Sex-specific effects of surface instability on drop jump and landing biomechanics

This study investigated sex-specific effects of surface instability on kinetics and lower extremity kinematics during drop jumping and landing. Ground reaction forces as well as knee valgus and flexion angles were tested in 14 males (age: 23±2 years) and 14 females (age: 24±3 years) when jumping and landing on stable and unstable surfaces. Jump height was found to be significantly lower (9%, p<0.001) when drop jumps were performed on unstable vs. stable surface. Significantly higher peak ground reaction forces were observed when jumping was performed on unstable versus stable surfaces (5%, p=0.022). Regarding frontal plane kinematics during jumping and landing, knee valgus angles were higher on unstable compared to stable surfaces (19-32%, p<0.05). Additionally, at the onset of ground contact during landings, females showed higher knee valgus angles than males (222%, p=0.027). Sagittal plane kinematics indicated significantly smaller knee flexion angles (6-35%, p<0.05) when jumping and landing on unstable vs. stable surfaces. During drop jumps and landings, women showed smaller knee flexion angles at ground contact compared to men (27-33%, p<0.05). These findings imply that knee motion strategies were modified by surface instability and sex during drop jumps and landings.

Effects of hyperbaric (6 ATA) pressure on voluntary and evoked skeletal muscle contractile properties

The objective of this study was to investigate the effect of 6 atmospheres of pressure (ATA) on plantar flexors' (PF) voluntary force and activation, force-frequency characteristics, and rate of torque development (RTD). Eight subjects performed PF isometric contractions. Muscle activation was monitored by electromyographic (EMG) activity (PF and dorsiflexors) and the interpolated twitch technique (ITT). Maximal evoked contractions of the PF were elicited at 1, 2, 3, 5, 10, 20, and 40 Hz. PF RTD was calculated with maximal voluntary, 1 and 40 Hz contractions. Hyperbaric pressures significantly decreased PF voluntary torque; 6.2%, ITT activation; 2.8% with a trend for a 19.1% decrease in EMG (p = 0.1). There were no significant differences in the dorsiflexors/PF EMG ratio. One Hz torque was potentiated 15.7% with an increased absolute RTD of 12.8%, but no change in relative RTD. The results suggested hyperbaric-induced decreases in PF activation contributed to voluntary torque loss. A lack of torque reduction with higher frequency tetanic stimulation (2-40 Hz) suggested that 6 ATA does not impair myofilament kinetics, whereas twitch potentiation may include changes in excitation-contraction coupling.

Comparison of interpolation and central activation ratios as measures of muscle inactivation

The objective of this study was to investigate different methods of estimating muscle inactivation, derived from single and multiple voluntary contractions. Ten subjects performed maximal and submaximal leg extensor contractions to determine an interpolation (IT) or central activation ratio (CAR). A superimposed evoked force was compared with the force output of either a voluntary (CAR) or resting evoked contraction (IT ratio), or the ratios were inserted into regression equations (linear, polynomial, exponential). Linear-regression estimates of CAR using doublets and tetanus provided physiologically inaccurate values. Whereas IT ratios using doublets (IT-doublet) and tetanus (IT-tetanus) had a significant difference in only one interaction, IT-tetanus and CAR using a tetanus (CAR-tetanus) estimates provided the most extensive correlation within and between measures. Thus, tetanic stimulation superimposed upon single maximal or multiple contractions seems to provide the most valid measure of muscle inactivation when using the interpolated-twitch technique.

Role of disulfide bonds in the structure and potassium channel blocking activity of ShK toxin

ShK toxin, a potassium channel blocker from the sea anemone Stichodactyla helianthus, is a 35 residue polypeptide cross-linked by three disulfide bridges: Cys3-Cys35, Cys12-Cys28, and Cys17-Cys32. To investigate the role of these disulfides in the structure and channel-blocking activity of ShK toxin, a series of analogues was synthesized by selective replacement of each pair of half-cystines with two alpha-amino-butyrate (Abu) residues. The remaining two disulfide pairs were formed unambiguously using an orthogonal protecting group strategy of Cys(Trt) or Cys(Acm) at the appropriate position. The peptides were tested in vitro for their ability to block Kv1.1 and Kv1.3 potassium channels and their ability to displace [(125)I]dendrotoxin binding to rat brain synaptosomal membranes. The monocyclic peptides showed no activity in these assays. Of the dicyclic peptides, [Abu12,28]ShK(3-35,17)(-)(32) (where the subscript indicates disulfide connectivities) had weak activity on Kv1.3 and Kv1.1. [Abu17,32]ShK(3-35,12)(-)(28) blocked Kv1.3 with low nanomolar potency, but was less effective (being comparable to [Abu12,28]ShK(3-35,17)(-)(32)) against Kv1.1. [Abu3, 35]ShK(12-28,17)(-)(32), retained high picomolar affinity against both channels. Corroborating these results, [Abu3,35]ShK(12-28, 17)(-)(32) had an IC(50) ratio relative to native toxin of 18 in the displacement assay, whereas [Abu17,32]ShK(3-35,12)(-)(28) and [Abu12, 28]ShK(3-35,17)(-)(32) had ratios of 69 and 390, respectively. Thus, the disulfide bond linking the N- and C-terminal regions is less important for activity than the internal disulfides. NMR analysis of the [Abu12,28] and [Abu17,32] analogues indicated that they had little residual structure, consistent with their significantly reduced activities. By contrast, [Abu3,35]ShK(12-28,17)(-)(32) had a moderately well-defined solution structure, with a mean pairwise root-mean-square deviation of 1.33 A over the backbone heavy atoms. This structure nevertheless showed significant differences from that of native ShK toxin. The possible interactions of this analogue with the channel and the distinction between native secondary and tertiary structure on one hand and global topology imposed by the disulfide bridges on the other are discussed.