The lower extremity, particularly at the ankle and knee joints, is vulnerable to injuries during movements involving repetitive landings. One major reason is that during those landing activities, e.g., landings after a basketball layup, a volleyball block jump or a gymnastics somersault, the lower extremity is exposed to vertical ground reaction forces (GRFs) amounting to 3.5-11 times body weight (BW) (Puddle and Maulder, 2013). The lower extremity plays a crucial role in attenuating these impacts (Kim et al., 2017). Consequently, related overuse damages, e.g., stress fracture (James et al., 2006), patellar tendinopathy (Rosen et al., 2015) and internal derangement of the knee joint (Granata et al., 2002; Tsai et al., 2016), often result from the accumulation of these repeated high impacts (Macdermid et al., 2017).
Adjustments in the landing patterns of the lower limbs, e.g., changes in leg geometry and joint torque or stiffness, can be beneficial for mediating the magnitude of the impact forces, joint loading and energy dissipation (Rowley and Richards, 2015; Yeow et al., 2011). For instance, DeVita et al. (1992) reported a redistribution of joint energy absorption in the lower extremity during a soft landing with a greater peak knee flexion compared with a stiff landing. However, these altered landing strategies are negatively affected by neuromuscular fatigue associated with prolonged exercise and, such fatigue may place athletes at a higher risk of landing-related injury (Murdock and Hubley-Kozey, 2012; Tamura et al., 2016). Brazen et al. (2010) observed a greater peak GRF after fatigue, whereas Cortes et al. (2014) reported a more erect landing posture after fatigue; both were considered as risk factors for anterior cruciate ligament injury. Moreover, studies on the effects of fatigue during landing activities have demonstrated different responses in both GRF characteristics and lower extremity control strategies (James et al., 2010; Nikooyan and Zadpoor, 2012). While Coventry et al. (2006) illustrated an increase in knee flexion after fatigue, a study reported no significant differences in knee flexion in women in post fatigue conditions (Kernozek et al., 2008), or even decreased knee flexion, as mentioned previously (Cortes et al., 2014). Collectively, numerous studies have shown that neuromuscular fatigue can affect the landing strategy of the lower extremity, mostly in a detrimental manner (Madigan and Pidcoe, 2003; Murdock and Hubley-Kozey, 2012). Multifactorial causes underlie these different responses, and further studies beyond the analysis of the kinematic level are warranted to provide insight on the energy absorption / dissipation strategies and the underlying neuromuscular actions occurring during fatigue.
Developing reliable fatigue protocols is a key aspect in understanding the effects of fatigue on landing biomechanics. The components for designing a fatigue protocol in a laboratory setting include consistent fatigue levels, valid fatigue models and standardized landing styles (Barberwestin and Noyes, 2017; Ferraz et al., 2017). Studies have followed both short- and long-term fatigue protocols. The short-term protocols included consecutive vertical jumps (Chappell et al., 2005), short-distance sprints and shuttle runs (Sanna and O'Connor, 2008) and approximately 50% 1 repetition maximum pedal exercise of the lower limbs (Gehring et al., 2009). The long-term protocols mainly induced fatigue through long-term treadmill running (Dierks et al., 2010; Koblbauer et al., 2014). Moreover, a recent study reported that fatigue protocols involved combinations of forward sprints, lateral shuffles, pivoting and backward running (Webster et al., 2016). Various fatigue protocols have been established to more accurately mimic athletic activities occurring in real sports scenarios. Nevertheless, evidence is still limited regarding whether biomechanical changes in the lower extremity are related to the type, site, or severity of fatigue (Barberwestin and Noyes, 2017). Furthermore, inconsistent results in the shock attenuation of the lower extremity were found in various exercise-induced fatigue protocols (Coventry et al., 2006). As previously summarized, no scientific consensus has been reached regarding the effects of fatigue on specific biomechanical features, such as kinematics, kinetics, stiffness and energy dissipation. The lack of consensus is largely because of insufficient comparisons between different fatigue protocols.
Therefore, this study quantified the effects of two typical exercise-induced fatigue protocols (constant speed running [R-FP] and repeated shuttle sprint plus vertical jump [SJ-FP]) on joint mechanics, stiffness and energy absorption in the sagittal plane of the lower extremity during double-leg drop landings. We hypothesized that both fatigue protocols would alter GRFs, joint mechanics, stiffness and energy absorption. Specifically, participants would have more GRFs and joint range of motion, and less vertical stiffness and joint stiffness, as well as more energy absorption after fatigue. Moreover, aforementioned biomechanical variables would be more pronounced in SJ-FP than R-FP.
Considering the intensity of a series of fatigue tests, fifteen collegiate male medium distance runners (age: 20.9 [+ or -] 0.8 years; height: 1.76 [+ or -] 0.04 m; weight: 68.9 [+ or -] 5.5 kg), with an average of 4.2 [+ or -] 1.1 years of experience in track and field events were recruited to participate in the study. All athletes had no history of musculoskeletal injuries to the lower extremity in the previous 6 months and did not engage in strenuous exercise for 24 hours before the study. A posthoc power analysis was performed to indicate the statistical power. It revealed that a sample size of 15 was sufficient to minimize the probability of Type II error for our variables of interest (Faul et al., 2007). Each participant signed an informed consent form before the experiments. The study was approved by the Institutional Review Board of Shanghai University of Sports.
Kinematics were collected using a 16-camera infrared three-dimensional (3D) motion capture system (Vicon T40, Oxford Metrics, UK) at a sampling rate of 240 Hz. Thirty-six infrared retroreflective markers, each with a diameter of 14.0 mm, were attached bilaterally to both lower extremities to define hip, knee and ankle joints according to the plug-in gait marker set (Figure 1). GRFs were measured with two 90 x 60 x 10-cm 3D force plates (9287C, Kistler Corporation, Switzerland) at a sampling rate of 1200 Hz. The 3D kinematic and force plate data were synchronized using the Vicon system. The maximum vertical jump height of each participant was acquired with a Quattro Jump force plate (9290BD, Kistler Corporation, Switzerland), which was also used to monitor the vertical jump height during the procedure of inducing fatigue. A heart rate (HR) transmitter belt monitor (SS020674000, Suunto Oy, Finland) was attached to each participant's chest to continuously monitor their HR during the entire fatigue procedure. The intervention time was recorded by a stopwatch (ZSD-013, sienoc, USA).
The participants visited the laboratory on two separate days and completed bipedal drop landing (DL) tasks for one of the two exercise-induced fatigue protocols at each visit. A 1-week break period was required between visits to ensure that fatigue was eliminated, and the two protocols would not affect each other (Yeow et al., 2009). The order of the two protocols was randomized using a random number allocation table (Zhang et al., 2000).
At each visit, the participants were asked to complete the DL tasks from a height of 60 cm (Zhang et al., 2000). A successful trial required the participants to step off with either leg from a landing platform without jumping up or losing their balance and to land as naturally as possible with a toe-heel landing. Furthermore, the participants were instructed to perform the landing tasks with their arms on their hips to reduce the influence of swinging during landing. Before given practice trials to become familiar with the DLs, participants wore a spandex outfit with non-cushioning shoes (WD-2A; Warrior, Shanghai, China). After a regular warm-up routine and practice trials, five successful trials were acquired for analysis. The DL tests were performed before and after conducting the exercise-induced fatigue protocols.
Exercise-induced fatigue protocol with constant speed running (R-FP)
The participants were required to run on a treadmill at 4 m/s until they could not continue running (Garcia-Perez et al., 2013). They were considered to have achieved fatigue, and intervention was terminated when the following two criteria were met: 1) The HR of the participants reached 90% of their age-calculated maximum HR (maximum HR estimated as 220--age) (Ramos-Campo et al., 2017) and 2) the participants could not continue running (Quammen et al., 2012).
Exercise-induced fatigue protocol with shuttle sprint + vertical jump (SJ-FP)
Before executing the SJ-FP, the maximal height of the vertical jump for each participant was recorded. The fatigue protocol comprised five consecutive vertical jumps, followed by a series set of 6 x 10-m shuttle sprints (Figure 1) (Tsai et al., 2009). The participants were required to repeat the aforementioned sequence at least...