Scaling Constraints in Junior Tennis: The Influence of Net Height on Skilled Players' Match-Play Performance.

PURPOSE: The net height in tennis (0.91 m) is approximately 50% of a professional tennis player's height. Children are also expected to play with this net height, even though it is approximately 70% of the average 10-year-old's height. This study examined the immediate effect of lowering net height on the performance characteristics of skilled junior tennis players aged 10 years and younger. METHOD: Sixteen players were matched in 8 pairs of even tennis ability and same sex. Each pair played 25-min singles matches in 4 conditions that varied in net height (0.91 m, 0.78 m, 0.65 m, and 0.52 m). Match-play characteristics were analyzed via video replay. RESULTS: Results showed that lowering the net height to 0.65 m and 0.52 m led to players adopting a more attacking style of play, as evidenced by a significant increase in the number of winners without a commensurate increase in errors and more shots struck inside the baseline. Lower nets also led to a greater percentage of successful first serves. The lowest net (0.52 m), however, reduced rally length significantly and therefore decreased hitting opportunities. CONCLUSION: These results offer support for equipment scaling to enhance match-play performance for skilled junior tennis players. We propose that current net height recommendations for junior tennis should be revised.

Hydrogen sulfide (H2S) and hypoxia inhibit salmonid gastrointestinal motility: evidence for H2S as an oxygen sensor.

Hydrogen sulfide (H(2)S) has been shown to affect gastrointestinal (GI) motility and signaling in mammals and O(2)-dependent H(2)S metabolism has been proposed to serve as an O(2) 'sensor' that couples hypoxic stimuli to effector responses in a variety of other O(2)-sensing tissues. The low P(O2) values and high H(2)S concentrations routinely encountered in the GI tract suggest that H(2)S might also be involved in hypoxic responses in these tissues. In the present study we examined the effect of H(2)S on stomach, esophagus, gallbladder and intestinal motility in the rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) and we evaluated the potential for H(2)S in oxygen sensing by examining GI responses to hypoxia in the presence of known inhibitors of H(2)S biosynthesis and by adding the sulfide donor cysteine (Cys). We also measured H(2)S production by intestinal tissue in real time and in the presence and absence of oxygen. In tissues exhibiting spontaneous contractions, H(2)S inhibited contraction magnitude (area under the curve and amplitude) and frequency, and in all tissues it reduced baseline tension in a concentration-dependent relationship. Longitudinal intestinal smooth muscle was significantly more sensitive to H(2)S than other tissues, exhibiting significant inhibitory responses at 1-10 µmol l(-1) H(2)S. The effects of hypoxia were essentially identical to those of H(2)S in longitudinal and circular intestinal smooth muscle; of special note was a unique transient stimulatory effect upon application of both hypoxia and H(2)S. Inhibitors of enzymes implicated in H(2)S biosynthesis (cystathionine ß-synthase and cystathionine γ-lyase) partially inhibited the effects of hypoxia whereas the hypoxic effects were augmented by the sulfide donor Cys. Furthermore, tissue production of H(2)S was inversely related to O(2); addition of Cys to intestinal tissue homogenate stimulated H(2)S production when the tissue was gassed with 100% nitrogen (~0% O(2)), whereas addition of oxygen (~10% O(2)) reversed this to net H(2)S consumption. This study shows that the inhibitory effects of H(2)S on the GI tract of a non-mammalian vertebrate are identical to those reported in mammals and they provide further evidence that H(2)S is a key mediator of the hypoxic response in a variety of O(2)-sensitive tissues.