John B. Cronin

A review of research on the mechanical stiffness in running and jumping: methodology and implications

Mechanical stiffness (vertical, leg and joint stiffness) can be calculated during normal human movements, such as running and hopping. Mechanical stiffness is thought to influence several athletic variables, including rate of force development, elastic energy storage and utilization and sprint kinematics. Consequently, the relationship between mechanical stiffness and athletic performance is of great interest to the sport and research communities. Unfortunately, these relationships are relatively unexplored by researchers. For example, there are no longitudinal studies that have investigated the effects of strength or power training on mechanical stiffness levels (calculated during human running). In addition to reviewing the available literature on the relationships between mechanical stiffness (calculated during human running) and functional performance, this review focuses its discussion on the various equipment and methods used to calculate leg‐spring stiffness during human running. Furthermore, future implications are presented for practitioners and researchers based on both the limitations and the gaps in the literature reviewed. It is our hope that a better understanding of mechanical stiffness will aid in improving the methodological quality of research in this area and its subsequent effect on athletic performance.

Anthropometric, physiological and performance characteristics of elite team-handball players

Abstract The objective of this study was to provide anthropometric, physiological, and performance characteristics of an elite international handball team. Twenty-one elite handball players were tested and categorized according to their playing positions (goalkeepers, backs, pivots, and wings). Testing consisted of anthropometric and physiological measures of height, body mass, percentage body fat and endurance ([Vdot]O2max), performance measures of speed (5, 10, and 30 m), strength (bench press and squat), unilateral and bilateral horizontal jumping ability, and a 5-jump horizontal test. Significant differences were found between player positions for some anthropometric characteristics (height and percentage body fat) but not for the physiological or performance characteristics. Strong correlations were noted between single leg horizontal jumping distances with 5-, 10-, and 30-m sprint times (r = 0.51–0.80; P < 0.01). The best predictors of sprint times were single leg horizontal jumping with the dominant leg and the distance measured for the 5-jump test, which when combined accounted for 72% of the common variance associated with sprint ability. In conclusion, performance abilities between positions in elite team-handball players appear to be very similar. Single leg horizontal jumping distance could be a specific standardized test for predicting sprinting ability in elite handball players.

Altering the Length-Tension Relationship with Eccentric Exercise Implications for Performance and Injury

The effects of eccentric exercise on muscle injury prevention and athletic performance are emerging areas of interest to researchers. Of particular interest are the adaptations that occur after a single bout, or multiple bouts of eccentric exercise. It has been established that after certain types of eccentric exercise, the optimum length of tension development in muscle can be shifted to longer muscle lengths. Altering the length-tension relationship can have a profound influence on human movements. It is thought that the length-tension relationship is influenced by the structural makeup of muscle. However, the mechanism responsible for the shift in optimum length is not readily agreed upon. Despite the conflict, several studies have reported a shift in optimum length after eccentric exercise. Unfortunately, very few of these studies have been randomised, controlled training studies, and the duration of the shift has not yet been established. Nonetheless, this adaptation may result in greater structural stability at longer muscle lengths and consequently may have interesting implications for injury prevention and athletic performance. Both contentions remain relatively unexplored and provide the focus of this review.

Effects of weighted vests and sled towing on sprint kinematics.

In this study, we compared sprint kinematics of sled towing and vest sprinting with the same relative loads. Twenty athletes performed 30-m sprints in three different conditions: (a) un-resisted, (b) sled towing, and (c) vest sprinting. During sled towing and vest sprinting, external loads of 15% and 20% of body mass were used. Sprint times were recorded over 10 and 30 m. Sagittal-plane high-speed video data were recorded at 5, 15, and 25 m from the start. Relative to the un-resisted condition, sprint time increased (7.5 to 19.8%) in both resisted conditions, resulting mainly from decreased step length ( − 5.2 to − 16.5%) with small decreases in step frequency ( − 2.7 to − 6.1%). Sled towing increased stance phase duration (14.7 to 26.0%), trunk angle (12.5 to 71.5%), and knee angle (10.3 to 22.7%), and decreased swing phase duration ( − 4.8 to − 15.2%) relative to the un-resisted condition. Vest sprinting increased stance phase duration (12.8 to 24.5%) and decreased swing phase duration ( − 8.4 to − 14.4%) and trunk angle ( − 1.7 to − 13.0%). There were significant differences between the two resisted conditions in trunk, thigh, and knee angles. We conclude that sled towing and vest sprinting have different effects on some kinematics and hence change the overload experienced by muscle groups.

Influence of Running Velocity on Vertical, Leg and Joint Stiffness Modelling and Recommendations for Future Research

Human running can be modelled as either a spring-mass model or multiple springs in series. A force is required to stretch or compress the spring, and thus stiffness, the variable of interest in this paper, can be calculated from the ratio of this force to the change in spring length. Given the link between force and length change, muscle stiffness and mechanical stiffness have been areas of interest to researchers, clinicians, and strength and conditioning practitioners for many years.This review focuses on mechanical stiffness, and in particular, vertical, leg and joint stiffness, since these are the only stiffness types that have been directly calculated during human running. It has been established that as running velocity increases from slow-to-moderate values, leg stiffness remains constant while both vertical stiffness and joint stiffness increase. However, no studies have calculated vertical, leg or joint stiffness over a range of slow-to-moderate values to maximum values in an athletic population. Therefore, the effects of faster running velocities on stiffness are relatively unexplored. Furthermore, no experimental research has examined the effects of training on vertical, leg or joint stiffness and the subsequent effects on running performance.Various methods of training (Olympic style weightlifting, heavy resistance training, plyometrics, eccentric strength training) have shown to be effective at improving running performance. However, the effects of these training methods on vertical, leg and joint stiffness are unknown. As a result, the true importance of stiffness to running performance remains unexplored, and the best practice for changing stiffness to optimize running performance is speculative at best. It is our hope that a better understanding of stiffness, and the influence of running speed on stiffness, will lead to greater interest and an increase in experimental research in this area.

Effects of eccentric exercise on optimum length of the knee flexors and extensors during the preseason in professional soccer players

OBJECTIVE To assess the effects of eccentric exercise on optimum lengths of the knee flexors and extensors during the preseason in professional soccer. DESIGN Twenty-eight athletes from a professional Spanish soccer team (Division II) were randomly assigned to an eccentric exercise intervention group (EG) or a control group (CG). Over the four-week period two athletes from the control group suffered RF injuries and two athletes were contracted by other clubs. After these exclusions, both groups (EG, n=13; and CG, n=11) performed regular soccer training during the four-week preseason period. RESULTS After the four weeks, the optimum lengths of the knee flexors were significantly (P<0.05) increased by 2.3 degrees in the CG and by 4.0 degrees in the EG. The change in the EG was significantly (P<0.05) greater than that of the CG. The optimum lengths of the knee extensors were significantly increased only in the EG by 6.5 degrees . Peak torque levels and ratios of quadriceps to hamstring (Q/H ratios) were not significantly altered throughout the study for either group. CONCLUSION Eccentric exercise can increase the optimum lengths of both the knee extensors and knee extensors flexors during the preseason in professional soccer.

A biomechanical evaluation of resistance: fundamental concepts for training and sports performance.

Newton’s second law of motion describes the acceleration of an object as being directly proportional to the magnitude of the net force, in the same direction as the net force and inversely proportional to its mass (a = F/m). With respect to linear motion, mass is also a numerical representation of an object’s inertia, or its resistance to change in its state of motion and directly proportional to the magnitude of an object’s momentum at any given velocity. To change an object’s momentum, thereby increasing or decreasing its velocity, a proportional impulse must be generated. All motion is governed by these relationships, independent of the exercise being performed or the movement type being used; however, the degree to which this governance affects the associated kinematics, kinetics and muscle activity is dependent on the resistance type. Researchers have suggested that to facilitate the greatest improvements to athletic performance, the resistance-training programme employed by an athlete must be adapted to meet the specific demands of their sport. Therefore, it is conceivable that one mechanical stimulus, or resistance type, may not be appropriate for all applications. Although an excellent means of increasing maximal strength and the rate of force development, free-weight or mass-based training may not be the most conducive means to elicit velocity-specific adaptations. Attempts have been made to combat the inherent flaws of free weights, via accommodating and variable resistance-training devices; however, such approaches are not without problems that are specific to their mechanics. More recently, pneumatic-resistance devices (variable) have been introduced as a mechanical stimulus whereby the body mass of the athlete represents the only inertia that must be overcome to initiate movement, thus potentially affording the opportunity to develop velocity- specific power. However, there is no empirical evidence to support such a contention. Future research should place further emphasis on understanding the mechanical advantages/disadvantages inherent to the resistance types being used during training, so as to elicit the greatest improvements in athletic performance.

The Effects of Whole Body Vibration on Physical and Physiological Capability in Special Populations

Abstract The objective of this article was to systematically review the effects of whole body vibration (WBV) loading parameters on the elderly, postmenopausal women and neurological patients. Ten databases were searched for clinical trials using WBV training in special populations. To assess the methodological quality, the PEDro score was used. To compare effects, effects were converted into percentage changes and effect sizes. Four clinical and 10 randomized clinical trial papers were included. The average PEDro score was 4.93 (± 1.59). With 60-second intervention and 60-second rest periods, the most frequent vibratory stimulation loading parameters used were 3–6 Hz and 3 mm amplitude for multiple sclerosis and Parkinsons disease patients, and 30 Hz and 3–5 mm amplitude for all other conditions. Balance, stability and functional performance significantly improved ( p

The acute effects of hamstring stretching and vibration on dynamic knee joint range of motion and jump performance

OBJECTIVES To investigate dynamic knee joint range of motion (ROM) and jump performance following a single bout of passive hamstring stretching, hamstring vibration or a combination of both. DESIGN Knee joint dynamic ROM and jump performance were assessed prior to, immediately following and 10 min following stretching and vibration of the hamstring muscles. The study was a crossover design with all participants completing three interventions: (1) 3 x 30s static stretches of the hamstring muscles, (2) 3 x 30s bouts of vibration applied to the hamstring muscles, (3) a combination of the stretching and vibration protocols. SETTING The study was conducted in the muscle performance laboratory of Auckland University of Technology. PARTICIPANTS Ten male participants (mean+/-SD, age 22.7+/-3.6 yr, height 181.2+/-6.51 cm, mass 84.9+/-12.3 kg) with no musculoskeletal problems volunteered to participate in this study. MAIN OUTCOME MEASURES Dynamic knee joint ROM and jump height. RESULTS The only increase in dynamic knee joint ROM was between the pre and immediate post assessments in the stretching intervention (mean change 3 degrees or 2%, ES=0.4, p=0.011). There was no statistically significant interaction between intervention and time for any of the jump performance assessments. CONCLUSIONS A single bout of stretching produced a small, short-lived change in dynamic knee joint ROM but did not change jump performance. The addition of vibration alone or in combination with stretching did not influence dynamic knee joint ROM or jump performance.

Application of eccentric exercise on an Australian Rules football player with recurrent hamstring injuries.

STUDY DESIGN Case report. OBJECTIVE To assess an eccentric based intervention on an Australian Football player with recurrent hamstring injuries. CASE DESCRIPTION The athlete attempted several conventional rehabilitation programs in the past (e.g. physical therapy, manual therapy, acupuncture, active release, medial gluteal strengthening) with no sustained progress in regards to pain, soreness, or return to sport. OUTCOMES After the first three phases of the intervention (i.e. nine weeks), the optimum angle of peak torque during knee flexion decreased from 37.3 to 23.9 degrees in the injured leg, and from 24.3 to 20.3 degrees in the non-injured leg. After the first nine weeks, the optimum angles then remained constant for another 23 weeks. The optimum angle of peak torque was also shifted in the knee extensors by 3.9 degrees (injured leg) and 3.4 degrees (non-injured leg) after nine weeks and then remained constant for the remaining 23 weeks. Quadriceps to hamstring peak torque ratios (Q/H ratios) and peak torque during knee flexion and extension remained constant throughout the intervention. DISCUSSION An eccentric based intervention was shown to be safe and effective for altering the optimum angle of peak torque (i.e. shifting to longer muscle lengths) for this athlete with recurrent hamstring injuries.