CHAPTER 3 Structure and Function of Muscles MUSCLE ARCHITECTURE


Specifi c muscles are often described anatomically and individually by their proximal and distal attachments. The proximal attachment is called the origin; the distal attach- ment is the insertion. Muscle origins and insertions are labeled under the assumption that the proximal attach- ment is stabilized and the distal attachment is moving toward the proximal attachment performing a concentric contraction. Understanding these attachments and the movement they produce lays the foundation for studying human kinesiology. Because humans perform movement in a variety of patterns, the distal insertion is not always moving toward a proximal origin. For instance, during the closed-chain activity of moving from a standing posi- tion to sitting, the proximal attachments of the quadriceps muscles move toward the distal attachment as the quad- riceps eccentrically lower the body. Conversely, when moving from sitting to standing, the proximal attachment of the quadriceps moves away from the distal attach- ment during a closed-chain eccentric contraction. (Refer to Chapter 1 for a review of open and closed kinetic chain movements.) This same concept of the changing relationship of proximal and distal attachments can be observed during concentric contractions. When bringing the hand toward the mouth, the distal attachment of the biceps moves toward the proximal attachment. Stabiliz- ing the forearm and bringing the shoulder toward the hand during a chin-up exercise is a reversal of action of the biceps as the proximal attachment of the biceps moves toward the distal attachment (Fig. 3.9).


FIBER ARRANGEMENT


The basic architecture of an entire muscle and its rela- tionship to function depend in part on the arrangement of the muscle fi bers in terms of their angle of orientation, physiological cross-sectional area (PCSA), and length and the total length of the muscle. The PCSA refers to the area of the muscle that is perpendicular to the orientation of muscle fi bers—its cross-sectional width—and can be increased by an increase in the number of muscle fi bers or an increase in fi ber size. Fusiform muscles have fi bers that run parallel to the long axis of the muscle and its central tendon. They tend to be spindle shaped, wider at midbelly, and tapered toward the ends. In general, fusi- form muscles tend to have longer fi ber lengths and can produce a greater range of motion at the joint compared with muscles with other architectural arrangements. The biceps brachii is an example of a fusiform muscle. Strap muscles have parallel fi bers running the length of the muscle and have more of a fl attened shape that inserts into a wide fl at tendon. The rectus abdominis is an example of a strap muscle that is segmented with tendons.


59


Muscle fi bers that run at an angle to the long axis of the muscle have a pennate structure and are referred to as unipennate, bipennate, or multipennate, depending on the number of muscle fi ber groups. In this arrange- ment, fascicles fan out from a central tendon. Although the oblique orientation of pennate muscles decreases the amount of force exerted at the tendon, these muscles typically can generate larger forces than fusiform muscles because they have a greater PCSA of muscle fi bers. There is a direct relationship between the cross-sectional area and the muscle’s ability to generate tension. As a rule, the greater a muscle’s width, the greater its ability to produce force compared with a muscle of similar length with a smaller width. The quadriceps at the knee joint is an example of a muscle with a pennate structure. Figure 3.10 shows examples of the various types of muscle architecture.


FIBER TYPE


Muscles are also classifi ed according to how they func- tion metabolically and mechanically. The three primary types of muscle fi bers are type I (slow twitch), type IIA (intermediate twitch), and type IIX (fast twitch). Type I muscle fi bers tend to have a slower contraction rate and are fatigue resistant. These fi bers are considered pos- tural muscles because they can sustain contractions for long periods of time to help the body maintain posi- tions against gravity. They are also activated during low- resistance, high-repetition movements associated with aerobic activity. Type II muscle fi bers contract at faster rates for shorter periods of time and produce more force than type I muscle fi bers. The body relies on type II fi bers to produce larger movements against greater loads than those encountered by type I fi bers. Type II fi bers have a greater capability to produce power but fatigue more quickly than type I fi bers. Skeletal muscles throughout the body are composed of all of the muscle fi ber types to provide both postural stability and mobility. The vastus lateralis consists of 50% type I and 50% type II fi bers, although it can vary from 5% to 90%, depending on the person’s genotype. Table 3.3 compares the characteristics and function of each muscle fi ber type.


MUSCLE CONNECTIVE TISSUE


Connective tissue plays a role in muscle function by pro- viding support and structure and by reinforcing the inter- nal forces produced by the contractile components of muscle when opposing external forces. The entire muscle houses extracellular connective tissue composed mainly of collagen and elastin. This noncontractile connective tissue is considered the passive component of a muscle, providing both support and elasticity to the muscular system. Epimysium is tough connective tissue that sur- rounds the entire muscle and is resistant to stretch. It


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241  |  Page 242  |  Page 243  |  Page 244  |  Page 245  |  Page 246  |  Page 247  |  Page 248  |  Page 249  |  Page 250  |  Page 251  |  Page 252  |  Page 253  |  Page 254  |  Page 255  |  Page 256  |  Page 257  |  Page 258  |  Page 259  |  Page 260  |  Page 261  |  Page 262  |  Page 263  |  Page 264  |  Page 265  |  Page 266  |  Page 267  |  Page 268  |  Page 269  |  Page 270  |  Page 271  |  Page 272  |  Page 273  |  Page 274  |  Page 275  |  Page 276  |  Page 277  |  Page 278  |  Page 279  |  Page 280  |  Page 281  |  Page 282  |  Page 283  |  Page 284  |  Page 285  |  Page 286  |  Page 287  |  Page 288  |  Page 289  |  Page 290  |  Page 291  |  Page 292  |  Page 293  |  Page 294  |  Page 295  |  Page 296  |  Page 297  |  Page 298  |  Page 299  |  Page 300  |  Page 301  |  Page 302  |  Page 303  |  Page 304  |  Page 305  |  Page 306  |  Page 307  |  Page 308  |  Page 309  |  Page 310  |  Page 311  |  Page 312  |  Page 313  |  Page 314  |  Page 315  |  Page 316  |  Page 317  |  Page 318  |  Page 319  |  Page 320  |  Page 321  |  Page 322  |  Page 323  |  Page 324  |  Page 325  |  Page 326  |  Page 327  |  Page 328  |  Page 329  |  Page 330  |  Page 331  |  Page 332  |  Page 333  |  Page 334  |  Page 335  |  Page 336  |  Page 337  |  Page 338  |  Page 339  |  Page 340  |  Page 341  |  Page 342  |  Page 343  |  Page 344  |  Page 345  |  Page 346  |  Page 347  |  Page 348  |  Page 349  |  Page 350  |  Page 351  |  Page 352  |  Page 353  |  Page 354  |  Page 355  |  Page 356  |  Page 357  |  Page 358  |  Page 359  |  Page 360