Hamidollah Hassanlouei

Effect of exercise-induced fatigue on postural control of the knee

Muscle fatigue is associated with reduced power output and work capacity of the skeletal muscle. Fatigue-induced impairments in muscle function are believed to be a potential cause of increased injury rates during the latter stages of athletic competition and often occur during unexpected perturbations. However the effect of fatigue on functionally relevant, full body destabilizing perturbations has not been investigated. This study examines the effect of muscle fatigue on the activation of the quadriceps and hamstrings to fast, full body perturbations evoked by a moveable platform. Surface electromyographic (EMG) signals were recorded from the knee extensor (vastus medialis, rectus femoris, and vastus lateralis) and flexor muscles (biceps femoris and semitendinosus) of the right leg in nine healthy men during full body perturbations performed at baseline and immediately following high intensity exercise performed on a bicycle ergometer. In each condition, participants stood on a moveable platform during which 16 randomized postural perturbations (eight repetitions of two perturbation types: 8 cm forward slides, 8 cm backward slides) with varying inter-perturbation time intervals were performed over a period of 2-3 min. Maximal voluntary knee extension force was measured before and after the high intensity exercise protocol to confirm the presence of fatigue. Immediately after exercise, the maximal force decreased by 63% and 66% for knee extensors and flexors, respectively (P<0.0001). During the post-exercise postural perturbations, the EMG average rectified value (ARV) was significantly lower than the baseline condition for both the knee extensors (average across all muscles; baseline: 19.7±25.4μV, post exercise: 16.2±19.4 μV) and flexors (baseline: 24.3±20.9 μV, post exercise: 13.8±11.0 μV) (both P<0.05). Moreover the EMG onset was significantly delayed for both the knee extensors (baseline: 132.7±32.9 ms, post exercise: 170.8±22.9 ms) and flexors (baseline: 139.1±38.8 ms, post exercise: 179.3±50.9 ms) (both P<0.05). A significant correlation (R(2)=0.53; P<0.05) was identified between the percent reduction of knee extension MVC and the percent change in onset time of the knee extensors post exercise. This study shows that muscle fatigue induces a reduction and delay in the activation of both the quadriceps and hamstring muscles in response to rapid destabilizing perturbations potentially reducing the stability around the knee.

Delayed-onset muscle soreness alters the response to postural perturbations.

INTRODUCTION Eccentric contractions induce muscle fiber damage that is associated with delayed-onset muscle soreness and an impaired ability of the muscle to generate voluntary force. Pain and pathophysiological changes within the damaged muscle can delay or inhibit neuromuscular responses at the injured site, which is expected to have an effect on reflex activity of the muscle. PURPOSE The aim of the study was to investigate the reflex activity of knee muscles to rapid destabilizing perturbations, before, immediately after, and 24 and 48 h after eccentric exercise. METHODS Bipolar surface EMG signals were recorded from 10 healthy men with seven pairs of electrodes located on the knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis) and knee flexor muscles (the medial and lateral heads of the hamstring and the medial and lateral heads of gastrocnemius) of the right leg during rapid perturbations. RESULTS The maximal voluntary contraction force decreased by 24% ± 4.9% immediately after exercise and remained reduced by 21.4% ± 4.1% at 24 h and by 21.6% ± 9.9% at 48 h after exercise with respect to baseline. During the postexercise postural perturbations, the EMG average rectified value of the knee extensor muscles was significantly lower than baseline (P < 0.001). Moreover, the decrease in average rectified value over time during postexercise sustained contractions was greatest compared with the session before exercise (P < 0.0001). CONCLUSIONS Reflex activity in leg muscles elicited by rapid destabilizing perturbations is reduced after exercise-induced muscle soreness.

The effect of six weeks endurance training on dynamic muscular control of the knee following fatiguing exercise

The aim of the study was to examine whether six weeks of endurance training minimizes the effects of fatigue on postural control during dynamic postural perturbations. Eighteen healthy volunteers were assigned to either a 6-week progressive endurance training program on a cycle ergometer or a control group. At week 0 and 7, dynamic exercise was performed on an ergometer until exhaustion and immediately after, the anterior-posterior centre of pressure (COP) sway was analyzed during full body perturbations. Maximal voluntary contractions (MVC) of the knee flexors and extensors, muscle fiber conduction velocity (MFCV) of the vastus lateralis and medialis during sustained isometric knee extension contractions, and power output were measured. Following the training protocol, maximum knee extensor and flexor force and power output increased significantly for the training group with no changes observed for the control group. Moreover, the reduction of MFCV due to fatigue changed for the training group only (from 8.6% to 3.4%). At baseline, the fatiguing exercise induced an increase in the centre of pressure sway during the perturbations in both groups (>10%). The fatiguing protocol also impaired postural control in the control group when measured at week 7. However, for the training group, sway was not altered after the fatiguing exercise when assessed at week 7. In summary, six weeks of endurance training delayed the onset of muscle fatigue and improved the ability to control balance in response to postural perturbations in the presence of muscle fatigue. Results implicate that endurance training should be included in any injury prevention program.

Increased Fatigability Of Older Women Performing High-velocity Contractions Is Explained By Mechanisms Within The Muscle: 1805 Board #7 June 2, 1: 00 PM - 3: 00 PM.

METHODS: Twelve women volunteered to perform isometric leg extension muscle actions at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of maximal voluntary contraction (MVC) on a HUMAC NORM isokinetic dynamometer. The women were classified into lower strength (n = 6; 23.2 ± 1.7 y, 165.6 ± 7.6 cm, 59.3 ± 10.1 kg) and higher strength (n = 6; 22.8 ± 2.0 y, 164.6 ± 4.8 cm, 72.7 ± 12.2 kg) groups on the basis of their isometric MVC values (lower strength women MVC = 98.0 ± 20.6 Nm, higher strength women MVC = 169.2 ± 27.5 Nm). An accelerometer (EGAS S704 10_Rev C) was placed over the vastus lateralis to detect the MMG signal. The amplitude of the MMG signal was expressed as root mean square (RMS), while frequency data were expressed as mean power frequency (MPF). Torque (Nm) was recorded by the dynamometer. RESULTS: Polynomial regression analyses indicated that the relationships for MMG amplitude versus isometric MVC were quadratic for both the lower strength (R^2 = 0.987) and higher strength (R^2 = 0.964) women. However, for the lower strength women, MMG amplitude increased most between 60 and 100% isometric MVC, while for the higher strength women, MMG amplitude increased most between 10 and 60% MVC, then began to plateau. For MMG MPF, the relationships were cubic for the lower strength women (R^2 = 0.861) and linear for the higher strength women (R^2 = 0.902). CONCLUSIONS: The different torque-related responses for MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for lower vs. higher strength women. These results also suggest that isometric torque production is controlled by a combination of recruitment and firing rate, but that the reliance on each mechanism differed throughout the entire range of torque production between lower strength and higher strength women. Lastly, the torque-related patterns for MMG amplitude and frequency may also have been affected by differences in absolute torque, and thus muscle stiffness, between the lower strength and higher strength women.