Endurance running physiology studies have focused on aerobic capacity, metabolic systems and running economy as the limiting factors for performance (Costill et al., 1976). However, these properties cannot solely explain differences observed in endurance performance (Noakes, 2011; Nummela et al., 2006). Neuromuscular based theories propose that, during endurance exercise, the central nervous system integrates all feedback and consequently regulates the skeletal muscle recruitment, such that the levels of intensity and duration do not exceed the threshold where potential damage could occur to the brain, heart, and other vital organs, thus affecting the muscle's ability to produce power and speed. (Gibson, 2004; Noakes, 2000).
In the case of an endurance athlete, a high rate of fatigue development coupled with a heightened inability to tolerate fatigue, would decrease speed and negatively affect performance. Muscle fatigue can be characterized into central and peripheral components. Peripheral fatigue is due to changes occurring distal to the neuromuscular junction, while central fatigue has its origin proximal to the neuromuscular junction representing activities within the central nervous system (Gandevia, 1998). Research suggests that there are peripheral adjustments that take place at the skeletal muscle to counteract fatigue (Behm 2004) and enhance performance (Boullosa and Tuimil, 2009; Behm et al.., 2018; Hamada et al., 2000a). In addition to peripheral fatigue; reduced neural drive (central fatigue) to the contracting muscles is likely to compromise motor performance during running (Girard et al., 2008, Millet et al., 2003). Thus, the management of fatigue (central and peripheral), commonly known as pacing, becomes an important factor to consider for successful endurance performance.
PAP is a phenomenon that can enhance muscle force output and rate of force development as a result of muscle contractile history (Houston et al., 1987; Grange et al., 1993; Sale, 2004). This phenomenon is highly applicable to sports, and research has increasingly focused on methods for prompting PAP as a form of performance enhancement (Boullosa et al., 2018; Hamada et al., 2000a; Mitchell and Sale 2011). Due to the lack of applicable literature, scientists and performance professionals are often unaware of the potential PAP has to aid in both endurance training and competition. The aim of incorporating a PAP stimulus into an athlete's training or pre-competition program is to elicit an acute enhancement in muscle performance and consequently a higher jump, faster time, or further throw (Boullosa et al., 2018; Gullich and Schmidtbleicher, 1996; Hamada et al., 2000a; Mitchell and Sale, 2011).
Examining PAP in the endurance phenotype, a widely cited article by Sale (2004) suggests that PAP should have its greatest effect during submaximal contractions in which the motor units are firing at relatively lower frequencies, as is the case with endurance exercise. It follows then that if the levels of fatigue in a sport or activity are relatively lower than the levels of PAP, not only athletes performing maximal activities will benefit, but athletes performing submaximal activities could benefit from this mechanism. These lower firing rates occur during endurance activities and consequently help achieve an improved final force output (Behm, 2004; Sale, 2004).
With the evidence that PAP enhances subsequent endurance performance, the most plausible application of PAP to sport is its use as a warm-up technique prior to training or competition in order to acutely enhance contractile potential (Boullosa et al., 2018). Interestingly, few studies have examined the feasibility of this application in the endurance running population (Palmer et al., 2009; Silva et al., 2014; Skof and Strojnik et al., 2007) though many studies have investigated the enhancement of contractile activity during and after endurance performance activities (Boullosa et al., 2018; Hamada et al., 2000a). As PAP has been shown to persist for approximately 10 minutes (Houston et al., 1987; Grange et al., 1993), and other sports that involve endurance may have prolonged rest periods (e.g. half-time in soccer), it would be of interest to monitor potentiation following an exercise protocol. To our knowledge, there have been very few to no studies that have combined a PAP conditioning stimulus into a warm up while subsequently monitoring the central and peripheral alterations during and after a training-styled run session. This proposed study design offers insights into the confirmation of PAP in endurance athletes and attempts to investigate at what point during an endurance run PAP effects can be measured.
Therefore, the aim of the present study was to evaluate the neuromuscular alterations of a PAP conditioning stimulus on subsequent performance during and after a running time trial. Specifically, we compared the neuromuscular effects (i.e. electromyography, maximal voluntary contractions, evoked contractile properties, and performance measures) of an applicable (Gooyers et al., 2012), five repetition maximum (5RM) band-resisted squat jump on a subsequent 5 x 1 kilometer time trial to highlight possible central or peripheral changes. We hypothesized that performing a 5RM band-resisted jump squat protocol as part of a standardized running-specific warm-up would induce significant measurable PAP effects during the course of a subsequent 5 x 1 kilometer time trial run and up to 10 minutes post-run protocol.
A within subject (repeated measures) experimental study design was employed in this study. Twelve healthy male recreationally endurance-trained athletes completed two familiarization, and two intervention sessions in a randomized order separated by a minimum of 72 hours. The familiarization sessions included: 1) an incremental running test to volitional exhaustion (V[O.sub.2max]) and 2) a familiarization of testing equipment, estimation of the individual's 5RM, and testing of evoked contractile properties. The intervention sessions included a running-specific warm up, with either the intervention (4 x 5RM band resisted squat jumps) or control (no squats) condition, and a 5 x 1 kilometer time trial run. Tests were conducted immediately prior to the intervention, during a 3 minute recovery period between each kilometer, immediately following the 5 x 1 km run, and at minutes seven and ten, post-5 km run. These measures included the interpolated twitch technique (ITT) as a measure of muscle activation, contractile properties (peak twitch torque, rate of force development, percent voluntary activation, potentiated twitch force), maximum voluntary isometric contractions (MVIC) to determine peak ankle plantar flexor force, force produced in the first 100 ms (F100), 30-cm drop jump height (flight time, contact time and reactive strength index), rate of perceived exertion (RPE), and heart rate.
Based on a statistical power of 0.8, an effect size of 0.5 (Cohen, 1969, p.348), and an alpha level of p
Participants attended the laboratory on two separate occasions to 1) perform the Universite de Montreal Track Test (V[O.sub.2] consumption, ventilatory threshold, maximum aerobic speed, and maximum heart rate) and 2) to estimate 5RM squat, and establish evoked contractile properties measures. Due to the potentially exhausting effects of the familiarization sessions, they were separated by a 48-hour period.
Universite de Montreal Track Test (UMTT) & anthropometric measures: Upon arrival to the lab, the participant's height (cm) and weight (kg) was measured and age was recorded. The participants began the session with a 5-minute warm-up/familiarization period on a treadmill (Cybex 751T, Medway, Massachusetts) at a self-selected pace (recorded). Following the warm-up, the participants were connected to an indirect calorimetric system (MOXUS CD 3-A/S-3A, AEI Technologies, Pittsburgh, Pennsylvania) via a mask secured to the oral/nasal regions of the face. The participants were outfitted with a heart rate monitor (chest strap; Polar H10, Polar, Kempele, Finland), the test protocol was discussed and any questions answered. The test was conducted according to the protocol set out by Leger & Boucher (1980) in which the participant runs on a motor-driven treadmill at a constant slope of 1%. The test began at an initial speed of 7 kmx[hr.sup.-1] with a 1 kmx[hr.sup.-1] speed increase every two minutes until volitional exhaustion. After exhaustion was reached, a five-minute rest/recovery period was allowed before commencing a verification phase (to ensure a true VO.sub.2 maximum [V[O.sub.2max]]). During the verification phase (Rossiter et al., 2006), the treadmill was set at 105% of the recently determined maximal aerobic speed (MAS) and the participant ran until their limit of tolerance was reached. A self-selected pace cool down period followed and continued until the recovery heart rate reached 120 bx[min.sup.-1]. To further determine if the participants attained V[O.sub.2max] or true exhaustion, the respiratory exchange ratio (RER: >1.0), a plateau in V[O.sub.2] values at maximum intensity, Borg perceived exertion scales (>17/20) and maximum heart rate (approximately 220--participants' age) attained were also monitored. Results from this session were used to provide a descriptive statistic into the training status of the participants. MAS and maximum heart rate were used to compare and contrast the measured aerobic speed and heart rate reached during the running trials, enabling a better analysis of the metabolic properties during the trials.
Squat jumps: Based on the evidence demonstrating the potentiating effects of the squat, Rixon et al., (2007) have suggested that potentiation effects can depend on an individual's training background. Squats and other biomechanically similar exercises...