Repeated exercise stress impairs volitional but not magnetically evoked electromechanical delay of the knee flexors

Abstract

The effects of serial episodes of fatigue and recovery on volitional and magneticallyevoked neuromuscular performance of the knee flexors were assessed in twenty female soccer players during: (i) an intervention comprising 4x35s maximal static exercise; (ii) a control condition. Volitional peak force (PF V ) was impaired progressively (-16 % vs. baseline: 235.3±54.7 to 198.1±38 However, improved EMD E performance might identify a dormant capability for optimal muscle responses during acute stressful exercise and an improved capacity to maintain dynamic joint stabilty during critical episodes of loading. 2

Effects of neuromuscular fatigue on electromechanical delay of the leg extensors and flexors in young men and women

“…This study showed that voluntary EMD increased as a result of neuromuscular fatigue for the leg extensors and flexors in both men and women. The increased EMD is similar to the findings of previous studies that also showed fatigue‐inducing increases in EMD for leg extensors, leg flexors, adductor pollicis, plantar flexors, and biceps brachii muscles. Because EMD is a function of both the structural components of the muscle–tendon unit (i.e., SEC, tendon properties) and muscle membrane action potential propagation and EC‐C, the fatigue‐inducing increases in EMD we observed may be a consequence of alterations or impairments in these physiological mechanisms.…”

“…The concept of neuromuscular fatigue yielding an increase in voluntary EMD has been acknowledged by Zhou, who observed increased EMD immediately after performing maximum voluntary isometric contractions (MVICs) of the leg extensors. Moreover, previous studies assessing the time course of EMD after a fatiguing task have shown that increased EMD in leg extensors typically returns to baseline within 7–10 min post‐fatigue, while EMD of the leg flexors may remain elevated for up to 8–30 min . Although these findings provide support for potential differences in EMD recovery times between muscles, an increased EMD resulting from fatigue, regardless of the muscles being examined (leg extensors or flexors), may decrease performance and increase the risk of injury .…”

“…Moreover, previous studies assessing the time course of EMD after a fatiguing task have shown that increased EMD in leg extensors typically returns to baseline within 7-10 min post-fatigue, 6 while EMD of the leg flexors may remain elevated for up to 8-30 min. 6,17 Although these findings provide support for potential differences in EMD recovery times between muscles, an increased EMD resulting from fatigue, regardless of the muscles being examined (leg extensors or flexors), may decrease performance and increase the risk of injury. 3 It has been suggested that the increased risk of injury may be the result of increased muscle excitation and contraction time.…”

“…suggested that neuromuscular fatigue may elicit an increased injury risk due to inability of the muscles to contract properly to stabilize the knee joint during critical periods of mechanical loading. Because leg extensors and flexors are both critically important for sport performance and for knee stabilization to prevent anterior cruciate ligament and other lower extremity injuries, numerous research studies have focused on examining the effects of fatigue on EMD of leg extensor and flexor muscle groups . However, aside from Conchola et al ., who assessed voluntary fatigue on EMD for both the leg extensors and flexors in young and elderly men, previous studies have only assessed EMD for either the extensors or flexors .…”

“…Because leg extensors and flexors are both critically important for sport performance and for knee stabilization to prevent anterior cruciate ligament and other lower extremity injuries, numerous research studies have focused on examining the effects of fatigue on EMD of leg extensor and flexor muscle groups. 2,3,6,16,17,19 However, aside from Conchola et al, 6 who assessed voluntary fatigue on EMD for both the leg extensors and flexors in young and elderly men, previous studies have only assessed EMD for either the extensors 9,16,20 or flexors. 2,3,17 Moreover, although Conchola et al 6 observed different recovery patterns between muscles, further research is warranted as to whether differences in the time-course effects of fatigue on EMD exist between these large locomotor muscles.…”