1 Isotonic -a muscle fiber contraction in which force production is unchanged when the muscle contracts causing movement of an external load.  Isokinetic -a muscle fiber contraction in which force production of the contraction is sufficient to overcome the external load.
What Increases Your Risk. Most people get HIV by having unprotected sex with someone who has HIV. Another common way of getting the virus is by sharing needles with someone who is infected with HIV when injecting drugs. You have an increased risk of becoming infected with HIV through sexual contact if you: 1  Have unprotected sex (do not use condoms). 2  Have multiple sex partners. 3  Are a man who has sex with other men.  Have high-risk partner(s) (partner has multiple sex partners, is a man who has sex with other men, or injects drugs).
Risk by Age Group. In the United States, HIV is spread mainly through anal or vaginal sex or by sharing drug-use equipment with an infected person. Although these risk factors are the same for everyone, some groups merit special consideration because of their age. New infections are increasing among young men who have sex with men, especially young, black men men who have sex with men, for example, despite remaining steady overall.