VIBRATION OF THE BICEPS BRACHII TENDON REDUCES TIME TO FAILURE WHEN MAINTAINING LIMB POSITION DURING A FATIGUING CONTRACTION.

Abstract

Purpose/Hypothesis: Vibration reduces the tendon jerk, and the Hoffmann and stretch reflexes in the muscle exposed to the vibration (Bove et al, 2003;Cresswell and Ludieroscher et al, 2000;Lance et al, 1966), but does not alter the time to task failure when exerting a submaximal force against a rigid restraint (Cresswell and Ludieroscher 2000). Because the amplitude of the stretch reflex can be enhanced when the limb acts against a compliant load compared with a rigid restraint (Akazawa et al. 1983; De Serres et al. 2002), the purpose was to determine the influence of tendon vibration on the time to task failure when light loads were supported while maintaining limb position (position task) with the elbow flexor muscles. Number of Subjects: Twenty-five healthy adult men (22 ± 4 yr; range, 18–39 yr) performed the position task at 20% MVC force by maintaining a 90° elbow position until task failure. Materials/Methods: Subjects visited the laboratory for three sessions to perform the position task with two different levels of vibration (100 Hz, 2.5 N force, sub-threshold and supra-threshold for the tonic vibration reflex), and without vibration of the biceps brachii tendon. Results: MVC force prior to task performance was similar across the three sessions (313 ± 54 N, P = 0.83), indicating that the net torque exerted by the limb during the fatiguing contractions was similar. The EMG for the short and long heads of the biceps brachii, brachioradialis, and brachialis was similar across conditions (P > 0.05). Despite the similar criteria for task failure and a similar decline in MVC force (18.0 ± 8.0 %, P > 0.05), the time to task failure differed for each condition: supra-threshold vibration = 3.7 ± 1.4 min, sub-threshold vibration = 4.3 ± 2.1 min, and no-vibration conditions = 5.0 ± 2.2 min (P < 0 .001). The standard deviation (SD) for the vertical fluctuations in acceleration was greater at the start (0.27 ± 0.13 m/s2) and at 25% of task duration (0.28 ± 0.15 m/s2) for the supra-threshold condition compared with the no-vibration (0.13 ± 0.06 and 0.16 ± 0.09 m/s2;P = 0.03) condition, but was similar at task termination (0.83 ± 0.50 m/s2) for the three conditions. Conclusions: These findings indicate that both low and high levels of vibration applied to the biceps brachii tendon reduced the time to failure when maintaining limb position, which has implications for work-place activities. Clinical Relevance: Documenting evidence for early fatigue during tasks performed with prolonged vibration confirms the importance of afferent input during sustained tasks, and has implications for work and ergonomic environments.