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EFFECTS OF DEVELOPMENTAL STAGE ON KNEE KINEMATICS DURING DROP JUMPS
PhD LAZARIDIS S., PhD GALAZOULAS CH., PhD PANAGIOTIDOU K., PhD ALEXIOU K., PhD TSADIMAS CH., PhD ZAGGELIDIS G.,
PhD HATZOPOULOS D.
ARISTOTELES UNIVERCITY OF THESSALONIKI GREECE
DEPARTMENT OF PHYSICAL EDUCATION AND SPORT SCIENCE
Key words: kinematics, jumps, efects
Introduction
Executing a drop jump involves jumping down from a height and, upon landing, performing a maximal vertical jump. The lower extremities muscles perform a stretch- shortening cycle (SSC), during which it has been found that the eccentric phase (stretching) influences at a high level the subsequent concentric phase (shortening) (Cavagna et al., 1965). During this task, use of elastic energy and reflex activation of the neuromuscular system have been claimed that contribute to the production of power in the propulsive phase of the vertical jump. (Bosco et al., 1982b)
Relevant studies in the past examined the neuromuscular strategy of lower limb during a stretch shortening cycle performance (Arampatzis et al., 2001; Dietz et al., 1989; Gollhofer et al., 1998; Komi, 2000; Viitasalo et al., 1998). However, this performance has not widely studied in prepubertal population (Moritani et al., 1991). Much of the current biomechanical research has only focused on the landing after drop and this task mainly at adult population (Huston et al., 2001;McNitt-Gray et al., 2001; Santello et al., 2001; Ayalon et al., 1987). To date, no known research has compared the biomechanical characteristics of pre- and post- pubescent males performing a functional task such as drop jump.
For this reason the purpose of this study was to identify the biomechanical differences in a stretch shortening cycle task between adults and prepubertal population.
The fundamental question posed in this paper was whether the more compliant muscle –tendon unit that adopt children regarding adults (Kubo et al., 2001, Lambertz et al., 2003), and the more immature central nervous system in controlling complex movements (Croce et al 2004, Hass et al., 2004, Swartz et al 2005) lead to a different strategy in executing a functional task such as drop jump.
Methods
Subjects
Twenty subjects with no history of back or lower extremity injuries were divided into age grouping. Subjects were required to be within set age ranges for either pre- or post- pubescent, according to guidelines established by Tanner (1986). Prepubertal and adults parameters are presented in table 1.
Table 1
Average values of prepubertal and pubertal participants
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All children were non- systematic active participants within a youth sports program. All subjects were not physically active and did not follow a structured training program. All participants were free from any neurologic condition that could influence lower-extremity mechanics. Before testing, each participant read and signed a written informed consent. Parental written consent was obtained for the prepubescent participants before data collection. The experiment was performed according to the Ethics of the Aristotle University of Thessaloniki.
Test procedure
Participants were asked to remove their shoes and socks. At this point, a series of anthropometric measures were made including body mass, height, knee and ankle joint width, length of the foot using an anthropometer (Model 01291, Lafayette). To eliminate the possibility of any variability, all the anthropometric measurements were made by the same investigator. These anthropometric data (Body mass) were used then to normalise the body segment parameter calculations.
Upon completing the anthropometric measurements, participants warmed up by walking and jogging for 5 min on treadmill. Subjects were then allowed unlimited time to warm up, which generally included of light stretching. After the warm up, the subjects performed drop jumps (from 20cm height on a platform DJ20). The platform was placed approximately 8cm behind the back edge of the force plate. For control purposes the subjects were instructed to keep their hands on their hips during the drop jumps. After warming up trials the subjects were asked to perform 5 maximal jumps following the principle 'jump as high as you can' (Arampatzis et al 2001). The technique used for the drop was the bounce drop jump, according (Bobbert et al., 1987). The rest interval between trials was 2 min.
Instrumentation
All drop jumps were performed on a 0.6x0.6 Bertec force plate (Type 4060, Bertec Corporation, Columbus, OH) collecting at 1000 Hz, which provided ground reaction force data and served as a landing area.
A VICON 612 motion analysis system (Oxford Metrics Ltd., Oxford, Oxfordshire, UK) with six M3 cameras, sampling at 200 Hz was used to record the 3D trajectories of sixteen 14mm diameter reflective markers overlying anatomical landmarks of each lower extremity (posterior superior iliac spine, anterior superior iliac spine, lateral thigh, femoral epicondyle, lateral tibia, ankle, 5th metatarsal head and heel. All trajectories were filtered using the generalized cross-validated splines technique as reported by Woltring et al., 1986. The filtered markers were used to locate embedded coordinate systems for the pelvis, thigh, shank, and foot. Euler angles determined relative segmental motion and joint angles were calculated based on segmental motion of the distal segment about a fixed point or axis on the proximal segment (Davis et al., 1991). The six cameras were focused on the centre of the force plate at 90˚ angles. Prior to the kinematic evaluation, each participant was recorded in the standing position, which was used as reference for joint movement. The synchronization of the kinematics and dynamic data was achieved by starting the two measurement systems at the same time.
The reflective markers were placed over the two lower extremities as soon as the subject executed the familiarization drop jumps.
Data reduction
Each drop jump was divided into two phases braking and propulsion phase. These phases were determined using the changes in angular position of the knee and the relevant contact time as proposed by Aura and Viitasalo (1989).Specifically braking phase was determined until the deepest knee joint flexion and the rest contact period as propulsive.
The sum both of the two as total.
The flight times of the jumps were used to calculate the height of rise of the body centre of gravity as has been suggested by Asmussen and Bonde- Petersen (1974).
The vertical take-off velocity (V) of the centre of gravity is calculated by
1. V=0.5 (Tair x g) where g=acceleration of gravity (9.81m.s-²) and Tair = flight time in seconds
The height of the rise of gravity can be then calculated as follows:
Height (m) = V²/2g
During the dropping and contact phases the following variables were calculated. a) Knee flexion angle at the instant o touch down b) knee flexion angle at the deepest bending position c) landing phase duration defined as the time from initial foot-ground contact to toe-off (braking/propulsive/total). For the kinematic and force analyses the best height performance was selected.
Statistics
Calculations were performed with a SPSS/PC 10.0 for Windows statistical package including mean, standard deviation of the mean, Student's t-test for independent and paired samples. A two-way ANOVA. 2X2 with repeated measures was used and an a priori alpha level of 0.05 was set for all statistical tests. The two independent variables were the two age groups (pre-and post-puberty) and the two landing heights (20 and 40cm.).
The dependent variables included all the above mentioned kinematic and kinetics data.
Results
The two way ANOVA with repeated measures revealed significant main effects for age level and landing height for the kinematics of the knee and the kinetics in these pre- and post pubescent subjects.
Kinetics
Vertical Jump Height
There was a significant age effect in maximum vertical jump height (F1,18= 335,030, p<0,001). In DJ20 adults jumped almost 18cm. higher (33+ 3,4 vs.15+ 2,1 p=<0.05).
Table 2
Average values of jumping height after landing
(* indicate p<0.05, for all cases reported)
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Contact times
Prepubescent participants exhibited longer braking (F 1,18=11,384, p< 0,01), propulsive (F1,18=28,433, p<0,001) and total contact times F1,18= 28,471, p<0,001) than adults.
Table 3
Mean values for prepubertal and adults of the landing phase duration
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Kinematics
Prepubescent participants landed (tab 4, Fig 1) with more extended knee joint at the instant of touch down than their adult counterparts. (F1,18=4,646, p<0,05) and in fact landed with 5˚ greater knee extension in DJ20. Moreover, prepubescent presented more flexed angles at knee joint at the knee deepest position at both DJ conditions (F1,18=10,765, p<0,01) and as a result greater knee range of motion during the landing phase (F1,18=19,982, p<0,001).
Table 4
Average values on knee flexion at the initial contact, deepest knee flexion
and the maximal flexion
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Figure 1 - 3D kinematic depiction of the lower extremities of a prepubescent (left side) and a post pubescent participant (right side) during the execution of DJ20, at the instant of touch down (top panel) and deepest knee position (low panel)
Discussion
The presence of significant biomechanical differences between children and adults sinse the jump higher, present longer contact time, flex the knee less during initial contact and in a higher extent at the deepest flexion.
Several researchers (Quatman et al., 2006; McNair et al., 2000; Prapavessis et al., 1999; Hewett et al., 1996) have reported that increased jumping skill cold be affected by learning factors since they reported that adults are able to absorb landing forces. These studies imply that increased skill and aging leads to increased knee flexion (initial contact.) which is in accordance with our data. Similar to the results reported by Sigg et al., (2001) and Ayalon et al., (1987) our results suggest that the ability to modulate contact strategy upon impact and throughout landing increases with the process of aging, potentially due to various levels of contribution from physical maturation, skill development and experience. Children's, longer contact times and the greater knee range of motion at both drop jump conditions could be attributed to the more compliant muscle-tendon system which children posses (Kubo, Kanehisa, Kawakami, & Fukanaga, 2001; Lambertz et al., 2003; Lebiedowska, & Fisk, 1999; Lin, Brown & Walsh, 1997).
The better score (jumping height) that post pubescent achieved at both heights compared to prepubertals could be attributed to the differences that these two groups present on the levels of muscle strength (Davies, White & Young, 1983; Mero et al, 1981; Bosco et, al 1982; Komi & Bosco 1978) and probably due to the better absorption and recoil ability and of elastic energy during the stretch shortening cycle as this is expressed with shorter contact times. (Bosco et, al 1982; Mero et al, 1981; Komi & Bosco 1978;Cavagna et al., 1968)
Conclusions
Our findings demonstrate the existence of developmental differences between children and adults in knee kinematics during drop jumps. And suggest that kinetic and kinematic landing patterns of a functional task such as drop jump change with physical development.
Abstract
The purpose of this study was to identify the biomechanical differences in a stretch shortening cycle task between adults and prepubertal. Twenty subjects with no history of back or lower extremity injuries were divided into two equal age groups. The results demonstrated the existence of developmental differences between children and adults in knee kinematics during drop jumps. Compared with adults, the children demonstrated a landing pattern with significantly greater knee extension at the moment of touch, higher final knee flexion, and longer contact periods (p<0.05). Our results suggest that landing patterns in a drop jump affected by physical development.
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