9.4%) and tendon stiffness (8.1 ± 11.5%). This was accompanied by a significant correlation between the symmetry indexes of muscle strength and tendon stiffness (r = 0.44; p < 0.01; n = 67). Thus, the current findings give evidence for a uniform muscle-tendon unit adaptation in healthy elite track and field jumpers, which can be reflected as a protective mechanism to maintain its integrity to meet the functional demand.

Bouncing Back! Counteracting Muscle Aging With Plyometric Muscle Loading

The preservation of muscle power is crucial in aging for maintaining mobility and performing daily tasks. Resistance training involving high movement velocities represents a valid strategy to slow down the rate of sarcopenia, counteracting the loss of muscle mass and muscle power. Plyometric exercise may represent an effective training modality for increasing muscle power; however, its application in older populations has been sparingly investigated, as the high impact actions involved may reduce its feasibility for older individuals. By adopting a safer modality of plyometric training, we investigated if a 6-week plyometric training intervention could increase knee extensor muscle size, architecture, force and power in 14 young (YM, age = 25.4 ± 3.5 y; means ± SD) and nine older males (OM, age = 69.7 ± 3.4 y). Volunteers trained 3 times/week using a device similar to a leg press machine where the user was required to bounce against his body mass on a trampoline. Pre-to-post training changes in isometric maximum voluntary torque (MVT), leg extension power and vastus lateralis (VL) architecture were assessed. Muscle power increased in both groups (+27% OM -P < 0.001, 20% YM -P < 0.001), although the total external work performed during the training period was significantly lower for OM (i.e., ~-47%). Both groups showed significant increases in muscle thickness (MT) (+5.8 OM -P < 0.01 vs. +3.8% YM -P < 0.01), fascicle length (Lf) (+8% OM -P < 0.001 vs. +6% YM -P < 0.001), and pennation angle (PA) (+7.5% OM -P < 0.001 vs. +4.1% YM -P < 0.001). The current study shows that trampoline-based plyometric training is an effective intervention producing a rapid increase in muscle mass and power in both young and older individuals. The training modality used in this study seems to particularly benefit the older population, targeting the morphological and functional effects of sarcopenia in human muscle.

How do tendons adapt? Going beyond tissue responses to understand positive adaptation and pathology development: A narrative review

Understanding how tendons adapt to load is crucial to understanding how training can improve performance, minimise the risk of injury and aid rehabilitation. Adaptation is the positive response of an organism or tissue to benefit its function. In tendons, numerous tissue responses to load have been identified in vivo. Changes in tendon dimensions, structure on imaging, mechanical properties, and blood flow have been reported in response to mechanical stimuli. However, research has focused on tissue level changes with little understanding of how changes at the tissue level affect the person, their athletic performance or injury risk. Tendons can have a paradoxical response to load, load can induce positive adaptation, however it is also a major factor in the development of tendon pathology and pain. Tendon pathology is a risk factor for developing symptoms, yet the high rate of asymptomatic pathology suggests that the pathological tendon must adapt to be able to tolerate load. Similarly, there is mounting evidence to suggest that tendon remodelling or repair is not necessary for a positive clinical outcome following rehabilitation, suggesting that the tendon must adapt via other mechanisms. This narrative review synthesises evidence of how normal and pathological tendons adapts to load, and how this relates to adaptation of load capacity and function of the individual.

Effect of Training-Induced Changes in Achilles Tendon Stiffness on Muscle-Tendon Behavior During Landing

During rapid deceleration of the body, tendons buffer part of the elongation of the muscle–tendon unit (MTU), enabling safe energy dissipation via eccentric muscle contraction. Yet, the influence of changes in tendon stiffness within the physiological range upon these lengthening contractions is unknown. This study aimed to examine the effect of training-induced stiffening of the Achilles tendon on triceps surae muscle–tendon behavior during a landing task. Twenty-one male subjects were assigned to either a 10-week resistance-training program consisting of single-leg isometric plantarflexion (n = 11) or to a non-training control group (n = 10). Before and after the training period, plantarflexion force, peak Achilles tendon strain and stiffness were measured during isometric contractions, using a combination of dynamometry, ultrasound and kinematics data. Additionally, testing included a step-landing task, during which joint mechanics and lengths of gastrocnemius and soleus fascicles, Achilles tendon, and MTU were determined using synchronized ultrasound, kinematics and kinetics data collection. After training, plantarflexion strength and Achilles tendon stiffness increased (15 and 18%, respectively), and tendon strain during landing remained similar. Likewise, lengthening and negative work produced by the gastrocnemius MTU did not change detectably. However, in the training group, gastrocnemius fascicle length was offset (8%) to a longer length at touch down and, surprisingly, fascicle lengthening and velocity were reduced by 27 and 21%, respectively. These changes were not observed for soleus fascicles when accounting for variation in task execution between tests. These results indicate that a training-induced increase in tendon stiffness does not noticeably affect the buffering action of the tendon when the MTU is rapidly stretched. Reductions in gastrocnemius fascicle lengthening and lengthening velocity during landing occurred independently from tendon strain. Future studies are required to provide insight into the mechanisms underpinning these observations and their influence on energy dissipation.

Theoretical considerations for muscle-energy savings during distance running

We have recently demonstrated that the triceps surae muscles energy cost (EC_TS) represents a substantial portion of the total metabolic cost of running (E_run). Therefore, it seems relevant to evaluate the factors which dictate EC_TS, namely the amount and velocity of shortening, since it is likely these factors will dictate E_run. E_run and triceps surae morphological and AT mechanical properties were obtained in 46 trained and elite male and female distance runners using ultrasonography and dynamometry. EC_TS (J·stride^-1) at the speed of lactate threshold (sLT) was estimated from AT force and crossbridge mechanics and energetics. To estimate the relative impact of these factors on EC_TS, mean values for running speed, body mass, resting fascicle length (L_f), Achilles tendon stiffness and moment arm and maximum isometric plantarflexion torque were obtained. EC_TS was calculated across a range (mean ± 1 sd) of values for each independent factor. Average sLT was 233 m·min^-1. At this speed, EC_TS was 255 J·stride^-1. Estimated fascicle shortening velocity was 0.08 V_max and the level of muscle activation was 84.7% of maximum isometric torque. Compared to the EC_TS calculated from the lowest range of values obtained for each independent factor, higher AT stiffness was associated with a 39% reduction in EC_TS, 81% reduction in fascicle shortening velocity and a 31% reduction in muscle activation. Longer AT moment arms and elevated body masses were associated with an increase in EC_TS of 18% and 23%, respectively. These results demonstrate that a low EC_TS is achieved primarily from a high AT stiffness and low body mass, which is exemplified in elite distance runners.

Effects of plyometric and isometric training on muscle and tendon stiffness in vivo

The purpose of this study was to compare the effects of plyometric and isometric training on tendon properties during ramp and ballistic contractions and muscle stiffness under passive and active conditions. Eleven subjects completed 12 weeks (3 days/week) of a unilateral training program for the plantar flexors. They performed plyometric training on one side (PLY) and isometric training on the other side (ISO). Active muscle stiffness in the medial gastrocnemius muscle was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions. Passive muscle stiffness was also calculated from estimated passive muscle force and fascicle length during slow passive stretching. Stiffness and hysteresis of tendon structures were measured using ultrasonography during ramp and ballistic contractions. Passive muscle stiffness and tendon hysteresis did not change for PLY or ISO. Active muscle stiffness significantly increased for PLY, but not for ISO. Tendon stiffness during ramp and ballistic contractions increased significantly for ISO, but not for PLY. In addition, tendon elongation values at force production levels beyond 100 N during ballistic contractions increased for PLY. These results suggest that plyometric training (but not isometric training) enhances the extensibility of tendon structures during ballistic contractions and active muscle stiffness during fast stretching, and these changes may be related to improved performances during stretch‐shortening cycle exercises.

Imbalances in the Development of Muscle and Tendon as Risk Factor for Tendinopathies in Youth Athletes: A Review of Current Evidence and Concepts of Prevention

Tendons feature the crucial role to transmit the forces exerted by the muscles to the skeleton. Thus, an increase of the force generating capacity of a muscle needs to go in line with a corresponding modulation of the mechanical properties of the associated tendon to avoid potential harm to the integrity of the tendinous tissue. However, as summarized in the present narrative review, muscle and tendon differ with regard to both the time course of adaptation to mechanical loading as well as the responsiveness to certain types of mechanical stimulation. Plyometric loading, for example, seems to be a more potent stimulus for muscle compared to tendon adaptation. In growing athletes, the increased levels of circulating sex hormones might additionally augment an imbalanced development of muscle strength and tendon mechanical properties, which could potentially relate to the increasing incidence of tendon overload injuries that has been indicated for adolescence. In fact, increased tendon stress and strain due to a non-uniform musculotendinous development has been observed recently in adolescent volleyball athletes, a high-risk group for tendinopathy. These findings highlight the importance to deepen the current understanding of the interaction of loading and maturation and demonstrate the need for the development of preventive strategies. Therefore, this review concludes with an evidence-based concept for a specific loading program for increasing tendon stiffness, which could be implemented in the training regimen of young athletes at risk for tendinopathy. This program incorporates five sets of four contractions with an intensity of 85–90% of the isometric voluntary maximum and a movement/contraction duration that provides 3 s of high magnitude tendon strain.