Abstract
Eccentric contractions can affect musculotendon mechanical properties and disrupt muscle proprioception, but their behavioral consequences are poorly understood. We tested whether repeated eccentric contractions of plantarflexor muscles of one leg affected the dexterity of either leg. Twenty healthy male subjects (27.3 ± 4.0 yrs) compressed a compliant and slender spring prone to buckling with each isolated leg. The maximal instability they could control (i.e., the maximal average sustained compression force, or lower extremity dexterity force, LEDforce) quantified the dexterity of each leg. We found that eccentric contractions did not affect LEDforce, but reduced force variability (LEDSD). Surprisingly, LEDforce increased in the non-exposed, contralateral leg. These effects were specific to exposure to eccentric contractions because an effort-matched exposure to walking did not affect leg dexterity. In the exposed leg, eccentric contractions (i) reduced voluntary error corrections during spring compressions (i.e., reduced 0.5–4 Hz power of LEDforce); (ii) did not change spinal excitability (i.e., unaffected H-reflexes); and (iii) changed the structure of the neural drive to the α-motoneuron pool (i.e., reduced EMG power within the 4–8 Hz physiological tremor band). These results suggest that repeated eccentric contractions alter the feedback control for dexterity in the exposed leg by reducing muscle spindle sensitivity. Moreover, the unexpected improvement in LEDforce in the non-exposed contralateral leg was likely a consequence of crossed-effects on its spinal and supraspinal feedback control. We discuss the implications of these bilateral effects of unilateral eccentric contractions, their effect on spinal and supraspinal control of dynamic foot-ground interactions, and their potential to facilitate rehabilitation from musculoskeletal and neuromotor impairments.
Highlights
Successful performance of balance and locomotor tasks is mediated by the ability of the leg to control force vectors to regulate dynamic foot-ground interactions, which has been referred to as dexterity (Valero-Cuevas et al, 2003; Lyle et al, 2013; Lawrence et al, 2014)
The first 10 subjects (Exposure group) attended two separate sessions in which they were exposed to two different exercise modalities—eccentric contractions with the right leg (ECC) and walking (WALK)—after an initial baseline assessment of lower extremity dexterity (LED) test performance (Figure 4)
Subjects performed eccentric contractions with their right leg while their contralateral leg remained at rest (CONTRA)
Summary
Successful performance of balance and locomotor tasks is mediated by the ability of the leg to control force vectors to regulate dynamic foot-ground interactions, which has been referred to as dexterity (Valero-Cuevas et al, 2003; Lyle et al, 2013; Lawrence et al, 2014). The dexterity test for the leg has been associated with athletic skills such as agility (Lyle et al, 2015). Such dynamic regulation of foot-ground interactions becomes an especially critical skill during highly dynamic tasks such as single leg landing (Brown et al, 2004; Ross and Guskiewicz, 2004). Improving our understanding of the sensorimotor mechanisms that enable dynamic foot-ground interactions may be important for elucidating mechanisms of impaired balance and locomotion in health and disease
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