Thesis
The performance enhancement associated with dynamic stretch shorten cycle movements: an investigation into the development of testing devices, performance effects and underlying mechanisms
Southern Cross University, School of Exercise Science and Sport Management
Doctor of Philosophy (PhD), Southern Cross University
1998
Metrics
18 File views/ downloads
57 Record Views
Abstract
The purpose of this series of five investigations was to examine the performance benefits and underlying mechanisms currently reported as significant to dynamic stretch shorten cycle performance. The aim of the first study was to investigate the validity and reliability of a newly developed, in-vivo test, of lower body musculotendinous stiffness. Twenty three male subjects with at least twelve months weight training experience performed a series of quasi-static muscular actions in a supine leg press position during which a brief perturbation was applied. The resulting damped oscillations enabled each subject's maximal musculotendinous stiffness (k) for the lower body musculature to be estimated. To assess the individual's capacity to benefit from active stretch, subjects also performed both a static jump and a countermove jump. Statistical analysis revealed no significant difference between day one and two stiffness values (p<0.01), an inter-day reliability of r=0.94 and a coefficient of variance of 8%. It was further demonstrated that maximal stiffness was significantly correlated to both isometric and concentric rate of force development (r=0.50 and r=0.54, p<0.05; respectively), and inversely related to the percentage difference between vertical jumps with and without prior stretch (r=0.54). Such results are discussed with reference to the restitution of elastic strain energy, muscle potentiation and the interaction effects of elastic recoil on dynamic muscular function. It was concluded that the in vivo assessment of stiffness for the lower body using the oscillation technique is a valid and reliable measure of musculotendinous stiffness.
The second study investigated the relationship between musculotendinous stiffness, and the ability to perform dynamic stretch shorten cycle actions involving a range of eccentric loads. Twenty trained male subjects performed a series of quasi-static muscular actions in a supine leg press position during which a brief perturbation was applied. The resulting damped oscillations enabled each subject's maximal musculotendinous stiffness (k), for the lower body musculature to be estimated. All subjects also performed a countermovement jump (CMJ), and a series of drop jumps (DJ) from heights of 20, 40,. 60. 80 and 100 cm. When the jump heights of the nine most compliant (mean k=11/4 ± 2.7 kNm -1), and nine stiffest (mean k = 20.5 ± 2.5 Nm -1) subjects were compared, the stiff subjects demonstrated significantly poorer capacity to perform under the highest (DJ80 and DJ 100) eccentric loading conditions. It was hypothesised that the relatively greater forces transmitted from the skeletal system to the musculature of the stiff subjects, reduced their ability to attenuate the higher eccentric loads due to less effective contractile dynamics and greater levels of reflex induced inhibition.
The third study was performed to determine the reliability and validity of a new isokinetic squat device, in comparison to knee extension tests performed using a Cybex II Isokinetic Dynamometer. Twenty nine athletic male subjects performed a series of isokinetic squat tests at 0.4 ms -1, knee extension tests at 1.05, 2.09 and 3.14 rad.s -1, as well as a 6 s stationary cycle test which was used as the measure of functional performance. The squat tests included a purely concentric squat without pre-load, a test with pre-load and a stretch shorten cycle test. Two trials of each test were performed on one testing occasion. Intraclass correlation coefficients (r = 0.89 to 0.96) and coefficient of variations (3.1 to 8.7%) were determined between trials and indicated that all tests were highly reliable. The velocity characteristics of the newly developed system demonstrated that it was an effective isokinetic device, with the mean velocity of 0.41 ms -1 varying within narrow limits, a relatively small velocity overshoot and an isokinetic portion of movement of approximately 80%. The squat tests demonstrated a higher relationship to cycling performance (r = 0.57 to 0.65) as compared to the knee extension tests (r = 0.45 to 0.51). This difference was amplified when a more homogenous group of subjects were examined. Further, the squat tests were superior to the knee extension tests in discriminating between differing levels of cycling performer. These differences were believed to be due to the greater specificity of the squat movement, in comparison to the knee extension, to the performance of interest.
Having validated the isokinetic squat in the previous study, this device was subsequently used to isolate any difference muscular contraction history may have on concentric work output. Forty trained male subjects performed 3 separate isokinetic concentric squats which involved differing contraction histories: (1) a concentric-only (CO) squat; (2) a concentric squat preceded by an isometric pre-load (IS); and (3) a concentric squat preceded by an eccentric stretch (SSC). Over the first 300 ms of the concentric movement, work output (means ± SD) for both the SSC and IS conditions was significantly greater (154.8±39.8 & 147.9±34.7 J, respectively; p<0.001) when compared to the CO squat (129.7±34.4 J). In addition, work output following the SSC test over the first 300 ms, was also significantly larger than for the corresponding period following the IS protocol (p< 0.05). There was no difference in normalised, integrated EMG between any of the conditions. It was concluded that concentric performance enhancement derived from a preceding stretch of the muscle-tendon complex was largely due to the attainment of a higher active muscle state prior to the start of the concentric movement. However, it was also hypothesised that contractile element potentiation was a significant contributor to stretch induced muscular performance under these conditions.
The final study attempted to examine the possible effects of variations in eccentric movement speeds on the subsequent concentric work output. In this context, the phenomenon of resonance was pursued. To investigate the effects of eccentric velocity on concentric muscular work, 20 resistance trained males performed a cyclic series of SSC contractions, using a short range (10 cm) bench press action. Three different isokinetic eccentric velocities (0.1, 0.3 & 0.5 ms -1) were used, whereas the concentric phase was constant and consisted of a purely isoinertial movement at 50 % of each subject's !RM. The natural frequency for a quasi static bench press movement wasdetermined using a free oscillation technique, and surface EMG for pectoralis major (pecs.) and triceps brachii were also collected during testing and subsequently normalised (NIEMG). There was no significant difference between eccentric work over the three speeds. Concentric work was significantly greater at 0.3 & 0.5 ms -1, but there was no difference between these two speeds. Transitional force for 0.1 & 0.3 ms -1 was significantly lower than 0.5 ms -1, and yet there was no statistical difference between the slowest speeds. Eccentric NIEMG was significantly lower for pecs. during the 0.1 ms -1 test. Concentric NIEMG for the pecs. & triceps was significantly higher during the 0.3 ms -1 test and the ratio of concentric/eccentric NIEMG for the pecs. was significantly lower for 0.5 ms -1 condition. It would appear that at the slowest speed, concentric work output was reduced primarily because of low transitional force due to relatively lower eccentric muscle activation. At the intermediate speed, an increase in concentric work was accompanied by a corresponding increase in concentric NIEMG. For the fastest speed, a more efficient pattern of muscle usage was observed as the increased work output occurred without any corresponding increase in concentric NIEMG. In addition, at the fastest speed the mass-spring properties of the system were more pronounced than at the slower speeds and the natural and movement frequencies were most similar Therefore, the author's hypothesised that in an attempt to maximise performance at high eccentric velocities, the neuromuscular system shifts its emphasis from contractile work to sources arising from the relatively higher transitional force, such as elastic recoil and interaction effects associated with CE/tendon dynamics. This is in line with the suggestion that at the fastest speed, the mechanical properties of the system approached a state of resonance.
Details
- Title
- The performance enhancement associated with dynamic stretch shorten cycle movements: an investigation into the development of testing devices, performance effects and underlying mechanisms
- Creators
- Andrew D. Walshe
- Contributors
- Greg J Wilson (Supervisor) - Southern Cross University
- Awarding Institution
- Southern Cross University; Doctor of Philosophy (PhD)
- Theses
- Doctor of Philosophy (PhD), Southern Cross University
- Publisher
- Southern Cross University, School of Exercise Science and Sport Management
- Number of pages
- xxxvii, 295
- Identifiers
- 991012931100402368
- Copyright
- © Andrew D. Walshe 1998
- Academic Unit
- Faculty of Health
- Resource Type
- Thesis