The text discusses two main topics: the role of crossbridges in muscle stiffness and a new classification system for HIV-1 based on co-receptor usage. The first part explains that in a rigor muscle, all crossbridges contribute to stiffness. If many are in 'slack' states, the calculated duty ratio would be lower, potentially explaining experimental discrepancies. The experiments of Higuchi and Goldman are consistent with a small working distance, while the experiments of Yanagida et al. remain difficult to explain. The second part introduces a new classification system for HIV-1 based on co-receptor use (CCR5 or CXCR4). This system provides a more accurate description of viral phenotype than existing classifications. The new system defines R5 viruses as using CCR5 but not CXCR4, X4 viruses as using CXCR4 but not CCR5, and R5X4 viruses as using both co-receptors. This classification system is more precise and avoids the inaccuracies of previous systems. It is flexible and can be expanded to include other co-receptors if they play a major role in tropism. The system also accounts for the ability of viruses to use co-receptors in different contexts, and authors are encouraged to clarify any ambiguities. The classification system is intended to be used in conjunction with data from the Los Alamos National Laboratory Sequence Database. The authors include several experts in the field of HIV research.The text discusses two main topics: the role of crossbridges in muscle stiffness and a new classification system for HIV-1 based on co-receptor usage. The first part explains that in a rigor muscle, all crossbridges contribute to stiffness. If many are in 'slack' states, the calculated duty ratio would be lower, potentially explaining experimental discrepancies. The experiments of Higuchi and Goldman are consistent with a small working distance, while the experiments of Yanagida et al. remain difficult to explain. The second part introduces a new classification system for HIV-1 based on co-receptor use (CCR5 or CXCR4). This system provides a more accurate description of viral phenotype than existing classifications. The new system defines R5 viruses as using CCR5 but not CXCR4, X4 viruses as using CXCR4 but not CCR5, and R5X4 viruses as using both co-receptors. This classification system is more precise and avoids the inaccuracies of previous systems. It is flexible and can be expanded to include other co-receptors if they play a major role in tropism. The system also accounts for the ability of viruses to use co-receptors in different contexts, and authors are encouraged to clarify any ambiguities. The classification system is intended to be used in conjunction with data from the Los Alamos National Laboratory Sequence Database. The authors include several experts in the field of HIV research.