The influence of muscle length on the fatigue-related reduction in joint range of motion of the human dorsiflexors

Local Muscle Fatigue and 3D Kinematics of the Cervical Spine in Healthy Subjects

The authors aimed to further explore the effects of local muscle fatigue on cervical 3D kinematics and the interrelationship between these kinematic characteristics and local muscle endurance capacity in the unimpaired cervical spine. Twenty healthy subjects (38 ± 10 years; 5 women) performed 2 × 10 maximal cervical flexion-extension movements. Isometric muscle endurance tests (prone/supine lying) were applied between sets to induce local muscle fatigue quantified by Borg scale rates of perceived exertion (RPE) and slope in mean power frequency (MPF; surface electromyography; m. sternocleidomastoideus, m. splenius capitis). Cervical motion characteristics (maximal range of motion [ROM], coefficient of variation of the 10 repetitive movements, mean angular velocity, conjunct movements in transversal and frontal plane) were calculated from raw 3D ultrasonic movement data. Average isometric strength testing duration for flexion and extension correlated to the cervical ROM (r = .49/r = .48; p < .05). However, Student's t test demonstrated no significant alterations in any kinematic parameter following local muscle fatigue (p > .05). Although subjects' cervical muscle endurance capacity and motor output seems to be conjugated, no impact of local cervical muscle fatigue on motor function was shown. These findings underline the importance of complementary measures to address muscular performance and kinematic characteristics in outcome assessment and functional rehabilitation of the cervical spine.

Factors contributing to the fatigue-related reduction in active dorsiflexion joint range of motion

Reductions in active joint range of motion (ROM) are responsible for decreased work-generating capacity during fatiguing repetitive isotonic shortening contractions. Factors responsible for impairing the joint-angle-specific net torque developed during muscle shortening could include fatigue-induced torque loss, shortening-induced torque depression in the agonist muscle, and opposing passive tension of the antagonists, but these have not been systematically explored. Nine men (aged 25.8 ± 2.0 years) performed a maximal-effort fatiguing task that consisted of repetitive loaded shortening dorsiflexions through a 40° ankle joint ROM until active ROM decreased by 50%. Torque developed during contractile shortening, as well as passive opposing tension, was quantified before and after the reduction in active ROM. Before fatigue, and compared with maximum voluntary isometric contraction torque at the terminal ROM, shortening-induced torque depression in the agonist muscle and passive tension from the antagonists reduced net torque developed at the end of contractile shortening by ∼42% and ∼19%, respectively. After fatigue, a steepened ascending joint torque–angle relationship remained during contractile shortening, but neither muscle coactivation nor contractile slowing contributed to the fatigue-induced torque loss. Fatigue-induced torque loss, shortening-induced torque depression in the agonist, and passive tension in the antagonist greatly depressed net torque developed at the end of contractile shortening. These contributed to the fatigue-induced reduction in active ROM by impairing the ability of the dorsiflexors to generate sufficient torque to overcome the imposed load at the end of contractile shortening.

The development of peripheral fatigue and short-term recovery during self-paced high-intensity exercise

The time course of muscular fatigue that develops during and after an intense bout of self-paced dynamic exercise was characterized by using different forms of electrical stimulation (ES) of the exercising muscles. Ten active subjects performed a time trial (TT) involving repetitive concentric extension/flexion of the right knee using a Biodex dynamometer. Neuromuscular function (NMF), including ES and a 5 s maximal isometric voluntary contraction (MVC), was assessed before the start of the TT and immediately (<5 s) after each 20% of the TT had been completed, as well as 1, 2, 4 and 8 min after TT termination. The TT time was 347 ± 98 s. MVCs were 52% of baseline values at TT termination. Torque responses from ES were reduced to 33-68% of baseline using different methods of stimulation, suggesting that the extent to which peripheral fatigue is documented during exercise depends upon NMF assessment methodology. The major changes in muscle function occurred within the first 40% of exercise. Significant recovery in skeletal muscle function occurs within the first 1-2 min after exercise, showing that previous studies may have underestimated the extent to which peripheral fatigue develops during exercise.