Plantar Flexor Muscle Fatigue Research Articles

Effect of Fatigue on Neural Activity of the Ankle Musculature

Each year ankle injuries result in a significant amount of time loss from work and sport activity. A percentage of these eventually lead to long-term ankle dysfunction and instability. Some would suggest that fatigue, either central or peripheral, may play a role in inhibiting the ankle stabilizers from preventing sprains. Examining the neural activity of the ankle musculature in a fatigued state may enlighten clinicians and scholars and eventually help in finding the exact cause of chronic disability. PURPOSE: To examine neural activity in healthy, uninjured ankles pre and post fatigue. METHODS: A total of 9 male (age: 20.1 ± 1.5 yrs; height: 1.81 ± 0.09 m; mass: 84.4 ± 14.2 kg) and 10 female (age: 20.5 ± 2.0 y; height: 1.66 ±0.05 m; mass: 62.9 ± 3.7 kg) subjects volunteered to participate. Baseline isokinetic strength measurements were taken for the motions of plantar flexion/dorsiflexion and inversion/eversion at a velocity of 120°/s on two separate sessions in random order. Using a 50% decrement of baseline strength as a reference, fatigue was induced in each of these 4 motions individually. Post-fatigue isokinetic strength measurements were taken immediately after the fatigue event in a manner similar to that of the baseline test. Simultaneously, electromyographic activity was monitored from the tibialis anterior (TA), peroneals (PER), and gastrocnemius (GAS) muscles. Torque and median frequency (MF) data were calculated. Separate repeated-measures MANOVAs were performed for each of the 4 ankle motions with the conventional alpha level of 0.05. RESULTS: For all 4 motions, torque was greater in males than females, in eccentric vs. concentric muscle actions, and in the pre-fatigue vs. the post-fatigue state (p < 0.05). For ankle inversion, the MF decreased in the TA (P=0.000) pre vs. post. For ankle plantar flexion, the MF decreased in the PER muscles pre vs. post (P=0.007) and a time x gender interaction (P=0.032) was noted in the PER muscles. Specifically, the males decreased MF more than females pre vs. post, although males had a higher MF overall than females (P=0.004). CONCLUSION: In general, and as expected, MF decreased in fatigued muscles, although not statistically significant in all cases. The decrease in MF in the PER during plantarflexor fatigue may be of particular importance in ankle stability. The plantarflexors are often fatigued in jumping/landing/cutting activities, which are the same activities associated with ankle sprain risk. Decreased MF seen in the PER muscles correspond with a decreased firing rate. This may be of importance, especially with regard to their role in countering the violent moment seen with inversion ankle sprains.

Musculoskeletal Plasticity After Acute Spinal Cord Injury: Effects of Long-Term Neuromuscular Electrical Stimulation Training

Maintaining the physiologic integrity of paralyzed limbs may be critical for those with spinal cord injury (SCI) to be viable candidates for a future cure. No long-term intervention has been tested to attempt to prevent the severe musculoskeletal deterioration that occurs after SCI. The purposes of this study were to determine whether a long-term neuromuscular electrical stimulation training program can preserve the physiological properties of the plantar flexor muscles (peak torque, fatigue index, torque-time integral, and contractile speed) as well as influence distal tibia trabecular bone mineral density (BMD). Subjects began unilateral plantar flexion electrical stimulation training within 6 wk after SCI while the untrained leg served as a control. Mean compliance for the 2-yr training program was 83%. Mean estimated compressive loads delivered to the tibia were approximately 1-1.5 times body weight. The training protocol yielded significant trained versus untrained limb differences for torque (+24%), torque-time integral (+27%), fatigue index (+50%), torque rise time (+45%), and between-twitch fusion (+15%). These between-limb differences were even greater when measured at the end of a repetitive stimulation protocol (125 contractions). Peripheral quantitative computed tomography revealed 31% higher distal tibia trabecular BMD in trained limbs than in untrained limbs. The intervention used in this study was sufficient to limit many of the deleterious muscular and skeletal adaptations that normally occur after SCI. Importantly, this method of load delivery was feasible and may serve as the basis for an intervention to preserve the musculoskeletal properties of individuals with SCI.

Perturbation of the postural control system induced by muscular fatigue.

In this experiment, we induced muscular fatigue of ankle plantar-flexors to examine how it deteriorates the regulation of bipedal quiet upright standing. Postural stability was assessed in conditions with and without vision over 60 s period to examine not only classical postural variables (time- and frequency-domain analyses), but also structural variables (stabilogram-diffusion analysis). Muscular fatigue was induced with repeated plantar-flexion of both legs. With muscular fatigue, subjects exhibited an increased postural sway (faster center of pressure (CP) velocity, and greater CP mean and median frequency) and a decreased long-term scaling exponent compared with the control conditions. The fatigue conditions, however, did not modify the range of oscillations and the variability of the postural oscillations around the mean position of CP. The effects of muscular fatigue were similar with eyes open and eyes closed. These results suggest that fatigue did induce some changes in the control mode of postural stability, but the detection/action capabilities of the sensorimotor system remained partly efficient when the ankle plantar-flexors were fatigued. Furthermore, the decreased long-term scaling exponent observed with fatigue suggests that the control of upright stance operates in a less stochastic and more antipersistent manner when fatigue is present (i.e. past and future behaviors were more negatively correlated and thus more tightly regulated). Altogether, the present results suggest that, compared with the no-fatigue conditions, fatigue places higher demands on the postural control system by increasing the frequency of actions needed to regulate the upright stance.

Effects of isokinetic ankle fatigue on the maintenance of balance and postural limits

Yaggie JA, McGregor SJ. Effects of isokinetic ankle fatigue on the maintenance of balance and postural limits. Arch Phys Med Rehabil 2002;83:224-8. Objective: To quantify changes in balance parameters and ranges of postural control at the ankle after isokinetic fatigue. Design: Before-after trials, with a 5 × 6 repeated-measures design. Setting: General community. Participants: Twenty-four men (age, 24.9 ± 3.92y; height, 177.79 ± 6.36cm; weight, 80.78 ± 13.22kg) without ankle trauma within 2 years. Interventions: Fatigue of the plantarflexors and dorsiflexors was induced by isokinetic contractions. Balance was assessed by using a unilateral test (15-s quiet stance, 10-s lean test) on a force platform immediately before and at 0 (T0), 10 (T10), 20 (T20), and 30 (T30) minutes postfatigue. Main Outcome Measure: Mediolateral (ML) and fore-aft (FA) sway as well as ML and FA displacement were analyzed by analysis of variance with repeated measures for time (α =.05). Results: In quiet stance, ML sway was greater at T0, whereas total sway increased at all time points postfatigue (P <.05). For the lean test, FA sway increased at T0 and T10, and total sway increased at all time points (P <.05). Both ML and FA displacement significantly differed at T0 (P <.05). All sway parameters returned to baseline within 20 minutes. Conclusions: Isokinetic fatigue of ankle plantarflexors and dorsiflexors significantly influences sway parameters and ranges of postural control in healthy young men. These perturbations are transient, and recovery occurs within 20 minutes. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation