The maximum vertical ground reaction force was 8.32 BW for landing from 24.46 cm jump height (fig1.c.). This result was much higher than those found by Abian et al., (2008), who obtained 7.51 ± 2.38 BW for landings from 35.5 ± 4.5 cm height. This difference in GRF value might be because they analysed jumps made by applicants to a faculty of sports sciences during the tests for a long time, so were familiarized with the jump technique, including landing phase. However, this result was more in line with the result of Seegmiller et al., (2003), who analysed jump height made by recreational athletes (8.70 ± 2.21 BW at 30 cm jump height). The landing made first with the balls of the feet and then with the heels, produce lower ground reaction force to the joints compared with the landings with the feet flat (Ortega et al., 2010). Significant decrease in ankle angle during landing phase also support the idea that the participant in this study landed more on feet flat than with the balls of the foot. Many recent studies have also suggested that impact forces can be reduced by increasing the landing times by flexing at the hip, knee and ankles with the proper cordination (Bressel and Cronin, 2005; Reiser et al., 2006;). The hip angle during landing is not significantly reduced, which probably has resulted in increased GRF in this study. So, further studies should focus on the effective landing techniques while studying the effects of ground reaction forces in jumping activities.
The moment exerted about a muscle joint is the product of the force exerted by the muscle and its moment arm (Bobbert, Huijing, & Schenau, 1987). These in turn depend upon the joint angles or angular velocities. According to the force- velocity relationship, a larger force can be exerted at the lower shortening velocity when joint angles are reduced before the concentric shortening of the muscles and this yields a higher power output (Astrand, Rodahl, Dahl & Stromme, 2003). The higher minimal degree of knee and ankle joints compared to hip angle, and higher moment and power output generated by knee and ankle than hip joint in our study might be the possible effect of force-velocity relationship. So, there is much dominance of power output by knee and ankle joint than by hip joint in our study (fig 4. a, b, & c). This result has supported the idea of importance of knee and ankle joint power to lift the body upwards during vertical jump. According to the study done by Bobbert et al., (1986), the vertical jump performance following drop jump was better than following counter movement as power output generated by ankle and knee was greater during drop jump whereas hip joint generated more power in countermovement jump in their study, which failed to produce better vertical jump height. However recent studies (Vanezis et al., 2005; Arteaga et al., 2000) have suggested that the possession of the appropriate muscle charateristics and stretch loading conditions are more important than the joint angles or angular velocities and moreover jumping techniques. Although this study could not study the characteristics of muscle involved but it could support the idea of stretch loading effect on vertical jump performance. Since the participant uses arm swing during the performance in this study, the higher result of knee moment and power than other joints is in the agreement of the results of Vanezis et al., (2005), which shows higher knee power output and better performance following the countermovement jump with arm swing compared to the jump without armswing. According to Lees et al., (2004), the effect of arm swing results in a greater loading of the muscles during downswing which results in a greater moment during the early part of the ascent but a reduced output as energy is being stored in the muscles and tendons of the lower limb, especially in the quadriceps (large muscle group) and gastrocnemous (maximal weight bearing muscle group). This stored energy is rapidly released later during the late phase of the take-off, leading to enhanced power output and hence the better performance.
Another area of interest in this study was the result that the joint moment and power output is maximum during the early phases of the upward push-off of the body before the final prepartion for vertically lifting the body off the ground in case of all the joints (fig 3. A, b & c). This result is in agreement with the result presented by Linthorne, (2001), from his comparative analytical study between standing countermovement jump and squat jump on a force platform that the muscle performs maximum amount of work early in the upward phase of the countermovement jump as the levels of activation and force in the jumper’s leg muscles are high because the jumper has to slow and then reverse the initial downward motion. Linthorne, (2001), also reported that the more vigorous the preliminary downward phase, the higher the jump. However, our study could not compare the contribution of different speeds in downward phase in the vertical jump height performance. So, further studies should focus to develop the effective techniques and speed, especially during the countermovement phase for the efficient and better jump performance. But, it is still a topic of great dispute to report the most reliable and valid mechanisms responsible for the enhancement of force in a countermovement jump. Many studies have argued pre-stretching of the muscles is responsible for the muscles to develop a higher level of active state and force before starting to shorten whereas some authors have reported the evidence that the extra work is due to the release of elastic energy that has been stored in the muscles and tendons during the prestretch and others explain the enhancement through potentiation of the contractile proteins in the muscle (Linthorne, 2001).
Another fact that the knee moment has significantly increased at the early stages of push off phase whereas ankle moment is markedly increased just prior to the end of push off phase (Fig 3. b, c.), this result is in agreement with the study done by Babic and Lenarcic, (2007) that the activation of the biarticular gastrocnemius muscle prior to the end of the push-off enables the transportation of the power generated by the knee extensors from the knee to the ankle joint. This transformation of energy enhances a rapid extension of the foot which has a greater effect on the vertical velocity than the extension of almost straightened knee and thus enhance the better jump performance. The gastrocnemius muscle activation is also important in vertical jumping as it is connected to the foot by an elastic tendon because of which it has the ability to store elastic energy when stretched and to recoil this energy afterwards as a mechanical work and hence improving the performance (Asmussen and Bonde-Petersen, 1974 as cited in Babic et al., 2007).
In conclusion, the purpose of this study was to understand the effects of various joint and muscles parameteres such as joint angles, moments and forces on the vertical jump performance. The findings suggest that the capability to produce a greater muscle force and the techniques to enhance this force generation (especially countermovement and push off techniques) are the major factors in jump performance. However, this study could not include the alectromyographic study to rule out the particular role of different dominant muscle in the performance and also could not compare different jump techniques. So, further studies should focus on these aspects to better understand the major factors effecting in the countermovement jump performance. Also from this study we could understand the effect of landing techniques in the impact force that the lower limb joints has to withstand on landing. However, we could not experiment the different percentage of impact forces that the different joints has to withstand on landing. So further studies should focus on this aspect and landing techniques that will greatly support coaches and athletes to train for better and safer performance in the sporting activities.