Effects of two neuromuscular fatigue protocols on landing performance

Abstract

The purpose of the study was to investigate the effects of two fatigue protocols on landing performance. A repeated measures design was used to examine the effects of fatigue and fatigue protocol on neuromuscular and biomechanical performance variables. Ten volunteers performed non-fatigued and fatigued landings on two days using different fatigue protocols. Repeated maximum isometric squats were used to induce fatigue on day one. Sub-maximum cycling was used to induce fatigue on day two. Isometric squat maximum voluntary contraction (MVC) was measured before and after fatigued landings on each day. During the landings, ground reaction force (GRF), knee kinematics, and electromyographic (EMG) data were recorded. Isometric MVC, GRF peaks, loading rates, impulse, knee flexion at contact, range of motion, max angular velocity, and EMG root mean square (RMS) values were compared pre- and post-fatiguing exercise and between fatigue protocols using repeated ANOVA. Fatigue decreased MVC strength (p ⩽ 0.05), GRF second peak, and initial impulse (p ⩽ 0.01), but increased quadriceps medium latency stretch reflex EMG activity (p ⩽ 0.012). Knee flexion at contact was 5.2° greater (p ⩽ 0.05) during fatigued landings following the squat exercise compared to cycling. Several variables exhibited non-significant but large effect sizes when comparing the effects of fatigue and fatigue protocol. In conclusion, fatigue alters landing performance and different fatigue protocols result in different performance changes.

Introduction

Muscle fatigue commonly occurs during strenuous dynamic physical activities and results in altered performance. Typical performance changes during fatigue are reduced maximum voluntary muscle force and work capacity (Asmussen, 1979, Gandevia, 2001), altered movement control (Miura et al., 2004, Rodacki et al., 2002), and delayed reaction time (Zhou et al., 1998). Neuromuscular alterations that occur during fatigue also might increase the risk of injury (Arndt et al., 2002, Fahlstrom et al., 1998, Pettrone and Ricciardelli, 1987, Sharkey et al., 1995). Muscle fatigue is thought to alter the shock absorbing capacity of the muscle and the coordination of the locomotor system, potentially resulting in greater stress on passive structures (Frankel and Nordin, 2002). While both increased loading and altered control are thought to increase the risk of injury (Johnston et al., 1998, Nyland et al., 1994, Potvin and O’Brien, 1998, Wojtys et al., 1996), previous research on the effects of fatigue during locomotor activities has demonstrated different responses in both ground reaction force (GRF) magnitudes and lower extremity control strategies. The reason for these different responses is unknown and additional information is needed to provide insight about performance changes that occur during fatigue.

Previous research has shown that fatigue alters neuromuscular control. For example, during multi-joint upper extremity performance tasks, fatigue has been shown to result in a reorganization of the movement pattern in order maintain successful accomplishment of the task (Cote et al., 2002, Forestier and Nougier, 1998). Fatigue of selected trunk muscles during lifting has been shown to result in greater co-contraction of stabilizing muscles of the spine and greater bending moment variability (Potvin and O’Brien, 1998). Fatigue has been shown to result in poorer static and dynamic balance during postural-maintenance activities (Johnston et al., 1998) and altered lower extremity joint coordination strategies during jumping (Rodacki et al., 2002), both reflecting changes in control. In the knee joint, dynamic stabilization mechanisms appear to be affected by fatigue of the knee musculature. For example, neuromuscular reaction times were slowed and anterior tibial translation increased during fatigue of the quadriceps and hamstring muscle groups (Wojtys et al., 1996). Similarly, electromechanical delay of the quadriceps increased during repeated maximal isometric knee extension exercise (Zhou et al., 1998), further indicating a change in neuromuscular control that may occur during fatigue. Alternations in neuromuscular control during fatigue could increase load on passive structures.

Fatigue alters GRF magnitudes during the impact and eccentric braking phases of locomotor activities. Changes in GRF reflect alterations in segmental control and joint and system stiffness (Denoth, 1986) and also could alter the load on passive structures. However, previous research has demonstrated contradictory directions of change in GRF magnitudes during fatigued locomotion. Some studies have reported decreases in GRF during the eccentric braking phase of running (Nummela et al., 1994), single limb landing (Augustsson et al., 2006, Madigan and Pidcoe, 2003), and maximum upper extremity drop jumping (Gollhofer et al., 1987b). Decreased GRF has been explained by an alteration in stiffness regulation and reduction in the storage and utilization of elastic energy (Horita et al., 1996, Nummela et al., 1994), which might be a function of altered stretch reflex responses (Gollhofer et al., 1987a). Conversely, other data have suggested that GRF magnitudes increase during fatigued hopping (Bonnard et al., 1994), landing (James et al., 2006, Pappas et al., 2007, Wikstrom et al., 2004), and sub-maximum drop jumping (Gollhofer et al., 1987b, Nicol et al., 1991), which has been explained by increased pre-activation (Nicol et al., 1991) of stabilizing musculature in order to increase joint or system stiffness and changes in body geometry at initial contact (James et al., 2006). The discrepancy in GRF responses suggest that the neuromuscular system is either affected differently under various fatiguing conditions or responds differently to the neuromuscular impairment, possibly optimizing on different performance factors.

Currently, there is limited evidence that suggests under which circumstances participants respond to fatigue with less stiffness and reduced GRF or more stiffness and increased GRF. It is unknown whether these changes are related to the type or site of fatigue, severity of fatigue, type or intensity of the exercise, differences in accommodation strategies, or other factors. Gollhofer et al. suggested that the specific fatigue response might be related to the intensity of the exercise (i.e., landing height) as they observed increases and decreases in GRF following sub-maximum and maximum upper extremity drop jumps, respectively (Gollhofer et al., 1987b). However, collective results from other studies involving sub-maximum and maximum efforts do not necessarily support this idea. The characteristics of the fatiguing activity have been suggested to influence knee proprioception (Miura et al., 2004) but the influence on landing performance remains unclear (Wikstrom et al., 2004). Therefore, the purpose of our study was to investigate the effects of two different protocols for inducing fatigue on landing performance. First, we hypothesized that two fatigue protocols, one utilizing maximum effort isometric squat exercise and the other involving prolonged sub-maximum cycling exercise, both would result in alterations in GRF, knee kinematics, and neuromuscular activity during landing. Second, we hypothesized that these two different protocols leading to fatigue would result in different performance changes in these same biomechanical performance measures.