Fatigue is thought to be a major risk factor in the development of knee joint injuries (9,25,36,41) as a higher incidence of knee injuries occur toward the end of both halves (25) or in the later part of competitive team games (25,41). Altered lower limb landing strategies have been observed in healthy athletes as a consequence of being fatigued (9,16,36) as well as in athletes suffering from (7,8,24,44) or at risk of developing the overuse knee injury patellar tendinopathy (17,34). It is remains unknown, however, whether fatigue-induced alterations to an athlete’s lower limb landing technique contribute to increased patellar tendon loading. This is of considerable importance because repetitive loading of the patellar tendon is thought to be a major extrinsic risk factor in developing patellar tendinopathy in sports such as soccer, basketball, and volleyball, which involve repetitive landings (13,32,45) and incur a high prevalence of patellar tendinopathy (33).
Although there has been extensive research investigating the effects of muscle fatigue on the technique used by athletes to land, the results of these studies are disparate. Between-study differences in the results are mainly due to differences in experimental design, including differences in the experimental movement task (30,46) and/or the protocol used to induce participant fatigue (20,46). Interestingly, research investigating fatigue effects (3,16,36,38) and/or possible mechanisms of patellar tendinopathy (7,16) have predominantly been restricted to examining vertical landing tasks, such as drop landings, as the experimental task. Experimental movement tasks, however, should replicate gamelike situations (5,16,23,37), and drop landings, in which the landing phase is isolated from the whole jump-landing task, have limited application as they are rarely performed in gamelike situations (16). A more common movement task in gamelike situations within many team sports is a jump-landing task (9,10,47), such as the stop–jump task, which uses the stretch–shortening cycle (SSC) muscle action. Furthermore, compared with the vertical landing phase, the horizontal landing phase of a stop–jump task places the highest load on the patellar tendon, which may potentially lead to patellar tendinopathy (18). Therefore, research investigating factors affecting the lower limb landing technique of athletes, with implications for developing patellar tendinopathy, should preferably use a holistic jump-landing task that incorporates a horizontal landing phase to ensure the most appropriate experimental framework to better identify possible risk factors associated with development of patellar tendinopathy in repetitive landing sports (18).
As the effects of fatigue are task dependent (19), differences in the protocols used to induce fatigue can also contribute to disparate results relating to fatigue effects and landing technique and make between-study comparisons inappropriate. The validity of previous fatigue protocols has been limited (36) by factors such as the range of lower limb motion typically displayed during dynamic landing tasks not being replicated in the experimental design and the rest intervals tested being inconsistent with the stop–start nature of gamelike conditions. Furthermore, a fatigue protocol requiring repeated use of the SSC offers an ideal model to investigate lower limb fatigue (39) as the SSC muscle action is characteristic of running and dynamic jumping and landing movements (29,39).
This study aimed to investigate the effect of lower limb muscular fatigue on the landing technique and patellar tendon loads generated by asymptomatic athletes with normal patellar tendons during the horizontal and vertical landing phases of the stop–jump task. The experimental movement and the fatigue protocol used in the present study overcame the limitations in experimental design of previous research. That is, the experimental movement was a holistic jump-landing task that was common to sports with a high incidence of patellar tendinopathy (3,10). It involved both a horizontal and a vertical landing phase and used the SSC muscle action, and the stop–jump movement is a sporting skill commonly performed in basketball, soccer, and volleyball (10). The fatigue protocol also involved a repetitive submaximal SSC exercise with the landing phase incorporating a lower limb range of motion typically displayed during a stop–jump task and rest intervals consistent with the stop–start nature of gamelike conditions. It was hypothesized that muscular fatigue would alter the participants’ landing technique and, in turn, increase their patellar tendon loading during the horizontal landing phase but not the vertical landing phase of the stop–jump task. The results of this study will increase our understanding of how fatigue-induced alterations to an athlete’s landing technique contribute to patellar tendon loading, with implications for the development of overuse injuries such as patellar tendinopathy.
Sixteen skilled male basketball (n = 7) and soccer (n = 9) athletes (mean ± SD; age = 22.4 ± 2.9 yr, height = 182.1 ± 8.7 cm, mass = 75.7 ± 10.1 kg) volunteered as study participants. With the highest prevalence of patellar tendinopathy (33), volleyball players would be the most suitable group to study. Volleyball, however, has a low participation level in Australia compared with sports such as soccer and basketball (1). As both soccer and basketball also have a high prevalence of patellar tendinopathy (33), and these sports have high participation levels within Australia (1), players from these sports were deemed suitable participants to study. All the participants were right leg dominant, based on their preferred kicking leg (22), and reported no history of traumatic lower limb injuries. Each participant’s patellar tendon morphology was documented as “normal,” as noted by the absence of any ultrasound abnormalities (12) detected by an experienced musculoskeletal radiologist using a 13-MHz linear array ultrasound transducer (Siemens Antares, Siemens AG, Germany). Ankle joint range of motion (4), height, body mass, and lower limb dimensions were also evaluated for each participant as descriptive characteristics of the participant cohort and for later input into inverse dynamic modeling. Written informed consent was obtained from each participant before data collection, and all the study’s methods were approved by the institutions Human Research Ethics Committee (HE06/205).
No less than 1 wk before data collection, participants performed a familiarization session of the full experimental protocol, which included a 5- to 10-min warm-up, a familiarization of the stop–jump task and the fatigue protocol, and completion of a full trial of the fatigue protocol. The participants then returned on a second occasion for data to be collected during the full experimental protocol, which involved the warm-up, followed by standardized trials of the stop–jump task both before and after the fatigue protocol (Fig. 1).
The stop–jump task consisted of a horizontal landing phase, immediately followed by a vertical landing phase (15). The horizontal landing phase required the participants to accelerate forward for four steps toward two force platforms, to stop and perform a simultaneous two-foot landing with their right and left feet contacting separate force platforms. The vertical landing phase immediately followed and required the participants to jump vertically upward to strike a ball suspended from the ceiling with both hands and to then perform a simultaneous two-foot landing a second time, with each foot again contacting separate force platforms. During the stop–jump task familiarization, the effort at which the participants performed the task was standardized by using a set starting position away from the force platform. The participants’ average approach speed was measured immediately before the preparation phase of the first landing, using infrared timing lights (OnSpot, University of Wollongong). Jump height effort was standardized among the participants by positioning the ball at the maximum height in which each participant could touch the ball with both hands after performing the stop–jump task during task familiarization.
The protocol used to induce lower limb muscular fatigue in the present study required the participants to perform three maximal SSC exercises (described in the next paragraph), followed by repeatedly performing sets of 30 submaximal SSC exercises, interspersed with a 30-s rest, on a custom-built sledge apparatus. Fatigue was deemed when the participant could no longer achieve 70% of their maximal rebound height for three out of five submaximal SSC exercises or when participants self-terminated the sets because they felt that they could no longer continue. On the basis of blood lactate changes previously observed during a soccer (31) or basketball game (35), an increase in postfatigue blood lactate levels to at least 6 mmol·L−1, analyzed using an Accusport blood lactate analyzer (Boehringer, Mannheim, Germany), was also measured at the end of the experimental protocol to confirm fatigue.
The SSC exercises were performed on a custom-built 23-kg sledge apparatus seat, which glided along a track inclined 23.6° from the horizontal (Fig. 1). To ensure validity of the fatigue protocol, each participant was seated so their hip flexion angle (40°) replicated hip joint angles typically displayed during the landing phase of a stop–jump task (18). For each maximal SSC exercise, the participants performed two submaximal SSC efforts, immediately followed by a maximal SSC effort. Each SSC effort required the participants to begin with both their feet contacting a metal base plate that was secured to the base frame of the sledge apparatus. From this starting position, the participants pushed off the metal base plate and accelerated upward along the gliding track of the sledge. For the maximal and submaximal SSC efforts, maximal rebound height or submaximal rebound height of at least 70% of their maximal rebound height was required (29,30), respectively, before the participant glided down the track to stop in a two-foot landing, with each foot again contacting the metal base plate. Maximum knee flexion (75°) during the landing phase of the SSC efforts replicated angles typically displayed during the landing phase of a stop–jump task (15,18). To ensure participants met the 75° maximum knee flexion criterion and reached 70% of their maximal SSC exercise rebound height during the entire SSC efforts (fatigue protocol), they were given real-time verbal feedback from the investigator based on two marks, which were set on the frame of the sledge apparatus and indicated that the participant had achieved each criterion.
During each stop–jump trial, the ground reaction forces generated during each landing were recorded (1000 Hz) using two multichannel force platforms (Type 9281B and Type 9253B; Kistler, Winterthur, Switzerland) embedded in the floor, with each platform connected to a multichannel charge amplifier (Type 9865A and Type, 9865B; Kistler). The participant’s three-dimensional lower limb motion was recorded (100 Hz) using an Optotrak® 3020 motion analysis system (Northern Digital, Waterloo, Canada). Infrared light-emitting diodes were placed on each participant’s dominant lower limb and pelvis, on the shoe at the first and fifth metatarsal head and mid-anterior foot, lateral malleolus, medial malleolus, lateral leg, anterior distal leg, anterior proximal leg, lateral femoral epicondyle, medial femoral epicondyle, lateral femur, anterior distal femur, anterior proximal femur, greater trochanter, anterior superior iliac spine, and iliac crest. To avoid losing view of the infrared light-emitting diodes, the participants wore minimal clothing (a T-shirt and shorts), and their own socks and athletic shoes were worn during testing.
Electromyographic activity was recorded bilaterally for vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM), biceps femoris (BF), semitendinosus (ST), tibialis anterior (TA), and medial gastrocnemius (MG) using two TeleMyo systems (Noraxon, Scottsdale, AZ). Following standard preparation (2), bipolar silver–silver-chloride surface electrodes (Ambu® Blue Sensor M, electrode size = 40.8 × 32 mm, detection area = 13.2 mm2; Noblesville, IN) were placed longitudinally on each muscle belly (interelectrode distance of 20 mm). A common reference electrode was located on the tibial tuberosity of each lower limb. The electromyographic signals for each lower limb were sampled (1000 Hz; bandwidth, 16–500 Hz) and relayed from two TeleMyo 900 battery powered transmitters (Noraxon), firmly fixed around the participant’s waist, to two TeleMyo 900 receivers. Kinetic, kinematic, and electromyographic data were time synchronized and collected using First Principles software (Version 1.00.2; Northern Digital).
Analysis of the kinematic and kinetic data was performed using Visual 3D software (Version 3; C-Motion, Germantown, MD). The raw ground reaction force data were initially filtered using a fourth-order zero-phase-shift Butterworth digital low-pass filter (fc = 50 Hz) before calculating the ground reaction force variables. The raw kinematic coordinates, ground reaction forces, free moments, and center of pressure data were then filtered using a fourth-order zero-phase-shift Butterworth digital low-pass filter (fc = 18 Hz) before calculating individual joint kinematics, net internal knee extension–flexion joint moments, and patellar tendon forces (6). The patellar tendon forces were calculated by dividing the net internal knee extension–flexion joint moment by the patellar tendon moment arm (40). Using a polynomial regression equation, the patellar tendon moment arm was estimated as a function of knee joint angle using the method of Herzog and Read (26).
The raw electromyographic signals were filtered using a fourth-order zero-phase shift Butterworth (high-pass fc = 15 Hz) to eliminate any movement artifact. To quantify temporal characteristics of the muscle bursts, the filtered electromyographic data were full-wave rectified, filtered with a 20-Hz fourth-order zero-phase shift Butterworth low-pass filter, and then full-wave rectified to create linear envelopes using customized software (LabVIEW 8; National Instruments, Austin, TX). The time of the onset of each muscle burst activity was determined using a threshold crossing of 8% of the maximum amplitude of the linear envelope for each muscle (15). Each individual muscle’s filtered signal was visually inspected to confirm the validity of the calculated results for the temporal characteristics of the muscle bursts to minimize the probability of error.
The two landing phases within the stop–jump task were firstly identified from the vertical ground reaction force trace: (i) the “horizontal” landing phase and (ii) the “vertical” landing phase (15). The primary outcome variable analyzed during these two landing phases was the peak patellar tendon force (FPT). The FPT calculated for each participant’s right (dominant) lower limb was selected for analysis as asymmetry between lower limbs in FPT has been observed during the horizontal landing phase of a stop–jump task (15). Secondary variables analyzed during the two landing phases included the peak vertical ground reaction force (FV) and peak anterior–posterior ground reaction force (FAP); ankle, knee, and hip joint kinematics; the net peak knee joint moment; and the time of the onset and peak muscle activity of each of the seven lower limb muscles relative to the time of the FPT in each landing phase. The loading rate of the FV (LR FV, body weight per second) was calculated by dividing the FV by the time interval between initial foot–ground contact (IC) and the time of the FV. The loading rate of the FPT (LR FPT, body weight per second) was calculated by dividing the FPT by the time interval between IC and the time of the FPT. The temporal events (IC, and at the time of the peak FV and FAP) were defined using the 18-Hz filtered kinetic data, with initial contact defined when the ground reaction force exceeded 30 N. The jump height (cm) attained by each participant during the stop–jump task was also calculated as the difference in the maximum vertical displacement of the greater trochanter marker minus the vertical displacement of the same marker measured while each participant stood motionless.
Mean and SD values were calculated for each kinetic, kinematic, and muscle activity variable during the horizontal and vertical landing phases in a nonfatigued (NF) and fatigued (F) condition. The data were then analyzed using a series of paired-samples t-tests to determine whether there were any substantial differences in the primary and secondary outcome variables between the two fatigued conditions. Magnitude-based inferences on the differences in the mean changes between the two fatigue conditions (NF and F) and precision of estimation were then used to avoid the shortcomings of research based on null-hypothesis significance testing (28). To show the magnitude of the effects, effect sizes were calculated and magnitudes were assessed using the following criteria: 0.19 = trivial, 0.20–0.49 = small, 0.50–0.79 = moderate, and >0.80 = large (11). Clear moderate or large effect sizes were defined as substantial. The 95% confidence limits, which define the range representing the uncertainty in the true value of the (unknown) population mean, were used to indicate the precision of estimates. Although the chance of inflation of a type I error increases when multiple statistical tests are conducted, no adjustment to the alpha level was deemed necessary (28), as such adjustments may increase the likelihood of type II errors (42). All statistical procedures were conducted using the Statistical Package for the Social Sciences (Version 15; SPSS Inc., Chicago, IL) and a customized Excel spreadsheet (27).
During the fatigue protocol, participants performed on average 14.5 ± 9.6 submaximal SSC sets and displayed a significant increase in postfatigue blood lactate level above the 6-mmol·L−1 fatigue criterion (NF = 3.5 ± 0.7 mmol·L−1, F = 8.0 ± 1.9 mmol·L−1, d = 6.76, P < 0.001, 95% confidence interval [CI] = 2.7).
Stop–Jump Task Variables
Despite attempts to standardize both the approach speed and vertical jump height achieved during the stop–jump task, fatigue led to a significant decrease in the participants’ mean approach speed (NF = 4.5 ± 0.4 m·s−1, F = 4.1 ± 0.4 m·s−1, d = 0.93, P < 0.001, 95% CI = 0.2). Despite being significant, the decrease in the mean vertical jump height attained (NF = 48 ± 6 cm, F = 45 ± 6 cm, d = 0.47, P = 0.02, 95% CI = 10) during the stop–jump task was deemed only trivial due to the small effect size and high 95% CI.
Patellar tendon loading
During the horizontal landing phase of the stop–jump task, participants generated a significantly lower FPT, LR FPT, and peak knee joint moment when fatigued compared with when nonfatigued (Table 1). In contrast, during the vertical landing phase, there were no significant differences between fatigue conditions in patellar tendon loading.
Ground reaction forces
When fatigued during the horizontal landing phase, the participants displayed a significantly higher FV and LR FV compared with when not fatigued. In addition, although participants used significantly shorter time from IC-FV when fatigued compared with when nonfatigued, the magnitude of the absolute difference was functionally irrelevant due to the small effect size and high 95% CI. During the vertical landing phase of the stop–jump task, the only significant difference was a significant increase in FAP during the fatigued condition compared with the nonfatigued condition (Table 1).
Horizontal landing phase joint kinematic data.
During the horizontal landing phase of the stop–jump task, the participants maintained similar ankle joint positions from IC to the time of the FPT during both the fatigued and the nonfatigued conditions (Table 2). In fact, the only significant difference in ankle kinematics that the participants displayed when fatigued was a slower ankle inversion velocity at the time of the FV compared with when nonfatigued. However, participants landed with significantly less knee flexion throughout the horizontal landing phase (IC, FV, and FPT) when fatigued compared with when nonfatigued, although at IC the magnitude of the absolute difference was deemed trivial due to the small effect size. In contrast, the range of knee joint motion from the time of IC to the time of FPT (NF = 50° ± 12°, F = 50° ± 10°, d = 0.03, P = 0.805, 95% CI = 6.4) was similar between fatigue conditions. When fatigued, the participants displayed significantly less hip flexion throughout the horizontal landing phase (IC, FV, and FPT), slower hip flexion velocity at IC, and faster hip adduction velocity at IC when they were fatigued.
Vertical landing phase joint kinematic data
During the vertical landing phase of the stop–jump task, the participants displayed a significantly less plantar flexed position of the ankle and a higher plantarflexion velocity at IC when fatigued compared with when nonfatigued (Table 3). Furthermore, the participants underwent a significantly smaller ankle plantarflexion-dorsiflexion range of motion from the time of IC to the time of the FPT (NF = 55° ± 7°, F = 49° ± 8°, d = 0.80, large, P = 0.005, 95% CI = 6) when fatigued. A significantly slower knee flexion velocity at IC, a slower knee adduction velocity at IC, and a faster knee abduction velocity at both the times of the FV were also all evident during the fatigued condition compared with the nonfatigued condition. However, the only between-fatigue condition difference in hip kinematics was hip abduction at IC, but the magnitude of the absolute difference was considered functionally irrelevant due to the small effect size.
Muscle activation patterns
During the horizontal landing phase, the participants used a relatively similar muscle recruitment strategy to stabilize their lower limbs during both the nonfatigued and fatigued conditions (Table 4). Nevertheless, when landing horizontally, participants displayed a later peak muscle burst activity for MG, BF, and VL (moderate) relative to the time of the FPT during the fatigued condition compared with the nonfatigued condition. In contrast, during the vertical landing phase, the participants used a relatively different muscle activation pattern strategy to stabilize their lower limbs when fatigued compared with when nonfatigued (Table 5). That is, when fatigued, participants displayed a later onset of TA, BF, and ST and a later peak muscle burst activity for TA, MG, and BF relative to the time of the FPT activity compared with when nonfatigued.
The primary extrinsic risk factor associated with patellar tendinopathy is repetitive landing (13,32,45). We originally hypothesized that fatigue would exacerbate this risk by increasing the magnitude of the repetitive loads sustained by the patellar tendon during the horizontal landing phase but not the vertical landing phase of the stop–jump task. In partial support of our hypothesis, fatigue did not alter patellar tendon loading during the vertical landing phase. In contrast to our hypothesis, however, when landing horizontally, both the FPT and the LR FPT significantly decreased during the fatigued condition, despite significant increases in both the FV and the LR FV generated during the same condition. Suggested reasons for this unexpected finding are discussed below. This result, however, confirms previous research (18) that FV is not a reliable indicator of FPT. That is, if FV had been used as a surrogate of FPT in the present study, it would have been errantly concluded that fatigue increased patellar tendon loading during the horizontal landing phase, when fatigue in fact reduced patellar tendon loading. It should be noted that this unexpected finding of a decrease in patellar tendon loading in athletes with normal patellar tendons may not be observed in asymptomatic athletes who are at risk of developing patellar tendinopathy (17,34) because they might be less able to adapt to changes evoked by fatigue, leading to either an unchanged or increased patellar tendon loading.
Most of the significant between-fatigue condition differences were evident during the horizontal landing phase rather than the vertical landing phase of the stop–jump task. This finding highlights the limitation of previous studies that have only used landings with a predominantly vertical motion as the experimental movement to examine lower limb landing strategies in relation to patellar tendinopathy (7,8,43,44). The lower magnitude of patellar tendon force during the vertical landing phase (18) may explain why most of the fatigue effects were observed during the horizontal landing phase. It also helps to explain some of the discrepancies in results previously found in research investigating how fatigue affects landing technique.
Consistent with previous research of a stop–jump task, participants in the present study displayed significantly less knee flexion when they were fatigued compared with when they were nonfatigued during the horizontal landing (9) and during an SSC exercise (29). This reduction in knee flexion during the horizontal landing phase of a stop–jump task when fatigued may be a compensatory effect of fatigue, in which the participants are unable to generate an adequate net knee extension moment upon landing to prevent the knee from collapsing. By landing with the knee more extended, there was less reliance on generating a high knee extensor moment when fatigued to stabilize the knee upon landing. As the direction of the patellar tendon force is a function of knee joint angle (26), landing with less knee flexion results in less compressive load on the patellar tendon.
Irrespective of this reduced knee flexion, using the SSC during a movement task increases the FPT because of the countermovement, which passively and then actively lengthens the muscles to take advantage of the muscle’s length–tension properties to enhance velocity and power in the subsequent takeoff phase (21). Our previous research has highlighted that in a stop–jump task, the need to prepare to perform a vertical jump immediately after the horizontal landing phase may cause the participants to use greater knee flexion than would typically be used in an isolated horizontal landing movement, in which the knee joint is relatively extended at IC (18). Furthermore, fatigue of the SSC causes participants to land with less knee flexion at IC, decreases the knee extensor positive peak power, and decreases the performance of an SSC exercise (29). We speculate that in the present study, landing with less knee flexion during the horizontal landing phase when fatigued reduced the participant’s efficiency in using the SSC muscle action during the subsequent takeoff phase, reflected in the decrease in net knee joint extension moment, but would have protected the knee by significantly reducing the magnitude of the patellar tendon load.
The FPT is derived by dividing the net knee extension–flexion joint moment by the patellar tendon moment arm and thus can be influenced by changes in knee flexion used in an activity. In this present study, variations in the patellar tendon moment arm could partially explain the between-fatigue condition differences in the FPT during the horizontal landing phase. That is, as the peak patellar tendon moment arm occurs at 29° of knee flexion (26), by the participants landing when fatigued with less knee flexion and at an angle that used a larger patellar tendon moment arm at the times of the FV and FPT, this could have, in turn, contributed to the decreased FPT and LR FPT compared with when nonfatigued. However, the significantly lower knee joint moment generated when fatigued was the primary contributor to the significantly lower FPT found during the horizontal landing phase compared with when nonfatigued. This decrease in knee joint moment could not be attributed to the higher FV and LR FV when fatigued compared with when nonfatigued, as these both increased and would consequently increase the knee joint moment. We speculate that the lower knee joint moment was attributed to the change in the orientation and position of the body segments, which in turn altered the direction of the vector of the resultant ground reaction force. This change was characterized by landing with less knee and hip flexion during the horizontal landing phase, which would position the participant’s center of mass closer relative to their base of support, necessitating smaller forward translation of the center of mass during the landing phase compared with when nonfatigued. By using this landing strategy when fatigued, the participants were less efficient in using the SSC and unable to generate an adequate net knee extension moment upon landing. Our previous research has shown that landing with less knee flexion and less hip flexion contributes to a significantly lower net knee extension joint moment (18). The patellar tendon forces were calculated using the net knee extension–flexion joint moment, which does not take into account muscular cocontraction. Therefore, the lower net knee extension joint moment may have resulted from the later peak VL muscle burst activity, which occurred after the time of the FPT. This, in turn, may have resulted in a lower knee extension moment and/or the later peak MG and BF muscle burst activity, which occurred after the time of the FPT, and may have resulted in a lower flexion extension moment. In addition, fatigue also led to the participants performing the stop–jump task with a slower approach speed, which may have also contributed to the lower net knee joint extension moment.
Consistent with previous landing studies, the use of a muscle activation strategy in preparation to land was evident during both landing phases (14–16), and differences existed in these muscle recruitment patterns between the two landing phases (15,18). During the horizontal landing phase of the stop–jump task, although the participants maintained a similar lower limb muscle recruitment order and time of onset, substantial between-fatigue condition differences were observed, including a later peak BF and VL activity relative to the time of the FPT. We speculate that although this later muscle recruitment when fatigued had a positive effect on reducing patellar tendon loading, it could also have a negative effect on dissipating the impact load and knee joint stability due to these muscles acting too late. The significantly later peak MG activity relative to the time of the peak FPT displayed when fatigued during the horizontal landing phase cannot be attributed to any ankle joint kinematics due to no significant between-fatigue condition differences. Nevertheless, the later BF and MG activity, which acts as eccentrically to control the rate of knee extension before IC, relative to the time of the peak FPT when fatigued compared with when nonfatigued, may have contributed to the decreased knee flexion at IC. The muscle activation strategy that may be attributed to the decrease in hip flexion displayed during through the landing phase remains unknown. That is, as BF is a two-joint muscle that can also act eccentrically to control the rate of hip flexion during landing, the delayed BF muscle activity relative to the time of the peak FPT could potentially act to increase hip flexion. However, when fatigued participants displayed decreased hip flexion compared with when nonfatigued. It remains unknown whether this decreased hip flexion displayed throughout landing when fatigued compared with nonfatigued condition was due to an earlier gluteus maximum or later tensor fasciae latae muscle activation strategy, which functions to control hip joint motion, as the activity of these muscles was not measured.
During the vertical landing phase of the stop–jump task, despite fatigue causing only minor changes to the kinetics and kinematics displayed by the participants, fatigue caused substantial alterations to the participants’ lower limb muscle activation strategies. Interestingly, muscle activity onset relative to the time of FPT was delayed for TA and the hamstring muscle group when fatigued but remained unchanged for the antigravity muscles, MG, and the quadriceps muscle group. This suggests that coactivation of the hamstrings/quadriceps and MG-TA muscles was delayed when fatigued, potentially leading to a moderate decrease in FPT and active stabilization of the knee and ankle, respectively, during the vertical landing phase of a stop–jump task. Furthermore, as the hamstring muscles act to assist in stabilizing the knee, to decelerate anterior tibial translation, and to internally/externally rotate the hip, the later onset of BF and ST, and later peak BF activity relative to the time of FPT when fatigued during the vertical landing phase may have contributed to the increased FAP compared with when nonfatigued. Nevertheless, it remains unknown whether the faster knee abduction velocity throughout landing when fatigued compared with nonfatigued condition was due to a delayed hip adductor muscle activation strategy, which functions to control lower limb alignment, as this was not measured. We also speculate that the later MG peak muscle activity relative to the time of FPT contributed to the less plantarflexion and a higher plantarflexion velocity at IC due to delayed MG eccentric control of ankle dorsiflexion during landing. It should be noted, however, that the differences in muscle recruitment order displayed during the two landing phases are likely to have been moderated by the preparation for the takeoff phase in the horizontal landing phase, whereas there was no follow-on movement after the vertical landing phase.
Fatigue was found to significantly decrease the FPT and the LR FPT during the horizontal landing phase but not vertical landing phase of a stop–jump task in asymptomatic athletes with normal patellar tendons. This reduction in patellar tendon loading when landing horizontally while fatigued was primarily attributed to the way the participants moderated their landing technique, including reducing their net knee joint extension moment as a consequence of using less knee flexion and at an angle that used a larger patellar tendon moment arm, less hip flexion, and less efficient use of the SSC during the horizontal landing phase. This decrease in patellar tendon loading when fatigued, caused by altered landing technique during the horizontal landing phase, may be an inherent protective strategy to potentially decrease patellar tendon loading during repetitive landing.
The authors acknowledge the New South Wales Sporting Injuries Committee (Australia) for financial support of this research.
Professor Cook was supported by the Australian Centre for Research into Sports Injury and its Prevention, which is one of the International Research Centres for Prevention of Injury and Protection of Athlete Health supported by the International Olympic Committee (IOC).
There is no conflict of interest for any of the authors.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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